Checklist for Choosing Batteries for Solar Power System

The following are the key parameters you should consider and evaluate while choosing a Battery.

  • Amp-Hour Rating
  • Reserve Capacity
  • Battery Cycle Life
  • Operating Temperature
  • Nominal Voltage

Key Parameters to Look for while Selecting Solar Batteries

Specification What to look for Notes Location where it is specified
Amp Hour Rating Amp Hour Rating should be selected according to the extent of back-up hours you desire from the battery. Battery Ah =
[Backup hours*Load (W)]    
(Battery Volts * Inverter Efficiency). The load is the electrical load you need. Battery volts and inverter efficiency are specified in the respective equipment.
On the battery
Reserve Capacity This is usually denoted in minutes. Choose batteries with Reserve Capacity (RC) that can last for long durations. Eg: RC @ 25 A = 160 min indicates that the battery can supply a steady current of 25 A for 160 minutes. This is better than another battery for which RC @ 25 A = 120 min. Data on reserve capacities will be provided on the battery for different amperes. Which battery is best thus depends on the current drawn by you. Datasheet
Battery cycle life Choose a battery with the highest number of cycles. For depth of discharge of 80%, look for the following number of cycles: For lead acid batteries >600 cycles, for Li-ion (Lithium ion)>1200 cycles, and for NiCad (Nickel Cadmium)>2000 cycles. The indicative number of cycles will be usually provided in the documentation/manual that comes along with the batteries. Datasheet
Operating temperature The range of operating temperature should be as wide as possible. An acceptable range is -40°C to 60°C.   Datasheet

Details of Parameters
Amp-Hour Rating

The most common and important battery rating is the AMP-HOUR RATING. It is a unit for battery capacity, obtained by multiplying a current in amperes by the time in hours of discharge. A battery which delivers 5 amperes for 20 hours delivers 5 amperes times 20 hours, or 100 ampere-hours.Amp-Hour Rating gives information about “the maximum current that can be drawn from the battery and run for required period of time”.Amp-Hour Rating should be chosen according to the necessity.

Reserve Capacity

Reserve capacity (RC) also referred to as Reserve capacity minutes (RCM), is a battery’s ability to sustain a minimum stated electrical load; it is defined as the time (in minutes) that a lead-acid battery will continuously deliver 25 amperes before its voltage drops below 10.5 volts. Reserve capacity gives information about “how long will the battery last”, given a particular current draw (or load).If heavy loads are involved, focus should be on choosing batteries with higher reserved capacity rating rather than the ones with AH rating.

  • Reserve capacity is sometime used to rate deep cycle batteries.

Battery Cycle Life
The number of charges and discharges of a cell or battery can provide before failure is called battery cycle life.

  • Battery cycle life must be as high as possible.

Operating Temperature

Actual working temperature of the battery will not be the temperature of its surroundings but higher, depending on the heat generated by the battery application.If the product operating temperature requirements exceed the battery operating limits it will be necessary to add heating or cooling arrangements.

  • If the battery is utilised outside the operating temperature, it may result in a permanent degradation in performance or complete failure.

Nominal Voltage

Nominal voltage is used to indicate the operating range of battery voltage. For a sealed Lead-Acid Battery the nominal voltage is 2V per cell.The inverter must have the same nominal voltage as battery.