BATTERY CHARGING SYSTEM FOR THE SP.500 In a previous issue we featured a trickle charging unit for the SP.500 which had been developed at Birmingham University. ... The control circuit is designed for use with a battery charger capable of supplying 6A at 12V and with two accumulators, one, 72 amp. hr. for the tungsten lamp (TL) and the other, 10 amp. hr., for the photocell amplifier (PA).