How To Calculate How Long A 9 Volt Battery Will Last

Known originally as "PP3" batteries, rectangular 9-volt batteries continue to be very popular with designers of radio-controlled (RC) toys, digital alarm clocks and smoke detectors. Like 6-volt "lantern" models, 9-volt batteries actually consist of a plastic outer shell that encases several small, cylindrical cells wired in a series. However, 9-volt batteries use different types of a cells (e.g. alkaline, lithium, nickel-cadmium), which have different output capacities To calculate the approximate lifetime of a battery powering a specific appliance, you simply need to know the appliance's power rating and the battery's capacity.

Step 1

Determine the power rating (in watts) for the appliance using the battery. In general, this information is printed on a label on the bottom or rear of the device. If you're not sure, visit the manufacturer's website, pull up the device's model number and search under "Technical Specifications."

Step 2

Divide the power rating by 9 volts. The result will be the number of amperes or "amps" that the appliance draws from the battery.

Step 3

Find the "capacity" for the 9-volt battery by checking the Technical Specifications section of the battery's packaging. Note: the battery's capacity will most likely be measured in milliampere-hours or "mAh."

Step 4

Divide the battery's capacity by 1000 to convert its units to ampere-hours or "AH."

Step 5

Divide the battery's AH capacity (from Step 4 ) by the amps drawn (from Step 2). The result is the amount of time (in hours) that the battery will be able to power the appliance.