Here we see the front panel. The left meter reads the battery voltage, and the right reads charging current. The center switch selects which of the batteries is currently in the circuit. In this case, we see that the NiCad D-cells are in the circuit, that the voltage applied is about 2.5 volts, and the charging current is about 25 mA (from ambient indoor light)
Finally, here's a schematic of the electrical hook-up. The circuitry for charging batteries is unregulated, and depends on the operator to disconnect the battery before it gets overcharged. The circuitry for the NiCads is an especially crude regulator, but it keeps the batteries from over-charging. Here, two series silicon rectifiers limit the voltage to about 1.3- 1.4 Volts (1.43 Volts is about ideal for a NiCad to terminate charge), A planned improvement is to add real regulators so that over-charging cannot occur. A 14 Volt fixed-voltage-regulator would be ideal for the lead-acid battery, and maybe something a little more sophisticated for the NiCads.
