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.
It is important to note that nearly 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 can vary considerably. NiCad's,
Nickel-Iron, and other types are found in some systems, but
are not common due to their expense and/or poor efficiency.
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 a few 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.
The lifespan of a 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 at most 18
months, as they are not totally dry (actually, a few are,
but hard to find, the vast majority are shipped with damp
plates).
These are
some general (minimum - maximum) typical expectations
for
batteries if used
in deep cycle service:
Starting: 3-12 months
Marine: 1-6 years
Golf cart: 2-6 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+
years
Telephone (float): 1-20
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): 3-25
years
NiCad: 1-20 years
-
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 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. 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 popular golf
cart battery is generally a "semi" deep cycle
- better than any starting battery, better than most
marine, but not as good as a true deep cycle solid
Lead plate, such the L-16 or industrial type. However,
because the golf cart (T-105, US-2200, GC-4 etc) batteries
are so common, they are usually quite economical for
small to medium systems.
-
Many (most?) Marine batteries
are usually actually a
"hybrid", and fall between the starting and
deep-cycle batteries, while 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. "Hybrid" types
should not be discharged more than 50%. 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 not be a true deep-cycle battery.
It is sometimes hard to tell, as the terms marine and
deep cycle are sometimes 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.
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) 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.
Nearly all large rechargeable batteries in common use
are Lead-Acid type. (There are some NiCad's 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.
NiCad's are also inefficient - typically around 65% - and
very expensive. However, NiCad's 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.
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.
Automotive batteries typically have plates about .040" (40/1000")
thick, while forklift batteries may have plates more than
1/4"
(.265" for example in the 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".
Most industrial deep-cycle batteries use Lead-Antimony
plates rather than the Lead-Calcium used in AGM or gelled
deep-cycle 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 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.
A few industrial batteries have special caps that convert
the Hydrogen and Oxygen back into water, reducing water loss
by up to 95%. The popular "HydroCaps" that we sell
for flooded batteries do the same job for conventional ("wet"),
golf cart, and fork-lift batteries. Lead-Antimony 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. There are trade offs. The Lead-Antimony
types have a very long lifespan, but higher self discharge
rates.
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.
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 discharged.
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.
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. 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. Click on the small graph to see a full size
chart of temperature vs. capacity.
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.
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 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.
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, 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.
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 batteries.
"False" Capacity
A battery can meet all the 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.
On the table below, you have to 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.
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 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".
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 requirements.
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
folks have chosen to rate their batteries at the 100 hour
rate - which makes them look a lot better than they really
are.
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