Battery Safety Terminal???
Recently I went to start my 1999 BMW 540i (after leaving it parked for just a few hours) and all electric power was gone, not even the smallest interior light would come on. Assuming a dead battery, I was about to try jump starting when I slammed the trunk lid shut and suddenly all electric power came back on like normal, but when I tried to start engine all power went off again. So I opened trunk and checked for loose battery terminals and both seemed to be tight, but as I wiggled the large red cable coming out of BST all power came back again! This time I was able to start engine and drive car home without a problem and the Check Engine light never came on.
In trying to determine cause of total power loss, my first thought was the battery safety terminal (BST) at battery end of +ve cable. From reading other posts I understand that this is an "explosive coupling " that "disconnects the main battery feed to the starter and alternator" but "does not disconnect power to the other systems". I assume that means once this coupling has "exploded" it cannot be reset and must be replaced. Is that true - does it really explode and break apart what is a very thick battery cable? Anyone know exactly how it works? Is it a kind of fusible link and does it mean that any 1999 540i would be totally disabled and have to be towed away to a BMW dealer after an accident?
Anyway there's no sign that my BST has exploded (I'd expect to see it burned out) and since in my case (a) all power was lost, not just power to starter and alternator (b) all power came back after either slamming trunk lid shut or jiggling the +ve cable, am I right to assume that in my case this explosive coupling has not irreversibly deployed and so is not at fault here? If so that leaves me wondering what to do if this happens again, as seems likely? I now have a multimeter in car, so I could check for voltage across the battery terminals (check for possible bad battery with loose internal +ve terminal - this battery was installed by Autozone last year). Could this be a problem with a short or a bad ground somewhere? (I'm not good with electrics) Any other thoughts/suggestions very welcome.
Now I wonder if this battery safety device could be triggered by a rear-end collision because coincidentally this car was hit in the rear bumper in a minor parking lot incident last year, when another vehicle backed into it. This was enough of a jolt to set off the alarm, but only caused superficial scratch damage to the bumper and certainly the air bags were not activated and there was no loss of electrical power at that time. Could being hit in the rear bumper trigger air bag deployment and/or BST?
More related questions:
1. Can someone advise what would happen if for any reason battery power is completely shut off while driving? Would you need to immediately put it in Neutral rather than coasting to a halt in Drive and would that damage the automatic transmission? (I've never had such a thing happen with any car and never wondered about it until now). This would seem to be very dangerous for something that's supposed to be a safety feature! Would it result in damage to the alternator, computer, voltage regulator or what? I read that you should never disconnect the battery while the engine is running,
2. Any special precautions needed when working around this BST? Any chance that it could explode just from jiggling the cable?
3. I read one post where someone cut out the BST and rejoined the cable directly to the battery terminal clamp - anyone think that's a viable option (short of having dealer replace the whole cable for $600) or would that cause some other problems, like with the wiring for the air bags? (don't want those going off!)