Nine-volt batteries produce 400 to 500 milliampere-hours at 8 milliamperes. Ampere-hour is a unit of battery capacity, while amperes measure electric current. An ampere-hour measures the amount of electric current a battery can supply in one hour before exhausting its storage of energy.
While a 9-volt battery should last one hour if supplying 400 to 500 milliamperes (mAh), other factors, such as heat produced and time elapsed, may contribute to different results. Milliampere-hour rates are specified given a current rate or a given time at which batteries have been tested. Assume eight hours if no time is specified.