Abstract: A method of monitoring the internal impedance of an accumulator battery in an uninterruptible power supply, and a UPS. In accordance with the invention, during the discharge of the battery to the load, the load current induced by the load and passing through the battery is measured or calculated, and the corresponding AC voltage component obtained on the battery voltage is measured, the value for the internal impedance of the battery being calculated from these values. The obtained impedance value is compared to a predetermined reference value to determine the condition of the battery.