Battery draining

On the odd occasion I've looked at the voltage with the engine running on my E39, it's always been 13.8v. 13 volts is too low.

That is very fast discharge. I would suspect a bad bridge in the alternator.

The normal way is to put an ammeter on the battery so you can read the current draw, then start pulling fuses one at a time until the draw goes away. Then start looking at everything powered by that fuse, one at a time.

But before doing this, I'd disconnect the alternator first, seeing how rapidly it's dropping. If it doesn't draw current with the alternator disconnected, it's time to rebuild the alternator.

--scott

Thanks for your reply. My web searches find several descriptions of how to remove the alternator, but it's not clear to me if I can easily disconnect it from the battery without removing several other bits first. I'm not much of a mechanic so I'm hoping for a simple solution. Any pointers?

Thanks. That gives me something to double check when I manage to get it running again.

It will help to have the wiring diagram... and it helps to have the REAL one and not the fake one in the Hayes manual.

There is a big red wire that comes out of the alternator, goes through a fusable link, and then takes a separate line to the battery positive terminal than the big red conection to the fuse box.

Disconnect it either at the battery connector side or the alternator site and put an ammeter in series. Measure current draw.

--scott

Indeed, but the rate the voltge is dropping something would be geting very warm!

Yes, and you'll probably get a nice spark when you disconnect it too!

--scott

Looking at the alternator, I see the pully and belt on the front, and at the back near the top is a plug connector with a wire running off. It doesn't look particularly thick. Is there another thicker wire somewhere I can't easily see?

I have managed to get it running. I took the battery out last night and charged it up. But it back in this evening and started the car. Let it run for a bit and measured the voltage at the battery whilst it was running - 14.10 volts. So looks like the alternator is charging the battery Ok.

Went for a nice drive. Measured the voltage immediately after stopping the engine - 13.10 volts, but starting to fall.

I'm beginning to wonder whether it was just a not-very-well charged battery giving me problems. I'm going to measure the voltage tomorrow morning and see how it's doing.

Thanks for all the help so far - much appreciated.

Below the multipin connector is a bolt with a lug on it, and that's where the alternator output is connected. Follow that line to the battery.

--scott

Yup - that's fine, but suggests the battery isn't fully charged.

Battery voltage immediately when taken off charge is pretty meaningless. Should be about 12.5 when left overnight.

The alternator on these cars is so large no normal use should allow the battery to go flat. Obviously, leaving the lights on overnight etc excepted. A good battery specialist should have an electronic battery tester. Of course it needs an honest operator. ;-) But this wouldn't rule out some form of parasitic load running down the battery when the engine isn't running.

Voltage the morning after was 12.2 volts. Seems alright to me. I beginning to think that my new battery just needed a good charge up. I'll see how it handles being left for a longer period. It not unusual for me to go a week between uses, and then sometimes the journey is quite short.

Thanks for all your help. Following a decent charge of the battery, all seems to be well. I'll know for sure after a week or so, but I now reasonably sure I was worrying over nothing. Still, I've learnt a thing or to that might come in handy later. Cheers.