Engine/Transmission (1998.5 - 2002) Truck will not start

[BCronk]

I have noticed my batteries getting weak the past few months, and now they have finally given up. So I proceed to remove the batteries and notice that when I unhook the cables there is a spark, when I hook the cable back up it sparks. Is this normal on a 24V? What is taking power? Is this why my batteries where dead this morning? Actually one is totally flat the other one still has a little juice left. I don't want to put in new batteries tonight and go out to a dead truck in the morning.