BATTERIES

The amount of power that a PV system produces varies during the day and at night will be zero. As power is needed during the night as well as in the day in cases it is essential to have the ability to store energy which is not used during the day. This energy is stored in a battery for use at a later time. PV systems can be used to effectively charge batteries during the day.

Batteries come in many different sizes and technologies and some are only suitable for certain applications (the four most commonly used types are Lead-calcium, Lead-antimony 6%, Lead-antimony 2% and Nickel-cadmium). When purchasing a battery the following should be taken into account: 

Cycle Life
It is the number of times a battery can be effectively charged and discharged. In a PV system this is continuously happening so it is essential that the battery purchased has a high cycle life. Some types of batteries are designed with a longer cycle life than others.
Note: The cycle life for any type of battery will depend to what extent the battery is continuously discharged. A small amount of charge being removed will result in a longer cycle life.

Rated Storage Capacity
A battery's size is expressed by the amount of charge it can hold and is measured in terms of Amp-hours (Ah).
Example: A 100Ah battery can run a light which requires 2A of current for 50 hours.

Depth of Discharge (DOD)
It is the extent to which a normal battery should be allowed to discharge in normal operation. Going beyond the DOD of the battery can result in permanent damage.

Self-Discharge rate
If left unused, all batteries will slowly lose their charge but some types will self-discharge faster than others.

Discharge Rate
It is the period of time in which a battery is designed to deliver its full storage capacity. A 100Ah battery with a discharge rate of 50 hours is designed to deliver a current of 2A. The available storage of a battery will be greater if it is discharged over a longer period or less if the discharge rate is increased.

Charge Rate
Charging a battery with a high current causes excessive heat to build up and can cause damage. Normally a battery can be safely charged with a current equal to that of one tenth of its Ah capacity. The lower the charge current, the greater the efficiency of recharging.

Temperature
The speed of the chemical reactions taking place in a battery is determined by it's temperature. Colder temperature yields a slower reaction and warm temperature will yield a faster rate of reaction and making charge available at a quicker rate.
Example: When starting a car on a cold morning the engine does not turn quickly. Warm that same battery and the improvement will be substantial.
Below is a comparison of short term power available from a fully charged battery at various temperatures.

Maintenance
Monitoring battery state of charge is the single largest responsibility of the system owner. The battery voltage should at least be kept at 50% of capacity for maximum battery life. Maintenance requirements will vary per battery type.