Many appliances require very short-term energy storage such as flywheels or capacitors. Batteries are used only in two most common cases, for power conditioning and as long-term storage. A battery storage system for a standalone PV system forms part of the critical aspect of cost. Other electrical parameters such as power rating, battery capacity, death of discharge and efficiency should be considered before implementing a standalone PV system.
The power rating of a battery, measured in Amperes (A) is defined as the maximum rate of charge and discharge.
Battery capacity is the maximum amount of energy that can be extracted from a battery without the battery voltage falling below a prescribed value. The battery capacity is measured in kilowatt-hours (kWh) or ampere-hour (Ah), at a constant discharge rate.
Depth-of-discharge is the percentage of the rated capacity withdrawn from the battery. Shallow cycling batteries should not be discharged more than 25% of the rated capacity, while up to 80% of the capacity of deep cycling (one that undergoes regular deep discharges) may be discharged.
Self-discharge is the extent to which a battery discharges itself without the presence of a connected load over time. This depends on the type of chemical reaction involved and the temperature of the battery.
The most common types of batteries used in PV systems are Lead-acid, Lithium ion and Nickel-Cadmium.