Interesting battery problem...

Greetings to you all. I'm new here.

After a couple of months of inactivity I went to turn over the ignition of my Mitsubishi Sigma GL '81 and realised the battery was flat. So I thought it could do with a recharging.

So I recharged it with my Arlec 12V 4A battery recharger for 6 hours. The manufacturers of the recharger recommend 12 hours for a full recharge. A couple of times during the recharging process the LEDs indicated it was in thermal overload mode.

The problem arises when I go to reconnect the battery to my vehicle. I connect it +'ve terminal first, then -'ve, and I know I've got the connections the right way around. There are some sparks when I start to connect the -'ve terminal but I just whack it on and then within a second theres smoke coming up from somewhere down there and the smell of burning rubber. I remove the -'ve terminal and run for it. I come back and stick my head under the bonnet when it's safe and it looks like there might be some melted wire insulation down there.

My questions are:
1. What causes this to happen?
2. Is it a good thing?

Not sure where your from, but there are many places where if you leave your car set untouched to long packrats like to eat the insulation off of wiring, this may have happend, I have seen many cars come through our service drive on a rollback that the wiring was destroyed by these little evil doers. The reason you get a spark when you connect the last terminal is because there is an electrical load on the system somewhere, I would say right in the middle of the smoke and burnt insualtion smell.

Lovely. You have a short-circuit somewhere, and you got yourself a big headache now, since a short-circuited wire gets very hot, melts is insulation, but also melts the insulation of all the wires around it, snowballing the amound of short-circuited wires. I can see you getting a new part of the harness.