Learn to determine the capacity of a battery in milliamp-hours (mAh) to know how much charge it holds when full. Keep in mind that the mAh unit indicates a milliamp, or one-thousandth of an amp, multiplied by an hour unit of time. This means that once the capacity of a battery is known, dividing its mAh rating by the milliamp current draw of the device results in how many hours the battery will last.
Fully charge the battery to be tested if it is rechargeable. Otherwise, use a new non-rechargeable battery.
Turn on the digital multimeter and switch its measurement dial to the direct current (DC) measurement setting, since all batteries produce DC current. The DC current setting is denoted by the capital letter "A" with straight lines above it. The "A" stands for the basic unit of current, the amp.
Plug the red probe of the multimeter into its positive port. Connect the black probe of the multimeter into its negative port. The negative port may be labelled "COM" for common ground.
Connect the red probe of the multimeter to the negative terminal of the battery. Connect the black multimeter probe to the positive terminal of the device powered by the battery using electrical wire stripped at both ends. Connect the negative terminal of the powered device to the positive terminal of the battery using a piece of electrical wire stripped at both ends. Write down the current reading from the multimeter screen and start the stopwatch. If the reading is in amps, multiply by 1000 to convert to milliamps. For example, assume the current reading was 200 milliamps.
Stop the timer when the device being power by the battery shuts off. The battery is now drained. Record the time in hours required to drain the battery. For example, assume it took 15 hours to exhaust the battery's energy.
Multiply the current reading by the time to arrive at the battery's capacity in milliamp-hours. Completing the example, you have 200 milliamps multiplied by 15 hours, your battery has a capacity of 3,000 mAh.