The batteries chosen for AO5 were "alkaline manganese" for their slow self discharge rate and relatively high capacity. They were a new type of battery with the patent only having been granted in 1960 but the chemistry is the same as the modern "alkaline" battery available in supermarkets and stores everywhere. However, each cell was considerably larger and enclosed in a cylindrical steel case.
The terminal voltage of alkaline cells is 1.5 volts and so 14 cells wired in series made a single 20 volt string. There were two such strings each supplying one of the two transmitters. The remainder of the electronics, that is, the command receiver and decoder, telemetry system and HI Keyer were supplied by both strings via isolating diodes. In this way, we were assured that one transmitter, telemetry and command system would continue to operate even if one battery string failed.
As you can see from the image taken during construction of the structure, individual cells were encased in epoxy resin. The isolating diodes are visible at the top of the image.
Union Carbide supplied the batteries but due to the long time between construction and launch, the batteries were replaced in the US prior to launch.