Thanks ill. That's about what I expected. I suppose I need to make sure that the capacity of the batteries I use stay well under what I expect the vehicle's to have. I would like to be able to leave these in to charge without leaving me unable to start the vehicle.
What happens if these batteries get plugged into a 12v cigarette plug wall adapter? Will it have the same effect? It just 'feels' odd to me to keep supplying a full battery with full voltage with no control over current. I suppose that's what happens in a running car, though.