You definitely have something shorted to ground. I've had an ignition switch short across the terminals( my old '40 did that) and I've had a dimmer switch do the same thing. The headlights and/or taillights can cause your type of problem. Recharge the battery and hook it back putting the ground cable on last. If it throws quite a spark pull it back off and pull all of the fuses and try it again. If it doesn't spark it's in one of your fused circuits. If you have a VOM check each circuit with the battery disconnected for a grounded circuit.