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GENERAL INFORMATION: BATTERIES
Types of Batteries
There are several different types of battery chemistry including liquid Lead-Acid,
Nickel-Iron (NiFe), Nickel-Cadmium (Ni-Cd), alkaline, and gel-cell. Batteries are either
sealed or vented. Simply, there are only two principal types of batteries: starting and
deep-cycle.
Starting batteries are designed for high cranking power, but not for deep cycling.
Used as energy storage, they will not last long in a deep cycle application. Starting
batteries use lots of thin plates to maximize the surface area of the battery. This allows
very high starting current but lets the plates warp when the battery is cycled. This
type of battery is not recommended for the storage of energy in solar applications.
However, they are recommended as starting battery for the back-up generator.
Deep cycle batteries are the type of battery best suited for use with inverters. The
physical dimension of the plates is thicker and the active material that holds the
charge is denser to increase cycle life. The “deep cycle” type of battery is designed to
have the majority of their capacity used before being recharged. They are available in
many sizes and in either “non-sealed” or “sealed” types.
Usual battery inverters are optimized for use with lead acid batteries that have
a nominal voltage of 2.0 volts per cell. Ni-Cd/NiFe batteries (also called alkaline
batteries) have a nominal cell voltage of 1.2 volts per cell. The nominal voltage of a
Ni-Cd / NiFe battery bank can be made the same as a lead acid bank just by juggling
the number of cells (10 cells for 12 volts, 20 cells for 24 volts and 40 cells for 48 volt
systems). However, the Ni-Cd/NiFe battery bank must be charged to a higher voltage
to fully recharge and will drop to a lower voltage during discharging compared to a
similarly sized lead acid type battery.
State of Charge (SOC) of the Battery
One important parameter that defines the energy content of the battery is the State
of Charge (SOC). This parameter indicates how much charge is available in the battery
referring to its capacity. It is the ratio of the difference of the rated capacity in Ampere
Hours (Ah) and the net Ah discharged or charged since the last full SOC. For example,
in a 100Ah capacity battery, if the net Ah discharged is 20 Ah, then the SOC is 80% ie
(100 Ah – 20 Ah) divided by 100 Ah. SOC is also called Residual Capacity.
Generally, the voltage of the battery cell is taken as the basis for calculating SOC
or the remaining capacity. Results can vary widely depending on actual voltage
level, temperature, discharge rate and the age of the cell and compensation for
these factors must be provided to achieve a reasonable accuracy. Fig 3.1 shows the
relationship between the Open Circuit Voltage and the Residual Capacity at constant