Abstract: A power supply having a battery and a battery charger is disclosed which includes a discharge circuit for automatically measuring the battery charge without taking the battery out of service and without draining the battery below the minimum safe charge level necessary to operate a piece of equipment connected thereto. The discharge circuit automatically electrically isolates the battery from the charging circuit and measures the battery characteristics as the battery discharges across the equipment load. Once the battery charge is measured, the results are stored in non-volatile memory and the battery is automatically recharged.