A line current power supply delivers operating current to a clock designed to be powered by a conventional electrochemical cell disposed in a power source compartment within the clock housing, and at the same time supplies recharging current to a rechargeable electrochemical cell for powering the clock in the absence of an external power supply. The rechargeable cell is contained within a cylindrical enclosure having a shape such that it can fit into the power source compartment of the clock, and having contacts allowing it to be substituted for the conventional electrochemical power source.