It is difficult to use the power generated by solar,
wind or hydro power sources directly, so the
electricity is usually stored in special batteries
for use when it is needed. These batteries are often similar
in chemistry to car batteries, but are designed differently.
Car and truck batteries are designed to give short bursts
of very high current to start the engine. They are not suited
for use as storage batteries because they are made to be
fully charged all the time and will have a very short life
if subjected to the deep discharge cycles (ie most
of the energy stored in the battery is used before recharging)
that are required with solar electric storage systems.
Solar storage batteries are often made from large
individual 2-volt cells connected together, although smaller
6 and 12 volt batteries are also available. By far
the most common type of battery is the lead-acid battery,
and these are usually of the flooded-cell type, though
sealed lead acid batteries are becoming more popular.
Nickel-cadmium (nicad) batteries are also available,
and while very expensive, do have the advantages of very
long life and more stable voltage during discharge.
The amount of energy that can be stored in a battery is
called its capacity, and is measured in amp-hours (Ah).
A 100Ah battery will deliver 1 amp of current for 100 hours,
4 amps for 25 hours, and so on, although battery capacity
will decrease with increasing discharge rates. Battery capacities
ranging from 1-2000 Ah or more are available.
For long battery life, it is desirable to use only a small
part of the total battery capacity before recharging. Each
time the batteries are run down and charged up, the batteries
undergo a charge/discharge cycle. If more than half the
battery's stored energy is discharged before it is recharged,
this is called 'deep cycling'. Lead acid deep cycle batteries
designed for solar storage will last anywhere from 300 to
5000 cycles (and up to 50,000 cycles for nicads), provided
the discharge is limited to about 20 per cent per cycle.
Solar systems normally do one shallow cycle per day, but
during 'low sun' periods may undergo much deeper discharges.
For long battery life, the shallow cycle should be less
than 20 per cent of battery capacity and the deep cycle
less than 80 per cent.
It is possible to damage batteries by overcharging them.
The maximum voltage that a battery should be charged to
is about 2.5 volts per cell or 15.0 volts for a 12 volt
battery. Some solar panels have an output voltage which
is claimed to be low enough to stop charging above 15 volts
and to be 'self regulating'. However, because their open
circuit voltage is still 18 volts or so, they will actually
continue to charge, with a much reduced current, until about
17 volts. Most conventional panels will deliver full power
up to about 17 volts and so need an external regulator.