Milliamp-hours, or mAh, are the units technicians use to measure a battery's useful operating lifetime, or how long the battery will last when it powers a normal electrical load. You can measure a battery's mAh rating using a multimeter, a resistor to act as a load and a stopwatch to measure elapsed time.
Check the meter's battery by setting the multimeter's control knob to the battery check position. If the meter's internal battery is depleted, replace it.
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Place the battery in the holder. For 9-volt batteries, use a snap-on battery clip with wire leads.
Slip the alligator clips onto the meter's probe tips.
Select a resistor appropriate for the battery's voltage and typical drain current using the table below:
- D battery, 200 milliamps drain current
- C battery, 100 ma
- AA battery, 50 ma
- AAA battery, 10 ma
- 9-volt, 15 ma
Calculate the resistance value by dividing the battery voltage by the current drain in amperes. For example, a 1.5-volt D battery with a drain of .2 amps gives 1.5 / .2 or 7.5 ohms. This is not a standard resistor value, however, so select the next greatest standard value, or 10 ohms.
Reset the stopwatch.
Connect the battery's positive terminal to one of the resistor's leads.
Set the multimeter's control knob to read direct current, or DC, in the 200-milliamp range.
Clip the positive or red meter probe to the unconnected resistor lead. Clip the negative or black probe to the battery's negative terminal. The meter should give a positive current reading.
Start the stopwatch.
Check the current reading about every hour. When the current is about 70 percent of the original reading, stop the stopwatch. For example, if the original current is 100 ma, stop the stopwatch when the meter indicates 70 ma.
Calculate the battery's milliamp-hour rating by multiplying the number of hours recorded on the stopwatch by the initial current reading on the meter. For example, if the meter initially read 90 milliamps and the stopwatch is at 10 hours, the milliamp-hour rating is 90 * 10 or 900 mAh.