You have already received a flood of information, but I missed where someone answered your question as to how to charge the batteries.
Allow me to repeat the information:
(1) Both transmitter and receiver batteries are charged simultaneously. A 12 volt charger is required.
(2) You will have trace the wiring visually, or best, with a volt or ohm meter. One side of the 2-pin plug goes to the battery negative - the charger negative goes to that pin,
the other pin of the 2-pin plug goes to a resistor and LED connected in series. Charger positive goes to that pin.
(3) The transmitter battery positive is connected to one of the 3-pin connectors. The 3-pin mating plug's wire goes to the receiver battery negative.
(4) The resistor/LED pair are connected to another connector on the 3-pin plug.
(5) The 3-pingmatinv plug's wire goes to the receiver battery positive.
(6) Addition wires are attached to the transmitter battery - negative to ground, positive to the switch and then the meter which in this case reads battery voltage. Some meters read output, this one doesn't.
You may chose to disregard this information in favor of all the expert advice previously received - and probably more to come. The source of this is from a schematic diagram prepared by Mrs Elliott and Krause of EK Products dated 4-27-76. Luck!