A DC power supply system in which the resistance of a battery is measured. The voltage of the DC power supply is reduced such that the battery supplies some current to the load, and the voltage and current are measured. The voltage of the DC power supply is further reduced such that the battery supplies increased power to the load, and the voltage and current are measured. A resistance value is computed, and may be compared with various pre-established criteria. The battery resistance may also be measured by comparing the charging time of a known resistive-capacitive circuit with the charging time established at a prior measurement epoch.