So I have come up with a few solutions, either I can wire the charge cable of the solenoid as usual, the the positive for the solenoid switch to straight 12v then the earth of that switch to something that will only earth when the engine is running, just gotta do some more poking, or buy a simple 12v timer thing from eBay, bout 8 bucks or so, programmable to 20 ins or so
