Batteries are a key component in a grid-tie with back-up or a
stand-alone renewable energy system that all of the other components rely on
for operation. Without proper maintenance, batteries can fail prematurely
and shut the whole system down. However, toiling over your battery bank with
a voltmeter, hydrometer and a gallon of distilled water every day is not
necessary. With simple monthly and quarterly maintenance procedures, your
batteries should last for a long time. On the other hand, neglecting your
batteries can drastically shorten their life span. The following statement
sums it up best, "few batteries die a natural death, most are murdered". The
following information is designed to tell you how to get the longest life
and best performance possible from your battery bank. Most of this
information is for flooded cell lead-acid batteries; alkaline (Ni-FE &
Ni-Cad) and sealed gel-cell battery charging characteristics are completely
Types Used in Solar Systems:
There are three types of batteries that are most popularly used in solar
electric systems. Each type has its pluses and minuses, so we will also
include the systems the individual types are best suited for.
Flooded Lead Acid:
Flooded lead acid batteries have the longest track record in solar electric
use and are still used in the majority of stand-alone solar systems. They
have the longest life and the least cost per amp-hour of any of the choices.
However the other side of the coin is, in order to enjoy these advantages,
they require regular maintenance in the form of watering, equalizing charges
and keeping the top and terminals clean. Some examples of flooded lead-acid
batteries used in solar electric systems are 6 volt golf-cart batteries, 6
volt L-16's and 2 volt industrial cells for large systems.
Absorbed Glass Mat Sealed Lead Acid (AGM):
AGM batteries are seeing more and more use in solar electric systems as
their price comes down and as more systems are getting installed that need
to be maintenance free. This makes them ideally suited for use in grid-tied
solar systems with battery back-up. Because they are completely sealed they
can't be spilled, do not need periodic watering, and emit no corrosive
fumes, the electrolyte will not stratify and no equalization charging is
required. AGM's are also well suited to systems that get infrequent use as
they typically have less than a 2% self discharge rate during transport and
storage. They can also be transported easily and safely by air. Last, but
not least, they can be mounted on their side or end and are extremely
vibration resistant. AGM's come in most popular battery sizes and are even
available in large 2 volt cells for the ultimate in low maintenance large
When first introduced, because of their high cost, AGM's
were mostly used in commercial installations where maintenance was
impossible or more expensive than the price of the batteries. Now that the
cost is coming down they are seeing use in all types of solar systems as
some of today's owners think the advantages outweigh the price difference
and maintenance requirements of flooded lead acid batteries.
Gelled Electrolyte Sealed Lead Acid:
Gelled lead acid batteries actually predated the AGM type but are losing
market share to the AGM's. They have many of the same advantages over
flooded lead acid batteries including ease of transportation, as the AGM
type, except the gelled electrolyte in these batteries is highly viscous and
recombination of the gases generated while charging, occurs at a much slower
rate. This means that they typically have to be charged slower than either
flooded lead acid or AGM batteries. In a solar electric system you have a
fixed amount of sun hours every day and need to store every solar watt you
can before the sun goes down. If charged at too high a rate, gas pockets
form on the plates and force the gelled electrolyte away from the plates,
decreasing the capacity until the gas finds its way to the top of the
battery and is recombined with the electrolyte. For use in a grid-tie with
back up system or any system where discharge rates are less than severe, gel
batteries could be a good choice.
Think of your batteries like a bucket of energy...
Batteries are simply a storage vessel for the direct current (DC) power
produced from your charging sources (solar modules, wind generator,
micro-hydro or generator/battery charger). If you aren't familiar with the
water to electricity analogy, please read the Basics of Electricity section.
If you don't have time to read that whole section, then just remember that
pressure = voltage and flow rate = amperage. The size of the bucket
determines how much water it will hold which is analogous to the amp-hour
storage capacity of a battery (bigger, heavier batteries hold more energy
like a larger bucket holds more water). If you connected a pressure gauge to
the bottom of a bucket and started filling it with water you would see the
pressure increase until the water reaches the top. The same holds true for a
battery as you put amperage or current into it, the voltage level rises.
Deep Cycle vs. Shallow Cycle
In battery lingo, a cycle on your battery bank occurs when you discharge
your battery and then charge it back up to the same level. A lead acid
battery is designed to absorb and give up electricity by a reversible
How deep a battery is discharged is termed depth of
discharge (DOD) while the state of charge (SOC) is 100% minus the DOD. This
means that a 25% DOD equals a 75% SOC. A shallow cycle occurs when the top
20% or less of the battery's energy is discharged and then recharged.
Automotive starting, lighting and ignition batteries (SLI) are of the
shallow cycle type and are not recommended for use in a photovoltaic system.
The lead plates inside an SLI battery are thin with a large overall surface
area. This design can produce a high amount of current in a very short time
(which is ideal for starting engines), but cannot be discharged very deeply
without damaging them and/or shortening their life span considerably.
Deep cycle batteries on the other hand can be repeatedly
discharged to 80% DOD and recharged without damaging them (although repeated
deep cycling will shorten the battery's life as compared to the same number
of shallow cycles). Deep cycle batteries have thicker lead plates which have
less overall surface area as compared to an SLI battery. Because of the
lessened availability of surface area for chemical reaction, deep cycle
batteries produce less current than a shallow cycle battery but can produce
that amount of current for a much longer period of time.
The depth of cycling has a good deal to do with
determining a battery's useful life. Even batteries designed for deep
cycling are "used up" faster as the depth of discharge is increased. It is
common practice for a system to be designed with deep cycle batteries even
though the daily or average discharging amounts to a relatively shallow
depth of discharge. To get the longest life out of your battery bank,
purchase deep cycle batteries and shallow cycle them.
Warm in the winter, Cool in the summer
The speed of the charging and discharging chemical reactions occurring
inside a lead-acid battery is governed by temperature and charge/discharge
current. The colder the temperature the slower the reactions and conversely
the warmer the temperature the faster the reactions. Hence a cold battery
will deliver less amperage in any given time frame as compared to a warm
battery. Most of us have experienced this effect when trying to start our
cars on a cold morning; the engine just doesn't turn over as quickly if at
all. Warm that same battery up and you will see a major improvement. (See
the bar graph of temperature effects below). The optimum temperature for a
lead-acid battery is around 77°F, but 60-80°F is acceptable. For this reason
we like to see batteries placed indoors or in a heated and ventilated space
to maintain them between 60° to 80°F. If you do install them in an unheated
space, battery capacity must be increased to compensate for this derating.
On the other extreme, high temperatures (110°F+) can drastically shorten the
life of the battery and should be avoided as well.
Batteries aren't 100% efficient
Energy is never consumed or produced, it merely changes form. The efficiency
of conversion is never 100% and in the case of new batteries averages around
90%. This means that if you want to discharge 100 watt-hours of energy from
a battery you must charge it with approximately 110 watt-hours of energy.
Due to impurities in the chemicals used for battery
construction, batteries will lose power to local action, an internal
reaction which occurs whether you are using the battery or not. This slow
discharging is termed self-discharge and its rates vary greatly among
battery types and increases along with temperature. The rate also increases
with the age of a battery, so much so that an old battery may require
significant amount of charging just to stay even. Even new batteries may
lose 1 to 2% of charge per day. Lead calcium grid batteries have the lowest
self-discharge rates, but are not designed for deep cycling applications.
Determining battery state of charge
Battery state of charge is determined by reading the static (i.e. not
charging or discharging) battery voltage or the specific gravity of the
electrolyte. The density or specific gravity of the sulfuric acid (H2SO4)
electrolyte of a lead-acid battery varies with the state of charge and
temperature. The density is lower when the battery is discharged and higher
as the cells are charged, (see the table below). This is because the
electrolyte is part of the chemical reaction, it changes as the chemical
reaction takes place. Specific gravity is read with a hydrometer which will
tell the exact state of charge. A hydrometer cannot be used with sealed or
Voltage meters are used to approximate battery state of
charge. They are relatively inexpensive and easy to use. The main problem
with relying on voltage reading alone is the high degree of battery voltage
variation through the working day. Battery voltage reacts highly to charging
and discharging. In a PV system we are usually charging or discharging and
many times are doing both at the same time. As a battery is charged the
indicated voltage increases and as discharging occurs, the indicated
These variations may seem hard to track, but in reality
they are not. A good accurate digital meter with a tenth of a volt accuracy
can be used with success. The pushing and pulling of voltage, once accounted
for by experience, can also help indicate the amount of charging or
discharging that is taking place.
comparing voltage readings to hydrometer readings, shutting off various
charging sources or loads and watching the resulting voltage changes, the
system owner can learn to use indicated voltage readings with good results.
Monitoring & Maintenance:
Monitoring battery state of charge is the single largest responsibility of
the system owner. The battery voltage should be kept at or above a 50% state
of charge at all times for maximum battery life (see the battery voltage
table). Be sure to keep the battery's electrolyte level at the marked full
level and never let the plates become exposed to the air. When refilling the
batteries, use only distilled water - not tap water. Water is the only
element used by your battery, you should never have to add additional acid
to your battery. Do not over-fill the batteries or fill when the batteries
are discharged. Over-watering dilutes the acid excessively and electrolyte
will be expelled when charging.
As batteries are charged they create bubbles of gas, produced when the
chemical reaction cannot keep up with the energy input. Some gassing is
necessary in flooded cell batteries. The amount and duration of gassing
varies from one battery to another. Gassing mixes the electrolyte and
compensates for the tendency of the electrolyte to stratify with the more
dense acid on the bottom. Gassing is the product of splitting water
molecules into hydrogen and oxygen. This consumes water and creates the need
for its periodic replacement.
A slight acid mist is formed as the electrolyte bubbles upon charging. This
mist is highly corrosive, especially to the metallic connectors on the tops
of the batteries. Inspect for corrosion and carefully clean these
periodically as needed with baking soda and water. Be sure not to get any
baking soda into the battery electrolyte as it will have a neutralizing
effect. Corrosion buildup can create a good deal of electrical resistance,
which can contribute to shortened battery life and the waste of power. It's
always a good idea to wear goggles and protective gear (goggles, rubber
gloves and apron) when working on your batteries as the sulfuric acid can
seriously damage your eyes and eat holes in your clothes.
Equalization is a controlled overcharging of a fully charged battery. This
overcharge mixes the electrolyte, evens the charge among varying battery
cells and reduces permanent sulfation of the battery plates. It is energy
invested in lengthening the life of the battery. Though the PV system
battery bank receives a good deal of cycling and gassing through normal
activity, equalization is a complement to this activity and as a rule of
thumb should be done every 60 to 90 days. The equalization process consumes
water and produces much gassing, so your batteries should be well ventilated
during this charging. Equalization charging voltages vary widely, as do
duration times, so the batteries should be monitored closely during this
process. Check periodically during the EQ process. You don't have to check
every cell each time, but watch any that show a high variation from the rest
of the cells. Keep checking the specific gravity of the electrolyte until
you receive three readings of 30 minutes apart which indicate no further
increase of specific gravity values. Keep a record of individual cell
voltages and specific gravity before and after equalizing. Equalization will
take your voltage to 15 volts or higher (30 volts on a 24 volt system) so
make sure any DC loads are disconnected before you begin.
The connections from battery to battery and on to the charging and load
circuits are critical. Before connecting your batteries together, be sure
that the interconnects and battery terminals are clean. When making your
series and parallel battery connections, be careful not to torque the
connecting hardware too tight as the battery's lead posts can break easily.
After all battery connections are made, go back to each battery terminal and
apply anti-corrosion coating or grease to minimize corrosion build up.
Torquing all bolts equally avoids variations in resistance. This variation
in resistance is the main reason we prefer to minimize the number of
parallel strings in the bank. Higher resistance values on one string of
batteries result in less charge to that string and consequently shorter
life. We also place the main negative and positive on opposing corners of
the battery bank. The goal is to keep the variation of resistance from one
parallel string to another to a minimum.
Like most things, you get what you pay for. Used lead acid batteries,
especially large two volt telephone type cells can be found for sale at some
very attractive prices. While used solar modules and inverters are usually
an acceptable risk, used batteries are not. Should you consider them? In our
experience it is difficult to know just how an older battery has been used
and cared for. Our recommendation on used batteries is to inspect them
carefully in person, get as much information as you can on them
(manufacturer, age, amp-hour capacity and type of system they were used in)
and have them load tested. Without load testing used batteries you are
really guessing as to their remaining life. If you are considering telephone
cells, realize that they are normally shallow cycle lead calcium grid
construction, and should not be used in a system designed for deep cycling.
DON'T SKIMP ON BATTERIES!
A correctly sized battery bank is vital to the proper functioning of your
system. Compromising on the battery bank can lead to poor performance and
dissatisfaction with the entire system. Do not skimp here.