How to Calculate Battery Amp Hours Article

Learn to estimate the capacity of a battery, in amp-hours, to know how much charge it needs for a specific task. Engineers frequently must determine the total available charge a battery must have to power a specific electronic device. It's important to note that the unit amp-hour indicates an amp multiplied by hour. Electrical current, in amps, represents an amount of current flowing past a given point each second, hence the amp-hour is a unit of charge.

Instructions

        1. Convert the electrical current needed by the device the battery will power into amps. For example, assume a device needs 550 milliamps of current. One amp contains 1,000 milliamps, so the conversion is achieved by dividing by 1,000. Performing this step, for the example, you have 550 milliamps divided by 1,000 milliamps per amp, or a current of 0.55 amps.
        2.  Change the total time the device needs to run on a single battery change into hours. Divide the minute portion of the time by 60, then add it to the hour portion. For example, assume a run-time of 15 hours, 40 minutes. Continuing the example, you have 40 minutes divided by 60 minutes per hour, or 0.67 hours. Adding 0.67 hours to 15 hours you arrive at 15.67 hours for the run-time.
        3. Multiply the current draw required by the run-time to arrive at the battery capacity needed in amp-hours. Completing the example, you obtain 0.55 amps times 15.67 hours, or a battery capacity of 8.62 amp-hours.

Tips & Warnings

    Dividing amp-hours by the current drawn, in amps, yields how long a battery will stay charged in hours.