Ampere-hours

• Battery capacities are typically measured in ampere-hours, or Ah. Twelve-volt batteries have varying capacities that can run on the low side, like 18 Ah, up to higher capacities, such as 225-Ah models. The amount of Ah refers to the number of hours a battery can provide an electrical current at its nominal discharge rate.
  
  

Actual Capacity

• The capacity of any battery refers to the maximum amount of energy that can be extracted from it. Rarely, though, can this maximum be reached. The actual capacity decreases and is determined by the number of times it has been recharged, the speed at which energy is discharged (a high discharge rate equals a lower actual capacity) and the temperature of the surrounding environment (higher temperatures equal a higher battery capacity).
  
  

Battery Testing

• Battery capacity is best tested using a temperature compensation hydrometer and a DC voltmeter. The first tests the specific gravity of a battery and the second tests its voltage. For a 12-volt battery, the volt measurement should actually be above 12 volts, but fewer than 13. The specific gravity should read 1.265 for a battery operating at 100 percent capacity. A drop in that number means the battery's capacity has been diminished.