Hi Alex and
to the forum.
With the GM controllers it is the full battery voltage that is being switched as shown in this diagram:
However, I'm not sure if this diagram is 100% accurate, as the e-lock wire appears to be connected directly from the battery
+ connection on the contactor, but I don't know whether the e-lock wire should also be switched via the throttle mounted e-lock switch as I've shown it here:
If a contactor is not fitted, the e-lock switch simply switches the 72V battery voltage to the e-lock wire on the controller, but when a contactor is fitted the e-lock switch switches battery voltage to energise the contactor's 72V coil.
If your vehicle already has a 12V battery for ignition and/or starting/lighting purposes, you could then use a contactor with a 12V coil powered from the existing 12V battery
(either through the existing ignition switch or via a separate switch).
Should you need to switch full battery voltage to a 72V contactor, most 12/24V switches will be capable of doing this, as they would only be switching a very low current of ~200mA
(~0.2A) at 72V to keep the contactor coil energised.
You could also wire in a 72V to 13.8V DC/DC converter to the switched output side of the contactor
(Battery supply to controller) to keep the 12V battery topped up while running on land and to power the lighting and horn etc.
Looking forward to hopefully seeing some pictures
(and eventually some video) of this unusual vehicle.
Alan