Job Specs: "Part MacGiver"

And dont ask "which part" - you know what I mean!

A little background. To collect our data we require a power source in our case it is a small 12v battery for every data acquisition unit (DAU). Normally I need 2 batteries on the line here at the remote and have 2 on charge in the dogbox all day. About every third day I swap them over.
The line we are recording at the moment is the largest we have done and all spare equipment is in use. Therefore I have no spare batteries here at the remote. To charge the batteries I have a charging harness that connects from a 12V regulated power supply to the batteries. However the same 12V power supply also runs other things in the dogbox and is a hassle to remove if I wanted to take it to my room to charge the batteries overnight there.
Solution: A 12V car battery.
Problem: Not enough amps to trigger the relay for the charging circuit built into the battery box.
Agro's Solution: Remove batteries from box, use a coathanger (or trouser hanger to be specific) to connect the batteries in parallel, connect the battery charger and hope the hotel room doesn't burn down while sleeping....