Battery capacity is measured in milliamps × hours (mAH). For example, if a battery has 250 mAH capacity and provides 2 mA average current to a load, in theory, the battery will last 125 hours. In reality, however, the way the battery is discharged has an impact on the actual battery life.
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Battery testers are electronic devices designed to test the remaining capacity of a battery's overall charge. Contrary to popular belief, they do not test the voltage – simply the remaining capacity. Every battery has a direct current.