The more electrolyte and electrode material there is in the cell, the greater the capacity of the cell. Thus a small cell has less capacity than a larger cell, given the same chemistry (e.g. alkaline cells), though they develop the same open-circuit voltage.

Because of the chemical reactions within the cells, the capacity of a battery depends on the discharge conditions such as the magnitude of the current, the duration of the current, the allowable terminal voltage of the battery, temperature and other factors.

The available capacity of a battery depends upon the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected.

The battery capacity that battery manufacturers print on a battery is the product of 20 hours multiplied by the maximum constant current that a new battery can supply for 20 hours at 68 F° (20 C°), down to a predetermined terminal voltage per cell.

A battery rated at 100 A·h will deliver 5 A over a 20 hour period at room temperature. However, if it is instead discharged at 50 A, it will run out of charge before the theoretically-expected 2 hours.

For this reason, a battery capacity rating is always related to an expected discharge duration—the standard duration is 20 hours.

t = \frac Q I

where

Q is the battery capacity (typically given in mA·h).
I is the current drawn from battery (mA).
t is the amount of time (in hours) that a battery can sustain.

The relationship between current, discharge time, and capacity for a lead acid battery is expressed by Peukert's law. The efficiency of a battery is different at different discharge rates. When discharging at low rate, the battery's energy is delivered more efficiently than at higher discharge rates.

In general, the higher the ampere-hour rating, the longer the battery will last for a certain load. Installing batteries with different A·h ratings will not affect the operation of a device rated for a specific voltage unless the load limits of the battery are exceeded. Theoretically, a battery would operate at its A·h rating, but realistically, high-drain loads like digital cameras can result in lower actual energy, most notably for alkaline batteries. For example, a battery rated at 2000 mA·h may not sustain a current of 1 A for the full two hours.