At 8 milliamperes, nine-volt batteries generate 400–500 milliampere-hours. While amperes measure electric current, ampere-hours are a measurement of battery capacity. A battery’s ability to produce electric current for an hour without running out of energy is measured in amperes.

A 9-volt battery should last an hour if it is delivering 400 to 500 milliamperes (mAh), but differing outcomes may be caused by other factors including the amount of heat produced and the amount of time that has passed. A current rate or a certain time at which batteries have been tested are used to specify milliampere-hour rates. If no time is given, take it to be eight hours.