The capacity of a battery relates to the time you can expect it to last and is measured in milliamp hours (mAh), not to be confused with voltage, which is the force of electrical power the battery produces. The mAh you can expect to get from a fully charged battery is often marked on the battery label, but if it isn't on the label, then you can obtain it from the battery manufacturer's website. The mAh declines as the battery gets used, so by using a multimeter you can calculate your battery capacity.
Read the fully charged capacity of your battery on the battery label or look on the manufacturer's website for your specific battery. For example, a nickel cadmium battery is likely to state 600 mAh and a lithium battery 2,400 mAh. Make a written note of the battery's mAh.
Turn on your multimeter and set it to measure mAh. Either turn the knob to the correct setting or press the appropriate button.
Place the metal tab on the end of the red wire from the meter onto the positive terminal of your battery; it's labelled "+." Place the sensor on the end of the black wire onto the negative terminal of your battery, labelled "-."
Read the meter display panel and make a written note of the measurement. If the battery is fully charged, it is the same as the figure you wrote down from the battery label. If the battery is not fully charged, the figure is less than the one from the battery label.
Use a calculator to divide the figure you wrote down from the meter by the figure from the battery label; then multiply the answer by 100 to get the percentage capacity in your battery. For example, if the meter reading is 600 mAh and the battery label is 2,400 mAh, 600 divided by 2,400, multiplied by 100, equals 25, so your battery capacity is 25 per cent. If the two figures you wrote down were the same, then your battery capacity is 100 per cent.
Use the figure you calculated earlier and you can work out how long your battery is going to last. Read the label on the device your battery powers to get its mAh rating. Divide the reading you got from the meter by the mAh rating on the electrical device, and multiply the result by 60 to get the minutes your battery will last. For example, if the meter reading is 600 mAh and the rating on the label of the device is 1,200 mAh, 600 divided by 1,200, multiplied by 60, equals 30, so your battery will last 30 minutes.