If you're drawing a lot of current, probably a little. But since you'll only be drawing a handful of amps I wouldn't bother.
Here's a quick pic of my ghetto conversion. I opened it up, desoldered most of the wires inside except the 12v, soldered the power on wire straight to ground and just ran two wires up to the two bolts through the case.
I use one of these: Charges lead acid, lithium polymer, nicad and nimh. It's getting pretty old now though.