My car is blowing up batteries! =/ Bad alternator?

[mikeote]

My car is blowing up batteries! =/ 2003 VW Passat 1.9 litre TDI, 140,000 miles.


Hey guys, have you seen this before? I was driving down the road and my car just died, no warning, no nothing every thing just turned off. Luckily I was able to pull over to a parking space. I checked the battery which had zero voltage so I thought the battery had an internal short. To verify I plugged in a small cheap car just starter which instantly exploded!


I thought that this was just because it was a cheapo jump starter which couldn't handle being plugged into a shorted battery. I took the bus to my local auto place and bought a new battery. As soon as I connected the battery cables to the battery there was a fizzing sound, so I disconnected the cables and checked the voltage on the battery which was previously 12 volts and now zero volts! =/


So I'm thinking that it's not a battery problem... After doing a little research I found that if a diode has failed inside the alternator, the alternator can flood the cars electrical system with alternating current; which is my best guess so far. The alternator was on it's way out any way (I got it from a scrap yard), when I revved hard a warning showed up on the dash "alternator workshop". I checked the alternator a few times when it was running, and when at idle it was fine; so I thought it was just losing a bit of power generating capacity when the car was revved hard so kept using it.

Can any thing else cause the instant death of a battery? Could it be a short in the system? I've ordered a multimeter (lost my last one) to check for AC in the system. If there is AC coming from the alternator how can I get rid of this to make it safe to remove the alternator?

PS. I definitely did not put the battery in backwards.

Thanks!
Mike

[Roverfan]

Sounds like a significant short is your immediate concern.
AC would send your electrical system wild but you don't have that symptom.
I would check the larger + wires first, to alternator, to starter to fuse box for any obvious problems. Perhaps one is pinched somewhere or has rubbed through or become detached, overheated etc

[Crasher]

Yes, I would disconnect the one to the alternator then the starter in turn but with the batteries earth off and then touch the earth clamp to the terminal to watch for obvious signs of distress... take great care here as whatever is shorting the battery out must be getting bloody hot, you are lucky it has not gone up in flames already.

[mikeote]

Thanks guys once I the new multimeter arrives I'll check the resistance between the battery cables to see if it shows a short. Crasher, yes I forgot to mention that the battery cables did get pretty hot; almost too hot to touch. I'll keep you updated.

Cheers!

[mikeote]

Sorry for the long delay with the update guys! Here's what's happened:

New multimeter arrived which showed continuity (0.02 ohms) between the battery cables.

Disconnected the alternator = no change.

Disconnected the starter motor (in fact had to remove it entirely as no access to the cables with it in the vehicle) = change to 0.20 ohms.

I realised that there is a smaller cable attached to the larger positive cable, I disconnected this which cured the short in the larger cable.

It's hard to see but I think the smaller cable goes directly to the relays under the steering wheel. Could it be the a relay which has failed and caused the high voltage side of the relay to stick closed causing the short? If not what else could it be? If you do think it's the relay where can I find the data which shows which relay is responsible?

Thanks!
Mike

[Crasher]

I won’t be able to look at the diagram until Monday to check but that would be the alternator cable, what colour is the thinner cable? Perhaps it has rubbed through?

[mikeote]

I won’t be able to look at the diagram until Monday.

Thanks bud!

[Crasher]

I edited my post

[VAG-Abound]

Trying to follow what you've found out...

Disconnecting alternator made no difference, so probably OK.

Large positive cable to starter motor was still shorting with motor removed. Somewhere it had a connection to a smaller cable, and it was this that was apparently providing short to ground. Where was this connected? Battery end? Starter motor end? And as Crasher asked, what colour? What did the connection look like an original VAG crimp or click together, or something else that may be after market?

Although now disconnected from main starter cable that smaller wire presumably still has a short to ground?

If starter relay was stuck starter motor would keep turning even if not needed. It sounds like you've managed to get a short between the high current starter motor cable and the small cable which has a path to ground without going through a fuse. To get hot, etc you're probably dumping 15A to 30A (200W-300W ) though a 1 Ohm or so path which may damage but not destroy that smaller wire if it is 2.5mm or so. Be sure to check and / or replace full length it even if it seems you've found a rub point that you can reinsulate.

BTW, you're mixing terminology a bit. Nothing is high voltage, just high current.

[mikeote]

Crasher: "what colour is the thinner cable?" It's black and though thinner than the larger cable it's not a thin cable, I think it contains several thinner cables which go to the relays. "Perhaps it has rubbed through?" Yeah definitely maybe, but to get access to the rest of that thinner cable I need to remove the ECU box. The ECU box's hex screws are all easy to get at apart from one which is at the rear of the box covered by the lower windscreen cowl, so you need to remove that cowl to get at it; which involves removing at least one of the windscreen wipers =/ So I'm checking every thing else first.