How is battery operating time calculated?

Battery capacity is measured in amp-hours (Ah) or milliamp-hours (mAh), which represents the total amount of electrical charge the battery can deliver over time. To calculate how long a battery will theoretically last, you simply divide this total capacity by the average current being drawn by the connected circuit. For example, if you have a 10 amp-hour battery and your radio draws an average of 2 amps, dividing 10 by 2 gives you an operating time of 5 hours. Dividing current by capacity yields the inverse of time, which is mathematically incorrect for this purpose. While internal resistance affects voltage drop under heavy load and overall efficiency, it is not the primary factor used in the basic formula to calculate battery operating time.