Battery capacity (Ah) is defined as a product of the current that is drawn from the battery while the battery is able to supply the load until its voltage is dropped to lower than a certain value for each cell.
Here below are some conservative power ratings for good quality alkaline-manganese dioxide batteries, which are available in your near by groceries.
| Battery Type | Capacity (mAh) | Typical Drain (mA) |
| AAA | 1000 | 10 |
| N | 650 | 10 |
| 9 V | 500 | 15 |
| AA | 2400 | 50 |
| C | 6000 | 100 |
| D | 13000 | 200 |
Lower the current drawn, better the capacity of a battery. To find out battery life, divide the capacity by the actual load current to get the hours of life.
The capacity of a battery is given in Ampere-hours (Ah) or milliampere-hours (mAh). For example, a battery with a capacity of 100mAh will be able to provide 100 mA to a circuit for 1 hour. Or 50 mA for two hours. Or 10 mA for 10 hours.
A circuit that draws 10 mA (milli Ampere) powered by a 9 volt rectangular battery will operate about 50 hours: 500 mAh /10 mA = 50 hours The cell voltage of alkaline cells steadily drops with usage from 1.54 volts to about 1 volt when discharged.
The voltage is near 1.25 volts at the 50% discharge point. Alkaline cells exhibit a slightly increased capacity when warmed and the capacity drops significantly at temperatures below freezing.
Mercury and silver oxide batteries have nearly twice the capacity as alkaline batteries of the same size but the current ratings are significantly lower. Alkaline batteries also have good shelf life making them ideal for home-made electronic projects.
Rechargeable batteries have less capacity than primary cells as shown in the following chart. This chart shows the capacity as a percentage of the capacity of an alkaline battery with the same dimensions.
| Battery Type | Capacity % |
| Lead Acid | 35 |
| Silver Zinc | 85 |
| Nickel Cadmium | 30 |