The amount of electrical energy that a cell can provide from new to the end of its useful voltage on load is called the cell capacity and is quoted in Ampere hours (A-h).
Capacity varies with the amount of current drawn from the cell, the greater the current the lower the capacity, therefore capacity is normally quoted at a standard rate. The 1hr rate is the internationally accepted standard for Nickel Cadmium cells, with 10 hr or 20 hr rates being used for Lead Acid cells.
A cell quoted at 40A h at the 10 hr rate will provide 4 Amps continuously for 10 hours.
A battery quoted at 40A h at the 1 hr rate will provide 40 Amps continuously for 1 hour.
A 40 A-h cell will only be able to provide a discharge current of 80 amps for approximately 20 minutes, not 30 minutes as may be expected by calculation. Similarly, it will be able to supply a discharge current of 20 amps for longer than the expected 2 hrs.
The capacity of a cell is also affected by its age, the older a cell, the lower its capacity, therefore the only way of determining actual capacity is to measure it.