Abstract: A power convertor has a partial series resonant convertor circuit for supplying power at a substantially constant level to a load of variable impedance, and a control circuit for controlling the operation of the convertor circuit. The control circuit includes sensing lines for sensing a variation in load impedance below a threshold value, and regulating means responsive to the sensing lines for reducing the power supplied to the load by lowering an input control signed to a voltage-controlled oscillator. The control circuit is arranged to operate the convertor circuit in a first normal load mode in which the impedance of the load is above the threshold value and a second reduced load mode in which the impedance of the load is below the threshold value so as to limit a rise in current through the load. The invention extends to a method of controlling a power convertor.

Abstract: A battery filler unit for automatically filling a battery container to a predetermined level consists of a vessel containing liquid to be dispensed into the battery. The vessel is supplied with liquid in a manner to maintain a constant head in the vessel. An orifice in the bottom of the vessel above a downpipe, extending in use into the top of a battery, ensures that an air seal is formed across the orifice by the action of surface tension of the dispensing liquid. As the container fills, air is trapped in the downpipe and dispensing of liquid from the vessel ceases when a pressure equilibrium is established, at which time the liquid level in the container will be at a predetermined desired level.