battery back up?
 

im making a car stand slash tire warmer stand slash shock holder(for when i re build my shocks) and i want to make it portable so im going to have a plug at the back that i can plug into my power supply. and also have a 10 cell in 3800 pack my first idea was to run the plug the ps and batteries in parralel but thought that either charge the batteries at 20 amps or drain the batteries when i should be running of the power supply. so i played with a relay trying to get it to switch over when i current runs throught the ps leads i couldnt get it to work.
any suggestions? i want it so that when i plug it in it runs of the ps and when i unplug it it runs of the batteries?
cheers, Josh P