How To Calculate AH For A Battery

Based on an appliance's wattage and the voltage of the attached battery, the appliance will draw a specific amount of current through the connecting wire in order to run properly. Because a battery is designed to maintain peak voltage throughout its life, the standard measurement unit of how long an appliance can run on a given battery is amp-hours or "AH." While manufacturers put an amp-hours rating on a battery, this value is based on a one-amp appliance. To determine the amp-hours rating of a battery for a specific level of current, you need to use Peukert's formula.

Step 1

Check the battery's label for the published amp-hours rating.

Step 2

Check the battery's label for its voltage.

Step 3

Check the appliance's owner's manual for its power rating (in watts). If you don't have this manual, visit the manufacturer's website and search the "Support" section for technical information.

Step 4

Divide the appliance's wattage (from Step 3) by the battery's voltage (from Step 2). The result is the current (in amps) that the appliance draws from the battery.

Step 5

Determine the "Peukert's number" for the battery. See "Resources" for a link to a table of Peukert's numbers for common batteries.

Step 6

Calculate the current draw (from Step 4) taken to the power of the Peukert's number (from Step 5).

Step 7

Divide the battery's published amp-hours rating (from Step 1) by the result from Step 6. This value represents the actual amount of time (in hours) for which the battery can support the appliance.

Step 8

Multiply the result from Step 7 by the appliance's current draw (from Step 4). This will given you the actual amp-hours rating for the battery when used with that particular appliance.