On my P38 i made up a box for battery that sits in the back of the car on the left hand side and run to heavy duty cables though to the bulkhead and the black one (-) connected to the engine mount on the engine side . the red one (+) I run to a winch solenoid mounted near the bulkhead so that i can isolate the battery and it dos'ent drain and another cable from the other side to the starter motor . I run a wire from the live side of the solenoid to an on/off switch mounted in the cab and back to the activation terminal on the solenoid so when battery needs charging you switch it on or when the main battery is flat the second one can be switched on and used as a jump start. I tried using a starter solenid instead of a winch one but with constant use the winding's burnt out.
 

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