Wet Cell Batteries

  • Deep-cycle wet cell batteries have thick lead plates that
    are alloyed with antimony, and are flooded with an
    electrolyte and water mix.

  • Wet cell batteries will give off gas as a natural result of
    charging, therefore the battery compartment should have
    some sort of ventilation. Wet cell batteries should never
    be installed in in living areas.

  • Through daily recharges, over time water loss will occur.
    Deep-cycle batteries water level should be checked
    monthly and topped off with distilled water.

  • High quality deep-cycle wet cell batteries will offer good
    performance and are available in many dimensional sizes
    and Ah capacities.

  • Ah or amp hour is a unit of measure for a batteries
    storage capacity obtained by the amount of amps a
    battery can be discharged multiplied by the amount of
    hours at 80 degrees fahrenheit. For example a 100ah
    rated RV battery means that 5 amps can be discharged
    for 20 hours before the battery will fall below 10 volts.

Wet Cell Battery State of Charge Rates

  • 100% charged = 12.6 volts
  • 75% charged = 12.4 volts
  • 50% charged = 12.2 volts
  • 25% charged = 12 volts
  • 0% charged = <11.8 volts

  • Hybrid Deep-cycle/Cranking type batteries are suitable
    for both engine starting and deep-cycle applications.
    Although not recommended, this type of battery will work
    as a coach storage battery, but will have a reduced life
    span if deeply discharged, which means that the
    batteries will have to be replaced sooner than a deep-
    cycle battery that is rated in Amp-hours. This hybrid type
    of battery is usually rated in cold cranking amps (CCA).
Gel Cell Batteries

  • Deep-cycle gel cell batteries are lead-acid batteries, but
    different in construction and chemistry than wet cell
    batteries. They offer unique features like: no
    maintenance, low self-discharge rate, and low internal
  • Gel cell batteries are sealed but the battery compartment
    still needs to be ventilated. These batteries will have
    slightly shorter life span than wet cell batteries. Gel cell
    batteries dimensional sizes and Ah capacities are more
    limited than wet cell.

Gel Cell Battery State of Charge Rates

  • 100% charged = 12.95 volts
  • 75% charged = 12.65 volts
  • 50% charged = 12.35 volts
  • 25% charged = 12 volts
  • 0% charged = <11.8 volts
AGM Batteries (Absorbed Glass Mat)

  • Deep-cycle AGM batteries are maintenance free lead-
    acid batteries. They are similar to gel cell batteries and
    have the performance and charge rate of wet cell
    batteries, although they are much higher in cost than wet
    cell batteries.

  • Like Gel Cell batteries, AGM batteries are sealed but still
    need to be ventilated. AGM batteries are available in
    limited dimensional sizes and capacities.

  • Do not confuse AGM or Gel Cell batteries with standard
    maintenance-free batteries. Maintenance-free batteries
    have calcium plates alloyed with lead and will not hold up
    to repeated discharging.

AGM Battery State of Charge Rates

  • 100% charged = 12.9 volts
  • 75% charged = 12.6 volts
  • 50% charged = 12.3 volts
  • 25% charged = 12 volts
  • 0% charged = <11.8 volts
Deep Cycle Batteries

  • For optimum battery performance only use deep-cycle batteries in an RV that has a Solar and/or Inverter system installed.
  • Deep-cycle batteries fall into three categories:

  • Gel cell
  • AGM Batteries (Absorbed Glass Mat)
  • Wet Cell (Flooded)

  • High quality deep-cycle batteries are usually rated in Amp-hours (Ah) as opposed to cold cranking amps (CCA).

Charging Deep-cycle Batteries

  • Because of the charging characteristics of deep-cycle batteries, a vehicles charging system will only charge most deep-
    cycle batteries 80% to 85% of their maximum capacity. Depending on your daily use, a solar system will charge most
    deep-cycle batteries to 100% full charge before the day is out during spring and summer months.

  • For longest battery life it is recommended that deep-cycle batteries be charged when the batteries capacity has reached
    50%. A solar battery charging system will automatically take care of this for you 365 days a year.

  • Deep-cycle batteries can be discharged to about 80% of capacity before damage may occur.
State of Charge Rates
Gel Cell  Batteries
AGM Batteries
Wet Cell Batteries
100% Charged
12.95 volts
12.9 volts
12.6 volts
75% Charged
12.65 volts
12.6 volts
12.4 volts
50% Charged
12.35 volts
12.3 volts
12.2 volts
25% Charged
12 volts
12 volts
12 volts
0% Charged
11.8 volts
11.8 volts
11.8 volts

A basic overview of batteries commonly used with
photovoltaic power systems.

These are nearly all various variations of Lead-Acid batteries.

These are sometimes referred to as "deep discharge" or
"deep cell" batteries. The correct term is deep cycle.

Battery - Definition

A battery can be any device that stores energy for later use.
A rock, pushed to the top of a hill, can be considered a kind
of battery, since the energy used to push it up the hill
(chemical energy, from muscles or combustion engines) is
converted and stored as potential kinetic energy at the
top of the hill. Later, that energy is released as kinetic and
thermal energy when the rock rolls down the hill. Not real
practical for everyday use though.

Common use of the word, "battery" in electrical terms, is
limited to an electrochemical device that converts chemical
energy into electricity, by a galvanic cell.

Galvanic Cell

A galvanic cell is a fairly simple device consisting of two
electrodes of different metals or metal compounds (an anode
and a cathode) and an electrolyte (usually acid, but some are
alkaline) solution. A "Battery" is two or more of those cells in
series, although many types of single cells are usually
referred to as batteries - such as flashlight batteries.

As noted above, a battery is an electrical storage device.
Batteries do not make electricity, they store it, just as a water
tank stores water for future use. As chemicals in the battery
change, electrical energy is stored or released. In
rechargeable batteries this process can be repeated many

Batteries are not 100% efficient - some energy is lost as heat
and chemical reactions when charging and discharging. If
you use 1000 watts from a battery, it might take 1050 or 1250
watts or more to fully recharge it.

Internal Resistance

Part - or most - of the loss in charging and discharging
batteries is due to internal resistance. This is converted to
heat, which is why batteries get warm when being charged
up. The lower the internal resistance, the better.  S lower
charging and discharging rates are more efficient. A battery
rated at 180 amp-hours over 6 hours might be rated at 220
AH at the 20-hour rate, and 260 AH at the 48-hour rate. Much
of this loss of efficiency is due to higher internal resistance
at higher amperage rates - internal resistance is not a
constant - kind of like "the more you push, the more it pushes

Typical efficiency in a lead-acid battery is 85-95%, in alkaline
and NiCad battery it is about 65%. True deep cycle AGM's
(such as Concorde and Deka) can approach 98%.

PV Applications

Practically all batteries used in PV and all but the smallest
backup systems are Lead-Acid type batteries. Even after
over a century of use, they still offer the best price to power
ratio. A few systems use NiCad, but we do not recommend
them except in cases where extremely cold temperatures
(-50 F or less) are common.  They are expensive to buy, and
very expensive to dispose of due the the hazardous nature
of Cadmium.

An important fact is that
ALL of the batteries commonly
used in deep cycle applications are Lead-Acid. This
includes the standard flooded (wet) batteries, gelled,
and AGM.
They all use the same chemistry, although the
actual construction of the plates etc varies.

NiCads, Nickel-Iron, and other types are found in a few
systems, but are not common due to their expense,
environmental hazards, and/or poor efficiency.

Major Battery Types

Batteries are divided in two ways, by application (what they
are used for) and construction (how they are built). The
major applications are automotive, marine, and deep-cycle.

Deep-cycle includes solar electric (PV), backup
power, and RV and boat "house" batteries.

The major construction types are flooded (wet),
gelled, and AGM (Absorbed Glass Mat).


AGM batteries are also sometimes called "starved electrolyte"
or "dry", because the fiberglass mat is only 95% saturated
with Sulfuric acid and there is no excess liquid.


Flooded may be standard, with removable caps, or the
so-called "maintenance free" (that means they are designed
to die one week after the warranty runs out).


All gelled are sealed and are "valve regulated", which means
that a tiny valve keeps a slight positive pressure.  

Nearly all AGM batteries are sealed valve regulated
(commonly referred to as "VRLA" - Valve Regulated
Lead-Acid).  Most valve regulated are under some pressure -
1 to 4 psi at sea level.

Lifespan of Batteries

The lifespan of a deep cycle battery will vary considerably
with how it is used, how it is maintained and charged,
temperature, and other factors. In extreme cases, it can vary
to extremes - we have seen L-16's killed in less than a year
by severe overcharging, and we have a large set of surplus
telephone batteries that sees only occasional (5-10 times per
year) heavy service that are now over 25 years old.

We have seen gelled cells destroyed in one day when
overcharged with a large automotive charger. We have seen
golf cart batteries destroyed without ever being used in less
than a year because they were left sitting in a hot garage
without being charged. Even the so-called "dry charged"
(where you add acid when you need them) have a shelf life
of 18 months at most. They are not totally dry - they are
actually filled with acid, the plates formed and charged, then
the acid is dumped out.

These are some typical (minimum - maximum) typical
expectations for batteries if used in deep cycle service.
There are so many variables, such as depth of discharge,
maintenance, temperature, how often and how deep cycled,
etc. that it is almost impossible to give a fixed number.
Starting: 3-12 months
Marine: 1-6 years
Golf cart: 2-7 years
AGM deep cycle: 4-7 years
Gelled deep cycle: 2-5 years
Deep cycle (L-16 type etc): 4-8 years
Rolls-Surrette premium deep cycle: 7-15 years
Industrial deep cycle (Crown and Rolls 4KS series): 10-20+
Telephone (float): 2-20 years. These are usually special
purpose "float service", but often appear on the surplus
market as "deep cycle". They can vary considerably,
depending on age, usage, care, and type.
NiFe (alkaline): 5-35 years
NiCad: 1-20 years

Starting Batteries

Starting (sometimes called SLI, for starting, lighting, ignition)
batteries are commonly used to start and run engines. Engine
starters need a very large starting current for a very short
time. Starting batteries have a large number of thin plates for
maximum surface area. The plates are composed of a Lead
"sponge", similar in appearance to a very fine foam sponge.
This gives a very large surface area, but if deep cycled, this
sponge will quickly be consumed and fall to the bottom of the
cells. Automotive batteries will generally fail after 30-150
deep cycles if deep cycled, while they may last for
thousands of cycles in normal starting use (2-5% discharge).

Deep Cycle Batteries

Deep cycle batteries are designed to be discharged down as
much as 80% time after time, and have much thicker plates.
The major difference between a true deep cycle battery and
others is that the plates are SOLID Lead plates - not sponge.
This gives less surface area, thus less "instant" power like
starting batteries need. Although these an be cycled down to
20% charge, the best lifespan vs cost method is to keep the
average cycle at about 50% discharge.

Unfortunately, it is often impossible to tell what you are really
buying in some of the discount stores or places that
specialize in automotive batteries. The golf cart battery is
quite popular for small systems and RV's. The problem
is that "golf car" refers to a size of battery (commonly called
GC-2, or T-105), not the type or construction - so the quality
and construction of a golf car battery can vary considerably
- ranging from the cheap off brand with thin plates up the
true deep cycle brands, such as Crown, Deka, Trojan, etc.
In general, you get what you pay for.

Marine Batteries

Marine batteries are usually a "hybrid", and fall between the
starting and deep-cycle batteries, though a few
(Rolls-Surrette and Concorde, for example) are true deep
cycle. In the hybrid, the plates may be composed of Lead
sponge, but it is coarser and heavier than that used in
starting batteries. It is often hard to tell what you are getting
in a "marine" battery, but most are a hybrid.

Starting batteries are usually rated at "CCA", or cold cranking
amps, or "MCA", Marine cranking amps - the same as "CA".
Any battery with the capacity shown in CA or MCA may or
may not be a true deep-cycle battery. It is sometimes hard to
tell, as the term deep cycle is often overused. CA and MCA
ratings are at 32 degrees F, while CCA is at zero degree F.
Unfortunately, the only positive way to tell with some
batteries is to buy one and cut it open - not much of an option.

Using a deep cycle battery as a starting battery
There is generally no problem with this, providing that
allowance is made for the lower cranking amps compared to
a similar size starting battery. As a general rule, if you are
going to use a true deep cycle battery (such as the Concorde
SunXtender) also as a starting battery, it should be oversized
about 20% compared to the existing or recommended starting
battery group size to get the same cranking amps. That is
about the same as replacing a group 24 with a group 31.

With modern engines with fuel injection and electronic
ignition, it generally takes much less battery power to crank
and start them, so raw cranking amps is less important than it
used to be. On the other hand, many cars, boats, and RV's
are more heavily loaded with power sucking "appliances",
such as megawatt stereo systems etc. that are more suited
for deep cycle batteries. We have used the Concorde
SunXtender AGM batteries in some of our vehicles with no

It will not hurt a deep cycle battery to be used as a starting
battery, but for the same size battery they cannot supply as
much cranking amps as a regular starting battery.

Battery Construction Materials

Nearly all large rechargeable batteries in common
use are Lead-Acid type.
 (There are some NiCads in use,
but for most purposes the very high initial expense, and the
high expense of disposal, does not justify them). The acid is
typically 30% Sulfuric acid and 70% water at full charge.
NiFe (Nickel-Iron) batteries are also available - these have a
very long life, but rather poor efficiency (60-70%) and the
voltages are different, making it more difficult to match up
with standard 12v/24/48v systems and inverters. The biggest
problem with NiFe batteries is that you may have to put in 100
watts to get 70 watts of charge - they are much less
efficient than Lead-Acid. What you save on batteries you will
have to make up for by buying a larger solar panel system.
NiCads are also inefficient - typically around 65% - and very
expensive. However, NiCads can be frozen without damage,
so are sometimes used in areas where the temperatures may
fall below -50 degrees F.

Most AGM batteries will also survive freezing with no
problems, even though the
output when frozen will be little or nothing.

Industrial deep cycle batteries

Sometimes called "fork lift", "traction" or "stationary" batteries,
are used where power is needed over a longer period of
time, and are designed to be "deep cycled", or discharged
down as low as 20% of full charge (80% DOD, or Depth of
Discharge). These are often called traction batteries because
of their widespread use in forklifts, golf carts, and floor
sweepers (from which we get the "GC" and "FS" series of
battery sizes). Deep cycle batteries have much thicker plates
than automotive batteries.

Plate Thickness

Plate thickness (of the Positive plate) matters because of a
factor called "positive grid corrosion". This ranks among the
top 3 reasons for battery failure. The positive (+) plate is
what gets eaten away gradually over time, so eventually
there is nothing left - it all falls to the bottom as sediment.
Thicker plates are directly related to longer life, so other
things being equal, the battery with the thickest plates will
last the longest. The negative plate in batteries expands
somewhat during discharge, which is why nearly all
batteries have separators, such as glass mat or paper, that
can be compressed.
Automotive batteries typically have plates about .040"
(4/100") thick, while forklift batteries may have plates more
than 1/4" (.265" for example in larger Rolls-Surrette) thick -  
almost 7 times as thick as auto batteries. The typical
golf cart will have plates that are around .07 to .11" thick. The
Concorde AGM's are .115", The Rolls-Surrette L-16 type
(CH460) is .150", and the US Battery and Trojan L-16 types
are .090". The Crown L-16HC size has .22" thick plates. While
plate thickness is not the only factor in how many deep
cycles a battery can take before it dies, it is the most
important one.

Most industrial (fork lift) deep-cycle batteries use
Lead-Antimony plates rather than the Lead-Calcium used in
AGM or gelled deep-cycle batteries and in automotive starting
batteries. The Antimony increases plate life and strength,
but increases gassing and water loss.  This is why most
industrial batteries have to be checked often for water level if
you do not have Hydrocaps. The self discharge of batteries
with Lead-Antimony plates can be high - as much as 1% per
day on an older battery. A new AGM typically
self-discharges at about 1-2% per month, while an old one
may be as much as 2% per week.

Sealed batteries

Sealed batteries are made with vents that (usually) cannot be
removed. The so-called Maintenance Free batteries are also
sealed, but are not usually leak proof. Sealed batteries are
not totally sealed, as they must allow gas to vent during
charging. If overcharged too many times, some of these
batteries can lose enough water that they will die before their
time. Most smaller deep cycle batteries (including AGM) use
Lead-Calcium plates for increased life, while most industrial
and forklift batteries use Lead-Antimony for greater plate
strength to withstand shock and vibration.

Lead-Antimony (such as forklift and floor scrubber) batteries
have a much higher self-discharge rate (2-10% per week)
than Lead or Lead-Calcium (1-5% per month), but the
Antimony improves the mechanical strength of the plates,
which is an important factor in electric vehicles. They are
generally used where they are under constant or very
frequent charge/discharge cycles, such as fork lifts and
floor sweepers. The Antimony increases plate life at the
expense of higher self discharge. If left for long periods
unused, these should be trickle charged to avoid damage
from sulfation - but this applies to ANY battery. As in all
things, there are trade offs. The Lead-Antimony types have a
very long lifespan, but higher self discharge rates.

Battery Size Codes

Batteries come in all different sizes. Many have "group"
sizes, which is based upon the physical size and terminal
placement. It is NOT a measure of battery capacity. Typical
BCI codes are group U1, 24, 27, and 31. Industrial batteries
are usually designated by a part number such as "FS" for
floor sweeper, or "GC" for golf cart. Many batteries follow no
particular code, and are just manufacturers part numbers.
Other standard size codes are 4D & 8D, large industrial
batteries, commonly used in solar electric systems.
Some common battery size codes used are: (ratings are
U134 to 40 Amp hours12 volts
Group 2470-85 Amp hours12 volts
Group 2785-105 Amp hours12 volts
Group 3195-125 Amp hours12 volts
4-D180-215 Amp hours12 volts
8-D225-255 Amp hours12 volts
Golf Cart & T-105180 to 225 Amp hours6 volts
L-16, L16HC etc.340 to 415 Amp hours6 volts

Gelled electrolyte

Gelled batteries, or "Gel Cells" contain acid that has been
"gelled" by the addition of Silica Gel, turning the acid into a
solid mass that looks like gooey Jell-O. The advantage of
these batteries is that it is impossible to spill acid even if they
are broken. However, there are several disadvantages. One
is that they must be charged at a slower rate (C/20) to
prevent excess gas from damaging the cells. They cannot be
fast charged on a conventional automotive charger or they
may be permanently damaged.

This is not usually a problem with solar electric systems, but
if an auxiliary generator or inverter bulk charger is used,
current must be limited to the manufacturers specifications.
Most better inverters commonly used in solar electric
systems can be set to limit charging current to the batteries.

Some other disadvantages of gel cells is that they must be
charged at a lower voltage (2/10th's less) than flooded or
AGM batteries. If overcharged, voids can develop in the gel
which will never heal, causing a loss in battery capacity. In
hot climates, water loss can be enough over 2-4 years to
cause premature battery death.

It is for this and other reasons that we no longer sell any of
the gelled cells except for replacement use. The newer AGM
(absorbed glass mat) batteries have all the advantages (and
then some) of gelled, with none of the disadvantages.

AGM, or Absorbed Glass Mat Batteries

A newer type of sealed battery uses "Absorbed Glass
Mats", or AGM between the plates. This is a very fine fiber
Boron-Silicate glass mat. These type of batteries have all the
advantages of gelled, but can take much more abuse. We
sell the Concorde (and Lifeline, made by Concorde) AGM
batteries. These are also called "starved electrolyte", as the
mat is about 95% saturated rather than fully soaked. That
also means that they will not leak acid even if broken.

AGM batteries have several advantages over both gelled and
flooded, at about the same cost as gelled:

Since all the electrolyte (acid) is contained in the glass mats,
they cannot spill, even if broken.

This also means that since they are non-hazardous, the
shipping costs are lower.

In addition, since there is no liquid to freeze and expand, they
are practically immune from freezing damage.

Nearly all AGM batteries are "recombinant" - what that means
is that the Oxygen and Hydrogen recombine INSIDE the
battery. These use gas phase transfer of oxygen to the
negative plates to recombine them back into water while
charging and prevent the loss of water through electrolysis.
The recombining is typically 99+% efficient, so almost no
water is lost.

The charging voltages are the same as for any standard
battery - no need for any special adjustments or problems
with incompatible chargers or charge controls. And, since
the internal resistance is extremely low, there is almost no
heating of the battery even under heavy charge and
discharge currents. The Concorde (and most AGM) batteries
have no charge or discharge current limits.

AGM's have a very low self-discharge - from 1% to 3% per
month is usual. This means that they can sit in storage for
much longer periods without charging than standard
batteries. The Concorde batteries can be almost fully
recharged (95% or better) even after 30 days of being totally

AGM's do not have any liquid to spill, and even under severe
overcharge conditions hydrogen emission is far below the
4% max specified for aircraft and enclosed spaces.

The plates in AGM's are tightly packed and rigidly mounted,
and will withstand shock and vibration better than any
standard battery.

Even with all the advantages listed above, there is still a
place for the standard flooded deep cycle battery. AGM's will
cost 2 to 3 times as much as flooded batteries of the same
capacity. In many installations, where the batteries are set in
an area where you don't have to worry about fumes or
leakage, a standard or industrial deep cycle is a better
economic choice.

AGM batteries main advantages are no maintenance,
completely sealed against fumes, Hydrogen, or leakage,
non-spilling even if they are broken, and can survive most
freezes. Not everyone needs these features.

Temperature Effects on Batteries

Battery capacity (how many amp-hours it can hold) is
reduced as temperature goes down, and increased as
temperature goes up. This is why your car battery dies on a
cold winter morning, even though it worked fine the previous
afternoon. If your batteries spend part of the year shivering
in the cold, the reduced capacity has to be taken into account
when sizing the system batteries. The standard rating for
batteries is at room temperature - 25 degrees C (about 77
F). At approximately -22 degrees F (-27 C), battery AH
capacity drops to 50%. At freezing, capacity is reduced by
20%. Capacity is increased at higher temperatures - at 122
degrees F, battery capacity would be about 12% higher.

Battery charging voltage also changes with temperature. It
will vary from about 2.74 volts per cell (16.4 volts) at -40 C to
2.3 volts per cell (13.8 volts) at 50 C.

This is why you should have temperature compensation on
your charger or charge control if your batteries are outside
and/or subject to wide temperature variations.

Charge Controls

Some charge controls have temperature compensation built in
(such as Morningstar) - this works fine if the controller is
subject to the same temperatures as the batteries. However,
if your batteries are outside, and the controller is inside, it
does not work that well. Adding another complication is
that large battery banks make up a large thermal mass.

Thermal mass means that because they have so much mass,
they will change internal temperature much slower than the
surrounding air temperature. A large insulated battery bank
may vary as little as 10 degrees over 24 hours internally,
even though the air temperature varies from 20 to 70
degrees. For this reason, external (add-on) temperature
sensors should be attached to one of the POSITIVE plate
terminals, and bundled up a little with some type of insulation
on the terminal. The sensor will then read very close to the
actual internal battery temperature.

Even though battery capacity at high temperatures is higher,  
battery life is shortened. Battery capacity is reduced by 50%
at -22 degrees F - but battery LIFE increases by about 60%.
Battery life is reduced at higher temperatures - for every 15
degrees F over 77, battery life is cut in half. This holds true
for ANY type of Lead-Acid battery, whether sealed, gelled,
AGM, industrial or whatever. This is actually not as bad as it
seems, as the battery will tend to average out the good and
bad times.

One last note on temperatures - in some places that have
extremely cold or hot conditions, batteries may be sold locally
that are NOT standard electrolyte (acid) strengths. The
electrolyte may be stronger (for cold) or weaker (for very
hot) climates. In such cases, the specific gravity and the
voltages may vary from what we show.

Cycles vs Life

A battery "cycle" is one complete discharge and recharge
cycle. It is usually considered to be discharging from 100% to
20%, and then back to 100%. However, there are often
ratings for other depth of discharge (DOD) cycles, the most
common ones are 10%, 20%, and 50%. You have to be
careful when looking at ratings that list how many cycles a
battery is rated for unless it also states how far down it is
being discharged.

For example, one of the widely advertised telephone type
(float service) batteries have been advertised as having a
20-year life. If you look at the fine print, it has that rating only
at 5% DOD - it is much less when used in an application
where they are cycled deeper on a regular basis.  Those  
same batteries are rated at less than 5 years if cycled to
50%. For example, most golf cart batteries are rated for
about 550 cycles to 50% discharge - which equates to about
2 years.

Battery life is directly related to how deep the battery is
cycled each time. If a battery is discharged to 50% every
day, it will last about twice as long as if it is cycled to 80%
DOD. If cycled only 10% DOD, it will last about 5 times as
long as one cycled to 50%. Obviously, there are some
practical limitations on this - you don't usually want to have a
5 ton pile of batteries sitting there just to reduce the DOD. The
most practical number to use is 50% DOD on a regular basis.
This does NOT mean you cannot go to 80% once in a while.

It's just that when designing a system when you have some
idea of the loads, you should figure on an average DOD of
around 50% for the best storage vs cost factor. Also, there
is an upper limit - a battery that is continually cycled 5% or
less will usually not last as long as one cycled down 10%.
This happens because at very shallow cycles, the Lead
Dioxide tends to build up in clumps on the the positive plates
rather in an even film. The graph above shows how lifespan
is affected by depth of discharge. The chart is for a
Concorde Lifeline battery, but all lead-acid batteries will be
similar in the shape of the curve, although the number of
cycles will vary.

Battery Voltages

All Lead-Acid batteries supply about 2.14 volts per cell (12.6
to 12.8 for a 12 volt battery) when fully charged. Batteries
that are stored for long periods will eventually lose all their
charge. This "leakage" or self discharge varies considerably
with battery type, age, & temperature. It can range from
about 1% to 15% per month.

Generally, new AGM batteries have the lowest
self-discharge rates, and old industrial (Lead-Antimony
plates) are the highest.

In systems that are continually connected to some type
charging source, whether it is solar, wind, or an AC
powered charger this is seldom a problem.

However, one of the biggest killers of batteries is sitting
stored in a partly discharged state for a few months. A
"float" charge should be maintained on the batteries even if
they are not used (or, especially if they are not used).
Even most "dry charged" batteries (those sold without
electrolyte so they can be shipped more easily, with acid
added later) will deteriorate over time. Max storage life on
those is about 2-3 years.

Batteries self-discharge faster at higher temperatures.
Lifespan can also be seriously reduced at higher
temperatures - most manufacturers state this as a
50% loss in life for every 15 degrees F over a 77 degree cell
temperature. Lifespan is increased at the same rate if below
77 degrees, but capacity is reduced. This tends to even out
in most systems - they will spend part of their life at higher
temperatures, and part at lower.

Myth: The old myth about not storing batteries on concrete
floors is just that - a myth. This story has been around for
100 years, and originated back when
battery cases were made up of wood and asphalt. The acid
would leak from them, and form a slow-discharging circuit
through the now acid-soaked and conductive floor.

State of Charge

State of charge, or conversely, the depth of discharge (DOD)
can be determined by measuring the voltage and/or the
specific gravity of the acid with a hydrometer. This will NOT
tell you how good (capacity in AH) the battery condition is -
only a sustained load test can do that. Voltage on a fully
charged battery will read 2.12 to 2.15 volts per cell, or 12.7
volts for a 12 volt battery. At 50% the reading will be 2.03
VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less.
Specific gravity will be about 1.265 for a fully charged cell,
and 1.13 or less for a totally discharged cell.

This can vary with battery types and brands somewhat -
when you buy new batteries you should charge them up and
let them sit for a while, then take a reference measurement.
Many batteries are sealed, and hydrometer reading cannot be
taken, so you must rely on voltage. Hydrometer readings may
not tell the whole story, as it takes a while for the acid to get
mixed up in wet cells. If measured right after charging, you
might see 1.27 at the top of the cell, even though it is much
less at the bottom. This does not apply to gelled or AGM

"False" Capacity

A battery can meet the voltage tests for being at full charge,
yet be much lower than it's original capacity. If plates are
damaged, sulfated, or partially gone from long use, the
battery may give the appearance of being fully charged, but
in reality acts like a battery of much smaller size. This same
thing can occur in gelled cells if they are overcharged and
gaps or bubbles occur in the gel. What is left of the plates
may be fully functional, but with only 20% of the
plates left... Batteries usually go bad for other reasons
before reaching this point, but it is something to be aware of
if your batteries seem to test OK but lack capacity and go
dead very quickly under load.  Be careful that you are not just
measuring the surface charge. To properly check the
voltages, the battery should sit at rest for a few hours, or
you should put a small load on it, such as a small automotive
bulb, for a few minutes. The voltages below apply to ALL
Lead-Acid batteries, except gelled. For gel cells, subtract .2
volts. Note that the voltages when actually charging will be
quite different, so do not use these numbers for a battery
that is under charge.

Amp-Hour Capacity

All deep cycle batteries are rated in amp-hours. An amp-hour
is one amp for one hour, or 10 amps for 1/10 of an hour and
so forth. It is amps x hours. If you have something that pulls
20 amps, and you use it for 20 minutes, then the amp-hours
used would be 20 (amps) x .333 (hours), or 6.67 AH. The
generally accepted AH rating time period for batteries used in
solar electric and backup power systems (and for nearly all
deep cycle batteries) is the "20 hour rate". (Some, such as
the Concorde AGM, use the 24 hour rate, which is probably a
better real-world rating).  This means that it is discharged
down to 10.5 volts over a 20 hour period while the total
actual amp-hours it supplies is measured. Sometimes ratings
at the 6 hour rate and 100 hour rate are also given for
comparison and for different applications. The 6-hour rate is
often used for industrial batteries, as that is a typical daily
duty cycle. Sometimes the 100 hour rate is given just to make
the battery look better than it really is, but it is also useful for
figuring battery capacity for long-term backup amp-hour

Why amp-hours are specified at a particular rate:
Because of something called the Peukert Effect. The Peukert
value is directly related to the internal resistance of the
battery. The higher the internal resistance, the higher the
losses while charging and discharging, especially at
higher currents. This means that the faster a battery is used
(discharged), the LOWER the AH capacity. Conversely, if it is
drained slower, the AH capacity is higher. This is important
because some manufacturers and vendors have chosen to
rate their batteries at the 100 hour rate - which makes them
look a lot better than they really are. Here are some typical
battery capacities from the manufacturers data sheets:
Battery Type100 hour rate20 hour rate8
Trojan T-105250 AH225 AHn/a
US Battery 2200n/a225 AH181 AH
Concorde PVX-6220255 AH221 AH183 AH
Surrette S-460 (L-16)429 AH344 AH282 AH
Trojan L-16400 AH360 AHn/a
Surrette CS-25-PS974 AH779 AH639 AH

State of Charge

Here are no-load typical voltages vs state of charge
(figured at 10.5 volts = fully discharged, and 77 degrees F).
Voltages are for a 12 volt battery system. For 24 volt
systems multiply by 2, for 48 volt system, multiply by 4. VPC
is the volts per individual cell - if you measure more than a
.2 volt difference between each cell, you need to equalize, or
your batteries are going bad, or they may be sulfated. These
voltages are for batteries that have been at rest for 3 hours
or more. Batteries that are being charged will be higher - the
voltages while under charge will not tell you anything, you
have to let the battery sit for a while. For longest life,
batteries should stay in the green zone. Occasional dips into
the yellow are not harmful, but continual
discharges to those levels will shorten battery life
considerably. It is important to realize that voltage
measurements are only approximate. The best determination
is to measure the specific gravity, but in many batteries this is
difficult or impossible. Note the large voltage drop in the last
State of Charge12 Volt batteryVolts per Cell

Battery Charging

Battery charging takes place in 3 basic stages: Bulk,
Absorption, and Float

Bulk Charge -

The first stage of 3-stage battery charging. Current is sent to
batteries at the maximum safe rate they will accept until
voltage rises to near (80-90%) full charge level. Voltages at
this stage typically range from 10.5 volts to 15 volts. There is
no "correct" voltage for bulk charging, but there may be limits
on the maximum current that the battery and/or wiring can

Absorption Charge:

The 2nd stage of 3-stage battery charging. Voltage remains
constant and current gradually tapers off as internal
resistance increases during charging. It is during this stage
that the charger puts out maximum voltage. Voltages at this
stage are typically around 14.2 to 15.5 volts.

Float Charge:

The 3rd stage of 3-stage battery charging. After batteries
reach full charge, charging voltage is reduced to a lower
level (typically 12.8 to 13.2) to reduce gassing and prolong
battery life. This is often referred to as a maintenance or
trickle charge, since it's main purpose is to keep an already
charged battery from discharging. PWM, or "pulse width
modulation" accomplishes the same thing. In PWM, the
controller or charger senses tiny voltage drops in the battery
and sends very short charging cycles (pulses) to the battery.
This may occur several hundred times per minute. It is called
"pulse width" because the width of the pulses may vary from
a few microseconds to several seconds.  Note that for long
term float service, such as backup power systems that are
seldom discharged, the float voltage should be around 13.02
to 13.20 volts.  Chargers: Most garage and consumer
(automotive) type battery chargers are bulk charge only, and
have little (if any) voltage regulation. They are fine for a quick
boost to low batteries, but not to leave on for long periods.
Among the regulated chargers, there are the voltage
regulated ones, such as Iota Engineering and Todd, which
keep a constant regulated voltage on the batteries.  If these
are set to the correct voltages for your batteries, they will
keep the batteries charged without damage. These are
sometimes called "taper charge" - as if that is a selling point.
What taper charge really means is that as the battery gets
charged up, the voltage goes up, so the amps out of the
charger goes down. They charge OK, but a charger rated at
20 amps may only be supplying 5 amps when the batteries
are 80% charged. To get around this, Statpower (and maybe
others?) have come out with "smart", or multi-stage
chargers. These use a variable voltage to keep the charging
amps much more constant for faster charging.

Charge controllers

A charge controller is a regulator that goes between the
solar panels and the batteries. Regulators for solar systems
are designed to keep the batteries charged at peak without
overcharging. Meters for Amps (from the panels) and
battery Volts are optional with most types. Some of the
various brands and models that we use and recommend are
listed below. Note that a couple of them are listed as "power

"Why 120 watts does not equal 120 watts".
Most of the modern controllers have automatic or manual
equalization built in, and many have a LOAD output. There is
no "best" controller for all applications - some systems may
need the bells and whistles of the more expensive controls,
others may not.

These are some of the charge controllers that we
recommend, but almost any modern controller will work fine.
Exact model will depend on application and system size and
Xantrex (All)
Morningstar (All)
Outback Power MX60 & 80
Blue Sky Energy (Solar Boost)
Using any of these will almost always give better battery life
and charge than "on-off" or simple shunt type regulators

Battery Charging Voltages and Currents:

Most flooded batteries should be charged at no more than the
"C/8" rate for any sustained period. "C/8" is the battery
capacity at the 20-hour rate divided by 8. For a 220 AH
battery, this would equal 26 Amps.

Gelled cells should be charged at no more than the C/20 rate,
or 5% of their amp-hour capacity.   The Concorde AGM
batteries are a special case - the can be charged at up the
the Cx4 rate, or 400% of the capacity for the bulk charge
cycle. However, since very few battery cables can take that
much current, we don't recommend you try this at home. To
avoid cable overheating, you should stick to C/4 or less.

Charging at 15.5 volts will give you a 100% charge on
Lead-Acid batteries. Once the charging voltage reaches
2.583 volts per cell, charging should stop or be reduced to a
trickle charge. Note that flooded batteries MUST bubble (gas)
somewhat to insure a full charge, and to mix the electrolyte.

Float voltage for Lead-Acid batteries should be about 2.15 to
2.23 volts per cell, or about 12.9-13.4 volts for a 12 volt
battery. At higher temperatures (over 85 degrees F) this
should be reduced to about 2.10 volts per cell.

Never add acid to a battery except to replace spilled liquid.
Distilled or deionized water should be used to top off
non-sealed batteries. Float and charging voltages for gelled
batteries are usually about 2/10th volt less than for flooded to
reduce water loss. Note that many shunt-type charge
controllers sold for solar systems will NOT give you a full
charge - check the specifications first. To get a full charge,
you must continue to apply a current after the battery voltage
reaches the cutoff point of most of these type of
controllers. This is why we recommend the charge controls
and battery chargers listed in the sections above. Not all
shunt type controllers are 100% on or off, but most are.

Flooded battery life can be extended if an equalizing charge
is applied every 10 to 40 days. This is a charge that is about
10% higher than normal full charge voltage, and is applied for
about 2 to 16 hours. This makes sure that all the cells are
equally charged, and the gas bubbles mix the electrolyte. If
the liquid in standard wet cells is not mixed, the electrolyte
becomes "stratified". You can have very strong solution at
the bottom, and very weak at the top of the cell. With
stratification, you can test a battery with a hydrometer and
get readings that are quite a ways off. If you cannot equalize
for some reason, you should let the battery sit for at least 24
hours and then use the hydrometer.  AGM and gelled should
be equalized 2-4 times a year at most - check the
manufacturers recommendations, especially on gelled.

Battery Aging

As batteries age, their maintenance requirements change.
This means longer charging time and/or higher finish rate
(higher amperage at the end of the charge). Usually older
batteries need to be watered more often. And, their
capacity decreases.

Mini Factoids

Nearly all batteries will not reach full capacity until cycled
10-30 times. A brand new battery will have a capacity of
about 5-10% less than the rated capacity.

Batteries should be watered after charging unless the plates
are exposed, then add just enough water to cover the plates.
After a full charge, the water level should be even in all cells
and usually 1/4" to 1/2" below the bottom of the fill well in the
cell (depends on battery size and type).

In situations where multiple batteries are connected in series,
parallel or series/parallel, replacement batteries should be the
same size, type and manufacturer (if possible). Age and
usage level should be the same as the companion batteries.

Do not put a new battery in a pack which is more than 6
months old or has more than 75 cycles.  Either replace with
all new or use a good used battery. For long life batteries,
such as the Surrette and Crown, you can have up to a one
year age difference.

The vent caps on flooded batteries should remain on the
battery while charging.  This prevents a lot of the water loss
and splashing that may occur when they are bubbling.

When you first buy a new set of flooded (wet) batteries, you
should fully charge and equalize them, and then take a
hydrometer reading for future reference.

Since not all batteries have exactly the same acid strength,
this will give you a baseline for future readings.

When using a small solar panel to keep a float (maintenance)
charge on a battery (without using a charge controller),
choose a panel that will give a maximum output of about
1/300th to 1/1000th of the amp-hour capacity.

For a pair of golf cart batteries, that would be about a 1 to 5
watt panel - the smaller panel if you get 5 or more hours of
sun per day, the larger one for those long cloudy winter days
in the Northeast.

Lead-Acid batteries do NOT have a memory, and the rumor
that they should be fully discharged to avoid this "memory" is
totally false and will lead to early battery failure.

Inactivity can be extremely harmful to a battery. It is a VERY
poor idea to buy new batteries and "save" them for later.
Either buy them when you need them, or keep them on a
continual trickle charge. The best thing - if you buy them, use

Only clean water should be used for cleaning the outside of
batteries. Solvents or spray cleaners should not be used.

Manufacturers Websites
US Battery Manufacturing Company - some good information
and data.
Crown Battery - A major manufacturer of industrial and deep
cycle batteries.
Trojan Battery - not a lot of real technical info here, but has all
the specifications.
Surrette - Specs and data on the Surrette deep cycle and
marine batteries
Concorde - specs and data on all the Concorde batteries,
including Lifeline.

Northern Arizona Wind & Sun
Lead Acid Batteries

  • Using an electrolyte consisting of sulphuric acid, these cells can store
    electrical energy in a relatively small space.  This energy is stored in chemical
    form within lead grids mounted inside the battery.

  • Currently, there are three common lead-acid battery technologies:

  • Gel Cell (VRLA)
  • AGM (VRLA)
  • Flooded (Wet Cells)

The two most common VRLA batteries used today are Gel Cell and
Absorbed Glass Mat (AGM) variety

  • Gel Cell batteries feature an electrolyte that has been immobilized using a
    gelling agent like fumed silica.

  • AGM batteries feature a thin fiberglass felt that holds the electrolyte in place
    like a sponge.

  • Neither AGM or Gel cells will leak if inverted, pierced, etc. and will continue to
    operate even under water.

Gel Cells

  • Gel Cells use a thickening agent like fumed silica to immobilize the electrolyte.
    Thus, if the battery container cracks or is breached, the cell will continue to
    function. Furthermore, the thickening agent prevents stratification by
    preventing the movement of electrolyte.

  • As Gel cells are sealed and cannot be re-filled with electrolyte, controlling the
    rate of charge is very important or the battery will be ruined.  Gel cells use
    slightly lower charging voltages than flooded cells and thus the set-points for
    charging equipment have to be adjusted.

AGM (Absorbed Glass Mat Batteries)

  • Absorbed Glass Mat (AGM) batteries are the latest step in the evolution of
    lead-acid batteries. Instead of using a gel, an AGM uses a fiberglass-like
    separator to hold the electrolyte in place.

  • The physical bond between the  separator fibers, the lead plates, and the
    container make AGMs spill-proof and the most vibration and impact
    resistant lead-acid batteries available.

  • AGMs use almost the same voltage set-points as flooded cells and thus can
    be used as drop-in replacements for flooded cells. Basically, an AGM
    can do anything a Gel-cell can, only better.  However, since they are also
    sealed, charging has to be controlled carefully or they too can be ruined.


  • There are some very compelling reasons to use VRLAs: (Valve-Regulated
    Lead Acid) Gel and Absorbed Glass Mat (AGM) batteries can dispense
    charge at a higher rate than flooded cells due to their lower Peukerts

  • Deep-cycle Flooded Cells cannot deliver more than 25% of their rated amp-
    hour capacity in amps without plummeting Available Capacity.

  • Deep-Cycle Flooded cell battery manufacturers recommend a 4 to 1 ratio
    between battery bank size and the largest load encountered on board.

  • AGM and Gel cell manufacturers recommend a ratio of at least 3 to 1, a
    significant difference for loads such as the engine starter or windlass.

  • Virtually no gassing under normal operating conditions: Unlike flooded cells,
    gel cells and AGMs are hermetically sealed and operate under pressure to
    recombine the oxygen and hydrogen produced during the charge process
    back into water.

  • You find VRLAs in the bilges of high end yachts such as Hinckley, Hans
    Christian, Island Packet, etc.. Every boat benefits from a low center of gravity
    over the keel (good for righting purposes) and the minimal venting
    requirements make it possible.

  • The ability to put VRLAs in the bilges (they can operate under water should
    you hole yourself) also lengthens their lives: For every additional 15
    degrees of heat over 77 deg F, lead acid battery life (regardless of
    type) is cut in half (batteries self-destruct with time, you can only
    slow that process).  Chances are, the bilges are the coldest place on board
    (outside the freezer) and the keel provides protection.

  • VRLAs can operate in any orientation (although you may lose some capacity
    that way) and even if a container is broken, a VRLA will not leak.

  • Proper (heavy duty) battery restraints are a must, regardless of battery type.

  • Gel cells and AGMs require no maintenance once the charging system has
    been properly set up. No equalization charges (usually), no electrolyte to
    replenish, no specific gravity checks, no additional safety gear to carry on
    board in order to protect yourself.

  • VRLAs can be load tested, however, proper charge control and protection is
    much more important with VRLAs because once fried it is impossible to revive

  • The charge acceptance of AGMs can burn up an alternator if the charging
    system is not adequate for extended runtimes at full power. The larger the
    battery bank and the harder the charger is made to work, the more important it
    is to ensure that the charging system can handle the currents for extended
    periods of time. This caveat does not really apply to low-duty applications like
    starter banks, since they usually need so little charge to be topped up.

  • Even the small alternators found in Jet Skis should be able to handle an AGM
    starter battery, as long as that battery is just used for that - starting.

  • On the other hand, if you need a large house bank and want to rely on a
    single charge source for much of the power, you may want to explore a high
    quality charge system from a respected company such as Ample Power,
    Balmar, Ferris, Hehr, JackRabbit Marine, SALT, etc.

  • Ensure that the alternator receives enough cooling air as a hot alternator will
    produce less energy than a cool one and last longer.

  • AGMs and to a lesser extent gel cell systems can benefit from using the
    thermal alternator protection offered by the Balmar MaxCharge series of   
    regulators, particularly if you expect to bulk charge your system for extended
    periods of time and don't have good engine compartment ventilation.

  • The higher charge efficiency of AGMs allows you to recharge with less
    energy:  Flooded cells convert 15-20% of the electrical energy into heat
    instead of potential power.  Gel-cells lose 10-16% but AGMs as little as 4%.

  • The higher charge efficiency of AGMs can contribute to significant
    savings when it comes to the use of expensive renewable energy
    sources (wind generators, solar panels, etc.) as your charging
    system can be 15% smaller (or just charge faster).

  • While flooded cells lose up to 1% per day due to self-discharge, VRLAs lose
    1-3% per month. Why employ a solar charger to trickle-charge your battery
    banks if you don't have to?

  • High vibration resistance: The construction of AGMs allows them to be used
    in environments where other batteries would literally fall to pieces. This is
    another reason why AGMs see broad use in the aviation and the RV industry.

  • Thus, there are some significant differences between battery types in terms
    of features and construction. However, there are also some very important
    figures to consider when it comes to choosing the right battery: Various
    capacities, cost, warranty, etc. The following table tries to summarize across
    brands using batteries as close to the 8D Group Size as possible

Wet Cells

  • Flooded or Wet Cells are the most common lead-acid battery-type in use
    today.  They offer the most size and design options and are built for many
    different uses. In the marine business, they usually are not sealed so the user
    can replenish any electrolyte the battery vented while charging the battery.
    Typically, the cells can be accessed via small ~1/2" holes in the top casing of
    the battery.   The plastic container used for flooded cells will have one or
    more cells molded into it.

  • Each cell will feature a grid of lead plates along with an electrolyte based on
    sulphuric acid. Since the grid is not supported except at the edges, flooded
    lead-acid batteries are mechanically the weakest batteries.  Since the
    container is not sealed, great care has to be taken to ensure that the
    electrolyte does not come into contact with you or seawater (chlorine gas!).
    The water needs of flooded cells can be reduced via the use of Hydrocaps,
    which facilitate the recombination of Oxygen and Hydrogen during the
    charging process.

Comparing physical attributes between VRLAs and Flooded Cells

  • Lifeline AGM (8D)West Marine Gel (8D)Inexpensive Trojan (2xT105)Premium
  •      Surrette 400 (HT8DM)Premium Surrette 500 (12CS11PS)
  •      Amp-hour capacity (20hr rate)255225225221342
  •      Warranty (Replacement/Pro rated)1/5 Years1.5/5 Years0.5/3 Years2/5
  •      Years3/7 Years
  •      Life Cycles (@ 50% DOD)1,0005005001,2503,200
  •      Initial Purch. Cost (USD/12V set)387449152246683
  •      Initial Purch. Cost (approx. $/Ah)$1.52$2.00$0.68$1.11$2.00
  •      Energy Density (Ah/in^3)0.1110.0980.1360.0970.076
  •      Weight Factor (Ah/lb)1.6141.4241.8151.3481.257
  •      Max. net replenishment during bulk charge, accounting for charge limits,
  •      efficiency and assuming a 400Ah battery bank1550A*177A85A85A85A
  •      I tried to level the playing field by selecting as many group 8D batteries
  •      as possible. The two exceptions are the Trojan T105's and the Surrette
  •      12CS11PS (no series 500 Group 8D battery is manufactured by Surrette
    for the marine market). The larger battery size is to the advantage of the
    Surrette, although it does not impact results greatly.
  • The Trojan T105's were used because I was not able to find ready pricing on
    the Trojan 8D. I would expect results to be somewhat comparable.
  • *Concorde Batteries used to claim no charge limit on its web-site, while
  • Windsun.com claims 4x amp-hour capacity. I limit charge current in the cost
    model to 100% of amp-hour capacity just to be on the safe side.  
  • Comparison of Battery Types using several different measurements

Energy Storage per unit Weight and Volume

  • Here is one of the classic comparisons that people like to make: How much
    charge the battery can store per unit weight and per unit volume. As you can
    see, the Trojan T105 comes out ahead in both departments due to its low
    weight and compact construction. However, this construction technique will
    also lead to a lower cycle and overall life.

Purchase Cost per unit Weight and Volume

  • As we can see from this chart, the purchase cost per amp-hour and
    purchase cost per cycle still make the Trojan T105 look like the most attractive

  • Thus, if you are strapped for weight, space, and cash, such a battery might
    be ideal. The Trojan product has thin lead plates that make these batteries
    lighter but also shorter lived. Rolls advertises very long pro rata warranty
    replacement periods for their premium line that are indicative of the confidence
    they place in their product.

  • Premium cells are handicapped by lower energy storage density but offer
    longer lives and greater resistance to the self destructive habits of lead acid
    batteries: Thicker lead plates and a more complicated product make it possible.

  • Hence, premium cells usually have a higher resistance to vibration, are easier
    to service, and have higher cycle lives than their budget competition. Many
    boat owners are willing to put up with the initial purchase price in return for
    reliability and not having to replace them every few years.

Marine Batteries

  • Most batteries sold  through marine hardware stores do not qualify as
    premium batteries. Batteries are not created equal and brand or price are not
    the primary indicator for quality. For example:Rolls/Surrette make a range of
    flooded batteries from the super-premium 500/CS series to the mid-range 300
    series that is meant to compete with Trojan, Exide, etc.  WestMarine is
    offering AGM batteries with a shorter warranty period and higher price than
    Lifeline AGMs.

  • Furthermore, consider that premium batteries usually only exist in non-
    standard form factors. For example, you will probably have to make some
    custom modifications to properly mount/restrain the tall and heavy
    Rolls/Surrette 500 series (18"+ high, min. 128 lb+ each).

  • However, life cycle costs are not just a function of the initial purchase costs.

  • You should also consider the fuel/engine wear savings of using VRLAs over  
    flooded cells.

  • AGMs offer the highest charge acceptance, efficiency, and a
    reasonably long life which makes them generally a better bargain.  
    Unfortunately, there are fewer shapes and sizes of VRLAs to choose from
    (relative to the flooded cell universe anyway), and less familiarity and
    presence world-wide.

  • On the other hand, VRLAs can be shipped anywhere by air.

  • Flooded cells have to be bought locally or delivered by surface

Mixing AGMs and Flooded Cells?

  • Gel cells have sufficiently different set points to make them totally incompatible
    with flooded cells or AGMs). Ideally, your house bank would consist of a
    number of identical batteries wired in series and/or parallel that were
    manufactured on the same day.

How to Save Money with AGMs?

  • There are many attributes that determine the true cost of a battery technology.
    Much like incandescent versus compact fluorescent light bulbs, your choice
    of battery technology may cost you less up front but will cost you more over
    the life of the product. For example, the faster, more efficient bulk charging
    that AGMs and gel-cells allow will lead to reduced wear and tear on your
    charge source (engine, gen-set, etc.).
How Lead Acid Batteries Work


  • Voltage is an electrical measure which describes the potential to do work.
    Systems that use voltages below 50V are considered low-voltage.


  • Current is a measure of how many electrons are flowing through a conductor.
    Current is usually measured in amperes (A). Current flow over time is defined
    as Amp-hours or Ah.


  • Power is the product of voltage and current and is measured in Watts. Power
    over time is usually defined in Watt-hours (Wh), the product of the average
    number of watts and time. Your energy utility usually bills you per kiloWatt-
    hour (kWh), which is 1,000 watt-hours.

What is a Lead-Acid Battery?

  • A lead-acid battery is a electrical storage device that uses a reversible
    chemical reaction to store energy. It uses a combination of lead plates or grids
    and an electrolyte consisting of a diluted sulphuric acid to convert electrical
    energy into potential chemical energy and back again. The electrolyte of lead-
    acid batteries is hazardous to your health and may produceburns and other
    permanent damage if you come into contact with it.

Deep Cycle vs. Starter Batteries

  • Batteries are typically built for specific purposes and they differ in
    construction. Broadly speaking, there are two applications that batteries are
    made for: Starting and Deep Cycle.

Starter Batteries

  • As the name implies, Starter Batteries are made to start internal combustion
    engines. They contain thin lead plates which allow them to discharge energy
    quickly for a short amount of time.

  • However, they do not tolerate being discharged deeply, as the thin lead plates
    degrade quickly under deep discharge and re-charging cycles. Most starter
    batteries will only tolerate being completely discharged a few times before
    being irreversibly damaged.

Deep Cycle Batteries

  • Deep Cycle batteries have thicker lead plates that enable them tolerate deep
    discharges. They cannot dispense charge as quickly as a starter battery but
    can also be used to start combustion engines. You simply need a bigger deep-
    cycle battery than if you had used a dedicated starter type battery.

  • The thicker the lead plates, the longer the life span, all things being equal.  
    Battery weight is a simple indicator for the thickness of the lead plates used in
    a battery. The heavier a battery for a given group size, the thicker the plates,
    and the better the battery will tolerate deep discharges.

  • Some "Marine" batteries are sold as dual-purpose batteries for starter and
    deep cycle applications. However, the thin plates required for starting
    purposes inherently compromise deep-cycle performance. Thus, such
    batteries should not be cycled deeply and should be avoided for deep-cycle
    applications unless space/weight constraints dictate otherwise.

Regular versus Valve-Regulated Lead Acid (VRLA) Batteries

  • Battery Containers come in several different configurations.
  • Flooded Batteries can be either the sealed or open variety.
  • Sealed Flooded Cells are frequently found as starter batteries in
    cars.  Their electrolyte cannot be replenished. When enough electrolyte has
    evaporated due to charging, age, or just ambient heat, the battery has to be

  • Deep-Cycle Flooded cells usually have removable caps that allow you to
    replace any electrolyte that has evaporated over time.

  • VRLA batteries remain under constant pressure of 1-4 psi. This pressure
    helps the recombination process under which 99+% of the Hydrogen and
    Oxygen generated during charging are turned back into water.

The two most common VRLA batteries used today are the Gel and
Absorbed Glass Mat (AGM) variety

  • Gel batteries feature an electrolyte that has been immobilized using agelling
    agent like fumed silica.

  • AGM batteries feature a thin fiberglass felt that holds the electrolyte in place
    like a sponge.

  • Neither AGM or Gel cells will leak if inverted, pierced, etc. and will continue to
    operate even under water.

Battery Cells

  • Battery Cells are the most basic individual component of a battery. They
    consist of a container in which the electrolyte and the lead plates can interact.
    Each lead-acid cell fluctuates in voltage from about 2.12 Volts when full to
    about 1.75 volts when empty. Note the small voltage difference between a full
    and an empty cell (another advantage of lead-acid batteries over rival

Battery Voltage

  • The nominal voltage of a lead-acid battery depends on the number of cells that
    have been wired in series. As mentioned above, each battery cell contributes
    a nominal voltage of 2 Volts, so a 12 Volt battery usually consists of 6 cells
    wired in series.

State of Charge

  • The State of Charge describes how full a battery is. The exact voltage to
    battery charge correlation is dependent on the temperature of the battery.
    Cold batteries will show a lower voltage when full than hot batteries. This is
    one of the reasons why quality alternator regulators or high-powered
    charging systems use temperature probes on batteries.

Depth of Discharge (DOD)

  • The Depth of Discharge (DOD) is a measure of how deeply a battery is
    discharged. When a battery is 100% full, then the DOD is 0%. Conversely,
    when a battery is 100% empty, the DOD is 100%. The deeper batteries are
    discharged on average, the shorter their so-called cycle life.  For example,
    starter batteries are not designed to be discharged deeply (no more than 20%
    DOD). If used as designed, they hardly discharge at all.

  • Engine starts are very energy-intensive but the duration is very short. Most
    battery manufacturers advocate not discharging their batteries more than 50%
    before re-charging them.

Battery Storage Capacity

  • The Amp-hour (Ah) Capacity of a battery tries to quantify the amount of
    usable energy it can store at a nominal voltage. All things equal, the greater
    the physical volume of a battery, the larger its total storage capacity.

  • Storage capacity is additive when batteries are wired in parallel but not if
    they are wired in series.

  • Most marine, automotive, and RV applications use 12V DC. You have the
    choice to either buy a 12V battery or to create a 12V system by wiring
    several lower-voltage batteries/cells in Series.

  • When two 6V, 100Ah batteries are wired in Series, the voltage is doubled but
    the amp-hour capacity remains 100Ah (Total Power = 1200 Watt-hours).

  • You may decide to wire batteries in series because a single 12V
    battery with the right storage capacity is simply too heavy to lift into

  • Batteries consisting of fewer cells (and hence lower voltage) in series can
    provide the same storage capacity yet be portable. It is not unusual to see
    solar power installations where the battery bank consists of a sea of 2V
    batteries that have been wired in series.

  • Two 6V, 100Ah batteries wired in Parallel will have a total storage capacity of
    200Ah at 6V (or 1200 Watt-hours).

  • Battery banks consisting of 12V batteries wired in parallel are often seen on
    OEM installations in boats and RVs alike. Such banks are simple to wire up
    and require a minimum of cabling. However, the wiring must have the capacity
    to deal with a full battery bank.  Each battery should be fused individually to
    ensure that a battery gone bad will not affect the rest of the bank.

  • Battery banks wired in Series-Parallel are even more complicated. Here, four
    6V cells are wired in two "strings" of 12VDC that were then wired in parallel.
    Using 6V, 100Ah batteries, this system will have a storage capacity of 200Ah
    at 12V or 2,400Wh.

  • Since such a system has more wiring, it is very important to group "strings"
    logically and to label everything. Furthermore, it is a very good idea to fuse
    every "string" of series-wired batteries to ensure that a problem in one part of
    the battery bank does not take the whole bank down.

  • Despite advances in instrumentation, the battery industry mostly still uses
    amp-hours as a capacity measure instead of watt-hours.

Available Capacity versus Total Capacity

  • Since batteries depend on a chemical reaction to produce electricity, their
    available Capacity depends in part on how quickly you attempt to charge or
    discharge them relative to their Total Capacity. The Total Capacity is frequently
    abbreviated to C and is a measure of how much energy the battery can store.
    Available Capacity is always less than Total Capacity.

  • Typically, the amp-hour capacity of a battery is measured at a rate of
    discharge that will leave it empty in 20 hours (a.k.a. the C/20 rate). If you
    attempt to discharge a battery faster than the C/20 rate, you will have less
    available capacity and vice-versa. The more extreme the deviation from the
    C/20 rate, the greater the available (as opposed to total) capacity difference.

  • However, as you will discover in the next section, this effect is non-linear.
    The available capacity at the C/100 rate (i.e. 100 hours to discharge) is
    typically only 10% more than at the C/20 rate. Conversely, a 10% reduction in
    available capacity is achieved just by going to a C/8 rate (on average). Thus,
    you are most likely to notice this effect with engine starts and other high-
    current applications like inverters, windlasses, desalination, or air conditioning

  • For example, the starter in an engine will typically quickly outstrip the capacity
    of the battery to keep cranking it for any length of time. Therefore it is better to
    allow time between engine start attempts to allow the engine starter to cool
    down, it also allows the chemistry in the battery to "catch-up". As the battery
    comes to a new equilibrium, its available capacity increases. A very elegant
    equation developed in 1897 by a scientist called Peukert describes the
    charging and discharging behavior of batteries.

The Peukert Effect

  • As you can see below, the Peukert equation in its simplest form consists of
    several factors.
  • Peukerts Equation: I n x T = Cmax where I is the current (usually measured in
    amperes) T is time (usually measured in hours) n is the Peukert number /
    exponent Cmax is the storage capacity of the battery measured in amp-hours
    at 1ampere draw. Usually, the C/100 capacity comes close to this.
  • Adding 10% to the 20-hour rating (also known as C/20) usually comes close

  • For a more accurate calculation, you need to modify the equation to account
    for the fact that most battery capacities are measured using higher currents
    than the 1 ampere draw that Peukert used. The folk at Smartgauge.co.uk have
    a good and comprehensive explanation on their site, with lots of examples.
    Thus, the equation would have to be re-written as (I x Hact / Cact)n x T =
    Hact, where Hact is the actual hour rating (i.e. over how many hours the
    battery was drawn down) and Cact is the available battery capacity (in amp-
    hours) at that draw. Most batteries' storage capacity is published assuming a
    constant 20-hour draw, i.e. with Hact being 20. Assuming 20-hour ratings, the
    equation would thus simplify down to (I x 20 / C20)n x T = 20

  • Either way, the available current is dependent on the rate of discharge and
    the Peukert exponent for the battery. The closer the exponent is to 1 (one),
    the less the available capacity of a battery will be affected by fast
    discharges. Peukerts numbers are derived empirically and are usually
    available from manufacturers. They range from about 2 for some flooded
    batteries down to 1.05 for some AGM cells. The average peukerts exponent
    is 1.2 though the exact number depends on the battery construction and

  • When the time comes to charge a battery, the Peukerts effect also comes into
    play. The capacity of a battery to absorb a charge during the bulk phase is
    also dependent on it's Peukerts number.

  • This is one of the reasons why AGM cells can be bulk charged at much
    higher rates than either Gel or Flooded cells.

Reserve Minutes

  • Reserve Minutes are a measure of how long your battery can sustain a load
    before it's available capacity has been completely used up. This measure is
    especially useful for folks who want to run inverters, fridges, and other large
    loads. The following chart has a logarithmic time scale (minutes) - hence, the
    non-linear nature of the Peukert effect is smoothed out quite a bit.

  • Batteries that have a high Peukerts Exponent will quickly run out of capacity
    with high loads. Here, the low-exponent battery will last over 100 minutes
    with a 50 ampere load, while the high-exponent battery will last about 20

  • Anytime you deal with large loads relative to the battery capacity
    available, chose a low-exponent battery. This is why many wheel-
    chairs and other electrically motorized vehicles use AGMs.

  • Starter batteries are built to have a low Peukerts exponent. Otherwise, they'd
    simply not be able to crank an engine for more than a few seconds. The thin
    plates that allow flooded cells to work as starter batteries also make them too
    fragile for deep-cycle use.

Conversion Efficiency

  • The conversion efficiency denotes how well a battery converts an electrical
    charge into chemical energy and back again. The higher this factor, the less
    energy is converted into heat and the faster a battery can be charged without
    overheating (all other things being equal). The lower the internal resistance of
    a battery, the better its conversion efficiency.

  • One of the main reasons why lead-acid batteries dominate the
    energy storage   markets is that the conversion efficiency of lead-
    acid cells at 85%-95% is much higher than Nickel-Cadmium (a.k.a.
    NiCad) at 65%, Alkaline (a.k.a. NiFe) at 60%, or other inexpensive
    battery technologies.

Battery Life

  • Battery manufacturers define the end-of-life of a battery when it can no
    longer hold a proper charge (for example, a cell has shorted) or when the
    available battery capacity is 80% or less than what the battery was rated for.

  • The life of Lead Acid batteries is usually limited by several factors: Cycle Life
    is a measure of how many charge and discharge cycles a battery can take
    before its lead-plate grids/plates are expected to collapse and short out. The
    greater the average depth-of-discharge, the shorter the cycle life.

  • Age also affects batteries as the chemistry inside them attacks the lead
    plates. The healthier the "living conditions" of the batteries, the longer they will
    serve you. Lead-Acid batteries like to be kept at a full charge in a cool

  • Only buy recently manufactured batteries, so learn to decipher the
    date code stamped on every battery... (inquire with manufacturer). The
    longer the battery has sat in a store, the less time it will serve you.

  • Since lead-acid batteries will not freeze if fully charged, you can store them in
    the cold during winter to maximize their life.

  • Construction has a big role in battery life too, some designs are better at
    preserving batteries than others and the suitability of a design for a given
    application plays a role also.

  • For example, flooded lead-acid cells will  typically fare worse than their
    VRLA cousins in operations that involve a lot of jerky motion - the immobilized
    plates in VRLA cells will be stressed less than suspended plates in cheap
    flooded cells.

  • Plate Thickness helps - the thicker the plates, the more abuse, charge and
    discharge cycles they can take. Thicker plates will also survive any
    equalization treatments for sulphation better. The heavier the battery for a
    given group size, the thicker the plates are, so you can use weight as one
    guide to buying lead-acid batteries.

  • Sulphation is a constant threat to batteries that are not fully re-charged.  A
    layer of lead sulphate can form in these cells and inhibit the electro-chemical
    reaction that allows you to charge/discharge batteries.  Many batteries can be
    saved from the recycling heap if they are Equalized

  • The design life of a battery depends in part on its construction, its type, the
    thickness of the plates, its charging profiles, etc. All these factors come
    together to determine just how long your battery may ultimately serve you.


  • Sulphation layers form barrier coats on the lead plates in batteries that inhibit
    their ability to store and dispense energy. The equalization step is a last resort
    to break up the Sulphate layers using a controlled overcharge. The
    process will cause the battery electrolyte to boil and gas, so it should be only
    done under strict supervision and with the proper precautions.

  • It is much more tricky to equalize a VRLA battery than a flooded battery with
    removable caps. However it apparently can be done as described at the
    Ample Power web site.


  • Batteries start to gas when you attempt to charge them faster than they can
    absorb the energy.
  • The excess energy is turned into heat, which then causes the electrolyte to
    boil and evaporate. The evaporated electrolyte can be replenished in batteries
    with removable caps such as most flooded deep-cycle batteries. Many car
    batteries are sealed and thus need to be replaced when their electrolyte
    evaporates over time.

  • Since AGM and Gel cells are always sealed, it is very important to
    guarantee they are not overcharged. The only way to ensure this is to
    use a temperature-compensated charging system. Such chargers use a
    temperature probe on the battery to ensure that the battery does not get too
    hot. As the battery heats up, the charging current is reduced to prevent
    thermal runaway, a very dangerous condition.

Thermal Runaway

  • This is a very dangerous condition that can occur if batteries are charged too
    fast. One of the byproducts of Gassing are Oxygen and Hydrogen. As the
    battery heats up, the gassing rate increases as well and it becomes
    increasingly likely that the Hydrogen around it will explode. The danger posed
    by high Hydrogen concentrations is one of the reasons that the American
    Boat and Yachting Council (ABYC) requires that batteries be installed in
    separate, well-ventilated areas.


  • The self-discharge rate is a measure of how much batteries discharge on
    their own. The Self-Discharge rate is governed by the construction of the
    battery and the metallurgy of the lead used inside. For instance, flooded cells
    typically use lead alloyed with Antimony to increase their mechanical strength.
    However, the Antimony also increases the self-discharge rate to 8-40% per
    month. This is why flooded lead-acid batteries should be in use often
    or left on a trickle-charger.

  • The lead found in Gel and AGM batteries does not require a lot of mechanical
    strength since it is immobilized by the gel or fiberglass. Thus, it is  typically
    alloyed with Calcium to reduce Gassing and Self-Discharge.

  • The self-discharge of Gel and AGM batteries is only 2-10% per
    month and thus these batteries require minimal maintenance.

Battery Group Size

  • Manufacturers for marine batteries make them in all sorts of sizes and
    voltages. Battery case sizes are typically denoted by a "Group Size" which
    has nothing to do with the actual size of the battery. For example, Group 8D
    batteries are much larger than Group 31 batteries.

  • The group size will merely indicate the approximate exterior dimensions
    (including terminals) and voltage of the battery in question. However, the
    exact dimensions can only be directly obtained from each manufacturer.
VRLA Batteries

  • The Valve Regulated Lead Acid (VRLA) battery utilizes a
    dilute sulfuric acid electrolyte which is immobilized to
    eliminate the hazards of spills and leakage which
    facilitates an oxygen recombination cycle.  

  • The oxygen recombination cycle eliminates the need to
    add water throughout the battery's life and improves its
    safety of operation.  

  • The VRLA battery also contains a self resealing
    pressure relief valve which prevents buildup of
    excessive pressure in the cell and prevents entry of
    outside air into the cell, thus extending the battery's shelf

  • Due to these advantages of no electrolyte spillage or
    maintenance, minimal gas evolution, extended shelf life
    and improved safety, the VRLA battery is often used for
    critical power applications and is rapidly displacing
    the traditional vented or wet lead acid cell.


  • The AGM VRLA battery utilizes a separator of glass
    fibers which serves to both isolate the negative and
    positive plates and act as a blotter to absorb all the
    electrolyte within the cell.  This AGM separator is
    somewhat fragile, highly porous and absorbent, and of
    very low resistance.  

  • The AGM separator is maintained under compression
    between the plates to assure complete contact with the
    plate surface since it provides the source of electrolyte
    essential to the cell's electrochemical reaction.  Actually
    the separator is not completely saturated with electrolyte
    and it is the 5 to 10% void space that allows the oxygen
    gas generated at the positive plate to diffuse to the
    negative plate where the oxygen recombination cycle

  • This system is also occasionally referred to as a
    starved electrolyte system in that there is more plate
    active material than what the limited amount of electrolyte
    can fully react.

Gelled Electrolyte VRLA

  • The gelled eletrolyte VRLA battery utilizes a robust
    plastic or glass leaf separator.  This leaf separator is not
    relied upon to absorb the electrolyte, since the electrolyte
    is gelled, but strictly performs the function of separating
    and resisting the development of shorts between the

  • In some designs, the leaf separator contains an integral
    glass mat retainer which lies against the positive plate
    active material and retains sloughed material and
    consequently improves the cell's cycle life.  

  • This durable leaf separator and the gelled electrolyte are
    of relatively high resistance and introduce additional
    voltage drop during high rate discharge.  The cell is
    completely filled to the top of the plates with the gelled
    electrolyte.  However, there are cracks and fissures in
    the gel between the plates that allow for the transport of
    the oxygen from the positive to the negative plate
    allowing for the oxygen recombination cycle.

VRLA Battery Capacity

  • The AGM VRLA battery typically contains more
    electrolyte and is of slightly higher specific gravity than
    the comparable Gelled Electrolyte battery (a
    percentage of the electrolyte is actually displaced by the
    gelling agent).  As a result, it will provide 7-10% more
    long duration capacity within the same container volume.

  • Due to the very low resistance of the AGM system, it
    exhibits much less internal voltage drop (IR drop) during
    discharge, resulting in higher terminal voltage and longer
    run times at high discharge rates.


  • In UPS - ininterruptable power systems, where high rate
    performance is the key criteria, AGM VRLA batteries are
    highly recommended.  Gelled electrolyte VRLA batteries
    may be used, but will not be as efficient.  
High Temperature

  • The AGM VRLA battery has a more efficient oxygen
    recombination cycle and a lower resistance than the
    GELLED ELECTROLYTE VRLA battery.  Therefore it will
    draw slightly more float current resulting in greater
    internal heat generation.  

  • To prevent premature failure or thermal runaway, its
    important to operate the VRLA battery in an environment
    in which it can dissipate heat at a rate faster than it is
    internally generated.  

  • This can be accomplished by operation in a cool
    environment and allowing separation between the
    batteries to facllitate air flow and improved heat
    dissipation.  Another method for reducing heat is by
    reducing the charging voltage and resulting float current
    at elevated temperatures to minimize the internal
    generation of heat.

  • The GELLED ELECTROLYTE battery has gel in complete
    contact with the plates, where the heat is generated, and
    the walls of the battery container where it is radIated.  

  • In comparison, the AGM battery has the heat conducting
    electrolyte absorbed in the separator.  While it is in good
    contact with the plates, it is not in complete contact with
    the interior walls of the container.  

  • As a result, the gelled electrolyte VRLA battery provides
    approximately 15% better heat conduction from the
    plates and superior heat dissipation to the environment.

Float Service

  • A battery is in float sevice when it is continually
    connected to the power source and the load so as
    to provide instant uninterrupted power in the event of
    failure of the primary power source.  

  • The float service life characteristics at 77-degrees
    Farenheit are essentially the same for the AGM and
    gelled electrolyte VRLA batteries.  

  • The AGM and Type A gelled electrolyte batteries will both
    provide 95-100+% rated capacity upon initial installation
    and charging and all other factors being equal, will
    provide the same float service life.  It is not the electrolyte
    immobilization technique that determines the float service
    life but the design of other components in the battery
    such as the electrolyte specific gravity, separators, plate
    grids and active materials.

  • Selection of the AGM OR TYPE A GELLED
    ELECTROLYTE  batteries for float service is determined
    by the high rate performance requirements vs.
    anticipated elevated operating temperatures.

Cycle Service

  • In cycle service the battery is deeply discharged as the
    primary power source for applications such as
    wheelchairs, golf carts and photovoltaic systems.

  • The battery is then recharged following discharge to
    restore its capacity for repeated use.  In typical cycle
    service applications this cycle is repeated frequently.

  • This repeated cycle is especially stressful on the positive
    plate active materials, causing them to shed from the
    grid.  Additionally gassing is accelerated and the grids of
    the positive plates suffer accelerated corrosion due to
    the degree of overcharge normally experienced with the
    higher voltage "cycle service" charging.

  • While the AGM and Type A gelled electrolyte
    batteries will provide good cycle service, the Type
    B gelled electrolyte battery is designed
    specifically to provide the longest service life in
    deep cycle applications.  

  • To extend the cycle life the type B gelled electrolyte
    system utilizes special separators with glass mat
    retainers to secure  the positive active material in place
    and a unique addition of phosphoric acid to the
    electrolyte.  The effect of the phosphoric acid is to
    strengthen the positive active material, thus making it
    more capable of enduring the stresses of deep cycling
    and minimizing paste shedding.  

  • The net result is that a Type B gelled electrolyte
    battery can provide approximately double the
    cycle life as that provided by the Type A gelled
    electrolyte VRLA batteris.