Batteries, voltage & Alternators.

92 735i E32
In my continuing saga of battery drain; I've gotten conflicting information (not strange really; considering all the places I've sought info)
Here's what I know:
I have a drain, when adding up the mA's across fuses; about a 170ma drain.
If Idrive the car, then stop; the battery has a voltage of just about 12.5-12.8 volts
If I put the battery on a charger, after driving, it jumps to 9 amps and I measure 14.8 volts at the battery..
Pulling the charger off; I can watch the volts drop slowly, but steadily slowing down around 12.5. This happens regardless of the battery being connected to the car or not.
If I let the car "sit" for 2 days; the battery will not be strong enough to start the car.
the battery is relatively new. (less than 4 months old)
Theories/opinions.
Max battery drain should be no more than 50ma -- I haven't been able to confirm this anywhere.
at 200ma; the battery should not be dead in 2 days
Possibly the alternator is bad & if the diode is blown; it is draining the battery when teh car is off.
The battery is bad and not holding a charge.
Questions:
Could the alternator really drain the battery? If so; is there an easy way to disconnect the alternator? one of the two bolts on teh alternator is 'blocked' by the oil filter.
Could a bad alternator have killed the battery?
What is a reasonable battery drain (range) to expect from all the computers & whatnots?
Based on the rapid decline of voltage held in teh battery when disconnected; the battery is probably shot, correct?
Thanks all! -josh
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Josh Assing wrote:

First, an overcharging alternator would kill a battery quickly, but the 14.8 volts you mention is about right. Second, while 170ma is a little high, it's probably not your problem. Third, measuring across the fuses only checks the fused loads, you need to pull a battery cable and measure between it and the battey, Lastly if it's still 170ma and it's dead in two days, the battery is bad no matter how old it is.
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Actually; that 14.8 is with a charger, not the alternator.. but.....

Right! Which is why we're thinking the alternator diode might be bad....

Thanks. -josh
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That's higher than I would expect.

You really need the battery to settle after being on charge to get anything meaningful from a voltage reading, but if it was this sort of value the next morning it means a good battery is in a reasonable state of charge.

That really depends on the output of the charger. But 9 amps at 14.8 volts suggests a large unregulated charger - if the battery is good. And would cause it to gas freely. Not good with low maintenance semi sealed designs.

Sounds like a faulty battery, I'm afraid. A fully charged battery with no load will 'settle' to about 13 volts overnight.

What's the voltage then? A good battery should start the car when only showing 11.5 volts or so.

Quite; 200 mA is only approx 10 amp/hr over two days, so a fraction of the battery capacity.

You'd be seeing several amps quiescent if that was the case.

First thing to do is to have the battery tested. There are modern testers which can give the state of the battery regardless of charge (unless totally flat). Somewhere that sells batteries should have one and do the test for free.
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After a charge, let it settle; it's down to 12.2 or so volts.

I can hear it gas out....

That's EXCELLENT news! No need to be affraid... A battery is cheap (actually free, just have to get off the island... the battery is still under warranty)

10. something...

Only over the battery -- I do not have a meter capable of that level

Will do.
Thanks josh
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Josh Assing wrote:

I've just finished fixing a charging/draining issue on my E30. It turned out to be a bad alternator/regulator which in turn killed one cell of the battery, which wasn't that old. Replacing both fixed the problem. If you start the engine and then connect a voltmeter across the battery terminals, what reading to you get? On mine it was fluctuating quite a lot, between about 11 and 14 volts. With a new alternator and battery the reading shows it as charging at a steady 14v and all is well again. -- Mark
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This is; at least, encouraging that I might be on the right track.

WIll test it today.
Thanks -josh
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That's certainly a faulty alternator - probably the regulator or even worn brushes. But 14 v peak won't damage the battery - it's normal. Occasionally a regulator will fail in a hard on condition and output some 17 volts. This can damage the battery if left too long by 'boiling' it dry.

I had to have the alternator on my E34 changed by my dealer after it died and I had to have the car towed while I carried on to my destination. They changed the battery too - although it was previously fine. It could be running it 'flat' because the alternator failed caused it also to fail, but I'm not convinced.
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I'm sure that the failing alternator did kill the battery in my case, as it was a strong battery before the fluctuations started and wasn't very old. I don't think there was ever any serious over-charging, my readings were never much over 14v, I just considered that it must have been the frequent fluctuations themselves which killed the cell.
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It's unlikely given the actual charging DC isn't a smooth voltage but a series of peaks and troughs.
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Regarding my potential alternator problem...
Using a volt meter vs. the OBC yeilds different results.
Volt meter - Engine idling 14.04 volts; rev engine to 2500 RPM's volts bounce between 13.6 and 14.4
OBC - Engine Idling bounces between 13.9 and 14.1 rev to 2500rpm -- bounces between 13.2 and 14.8
Turn engine off; volts (via volt meter on the battery) w/in a minute volts dropped to 11.99 volts (door open; interior lights on).
The volts varing that much don't seem to be that bad, is it?
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That's not good news. A fluctuation of up to 0.1 volt would be more like it.

The battery isn't fully charged, then.

I'd strip the alternator and check the condition of the brushes and slip rings. If OK, I'd next replace the regulator.
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Yikes. I guess I"m used to older american cars...

Nah; Used a battery charger, let it do it' sthing -- -did the same basic thing; just stopped at 12.2...

sadly; I'm on an island; I think I'll just swap alternators; it'd be faster & easier....
Thanks Dave. -josh
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