Abstract: Two SCR full-wave center-tapped rectifiers are connected in opposite polarity to battery charging terminals. The proper polarity rectifier is selected by a battery polarity sensing circuit that enables the firing of the SCRs of the proper rectifier. The sensing circuit has two transistors of opposite polarity. The transistor matching the polarity of the battery turns on and enables current to the proper SCR gates. The sensing circuit is controlled by an optical isolator that activates the circuit by shorting the polarized output of a diode bridge connected in the transistors's emitter circuit. This allows the optical isolator to control emitter current of either polarity.