Battery draining

I've a 2002 E39 with a battery problem. It started a while back during the cold weather. I wasn't using the car much, and the battery ran flat a
couple of times. I figured that was to be expected, and following a re-charge everything seemed fine. I checked the voltage when the car was at idle, adn it was about 13 volts or so, so I figured the alternator was working. However, more recently the battery has run flat more often. I don't use the car much, and the battery was over 9 years old, so I figured the cold weather had finally killed it off. I went a bought a replacement.
I still having problems. I ran flat again today - last used the car on Saturday. I had to unlock the boot with the key, adn the voltage was about 2.8 volts! I recharged it for about an hour, and then it read 11.6 volts. I tried to start the car and it didn't have enough juice - just got a ticking sound. Read the voltage again and it was down to 10.2 volts. A couple of minutes later and it was 8.6, and dropping!
Have I been sold a dud battery? It seems like something is draining the power. Are there any likely suspects? Or an easy way to work out the culprit?
Cheers
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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.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Thu, 22 Apr 2010 00:09:30 +0100, Dave Plowman (News) wrote:

Thanks. That gives me something to double check when I manage to get it running again.
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On Thu, 22 Apr 2010 18:51:55 GMT, smb wrote:

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.
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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.

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On Fri, 23 Apr 2010 23:06:20 +0100, Dave Plowman (News) wrote:

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.
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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
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On 22 Apr 2010 10:32:29 -0400, Scott Dorsey wrote:

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?
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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
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On 22 Apr 2010 16:48:53 -0400, Scott Dorsey wrote:

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?
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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
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On 23 Apr 2010 15:58:59 -0400, Scott Dorsey wrote:

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.
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wrote:

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

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