How is the capacity of a battery defined? What does the Ah capacity of a battery mean?
Battery capacity is normally defined in Ah (Ampere-hour) and represents the maximum amount of energy that can be extracted from the battery under certain specified conditions. However, the actual battery capacity during operation can vary significantly from the nominal “rated” capacity as actual capacity depends on the age, rate at which energy is being drawn from the battery, historical usage pattern (charging, discharging cycles followed) and temperature.
Lead acid battery capacity for Inverter/UPS applications is normally rated at a 20hr draw and is specified as C20 capacity of the battery. For e.g. C20 capacity of 100Ah means that the battery is capable of providing 5Amps of current when discharged by a load over 20hrs (5A*20h = 100Ah) under standard temperature conditions. This battery can be discharged at a faster rate (i.e. the load could draw a higher current from the battery, say 10A) but this ends up reducing the total energy (Ah) that can be extracted from the battery.
The capacity to which the lead acid battery is discharged before it is charged again (known as depth of discharge) also influences battery capacity and its life (number of charge/discharge cycles it can provide). The recommended depth of discharge for lead acid batteries should be less than 80%.