Thread: Jumper Lead Query

AFAIK, the reason to place the - to body is to reduce the possibility of sparks igniting hydrogen.

BUT, you don't have to link negatives as your second battery is already earthed, so you only need a positive lead.
I keep a positive 2 gauge wire across the front of my car but disconnected from either battery just for this eventuality.
Regards Philip A

The aux battery will have its own earth but unless that cable is already battery-cable size, fitting a jumper lead from the aux negative pole to the chassis would be necessary to avoid overheating the fixed wiring, and last fitted too.

If you have a dual battery system fully wired in, the earth cable for the auxiliary battery should be capable of carrying starter current to the main engine earth point. My choice for "jumper leads" is then a simple push button wired under the dash, this takes power from the aux battery and operates a soleniod relay under the bonnet to connect the two batteries. It could be the solenoid of the dual battery system if wired correctly. Then you don't have to even open the bonnet or exit the vehicle to jump start, just press the button, turn key and proceed.