Bad alternator or battery?

Hi everyone, looking for some input. I've got a 2000 540i with ~125k miles on it. Here's the deal:

Went out to dinner on the 31st, valet left my lights on and the car died. Jump started it and my battery light was going on/off depending on how hard I was accelerating. Got home to check the battery and the "magic eye" was black, meaning it needed a charge. I was able to drive ~80 miles with the battery light issue still going on. Under acceleration the light would go off and the dash/radio lights would brighten a bit.

I had it charge overnight and the magic eye was still not showing a full charge. Using test #9 it showed a voltage of 10.2 when the car was off, and it dropped to 8.9 at idle.

At work today the car needed to be jumped to start. To see if it could possibly be an alternator issue I let the car sit idle for about 20 minutes and it died, leading me to believe that the alternator wasn't charging the battery. I'm not sure if this would be due to a faulty alternator or if a bad cell could cause the battery to not charge (I read something along those lines in some other places).

I had the alternator replaced about 4 months ago. I don't know if it's just another bad alternator or if the battery is shot now. Any help would be much appreciated.

Simple alt test: run engine at 1000 RPM. If the voltage does not rise to at least 13.5 V. the alt may be bad. A proper alt test should be run.

After charging overnight, and setting a few hours, battery voltage s/b 12.5+

If the battery is more than 5-6 years old, it may be on the way out. Sounds like there is one dead cell. Have it load-tested.

Originally Posted by edjack

Simple alt test: run engine at 1000 RPM. If the voltage does not rise to at least 13.5 V. the alt may be bad.

After charging overnight, and setting a few hours, battery voltage s/b 12.5+

Thanks a lot, I will try that. I just replaced the battery with a new one. It was at 12.1 volts when the car was off and dipped to 11.2 during idle. The battery light issue was the same, light was on during idle and went off under acceleration.

Sounds like your alternator is bad, and that may have killed your battery, too. It happened to me. The "battery" light going on and off could also be a slipping belt.

Hope that helps

It looks like your alternator is not charging, could be the carbon brushes inside it are worn or a slipping belt.

You said you charged battery overnight, what type of charger did you use. Was it a 2 amp charger or a 10 amp charger? A 2 amp charger will require two days to charge the battery if it was dead.
Was your spare battery fully charged when you installed it?
Your alternator is water cooled and doesn't have carbon brushes in it.
If it was replaced 4 months ago it should be under warranty, take it back and have them check it out after you determine if the battery is good or bad.

simple test... when car is off use a multimeter and check what voltage you got on the battery.

it should be 12v or so

and then turn on your car and test the battery again with voltage meter.. you should be reading 13.5+ Volts or more.. it should not past 14.5 ... the job of the alternator is to charge the battery up .. without under charging it or over charging it...

in my case when I had a bad alternator .. when I started car.. my battery was 18V it was over charging the battery therefore killing it.

hope this helps.

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Originally Posted by JimLev

You said you charged battery overnight, what type of charger did you use. Was it a 2 amp charger or a 10 amp charger? A 2 amp charger will require two days to charge the battery if it was dead.
Was your spare battery fully charged when you installed it?
Your alternator is water cooled and doesn't have carbon brushes in it.
If it was replaced 4 months ago it should be under warranty, take it back and have them check it out after you determine if the battery is good or bad.

Thanks for everybody's help. To answer your questions:

It was a 10 amp charger. The battery was from 2007 so it needed to be replaced at some point anyway. The replacement battery is brand new and fully charged.

The alternator is under warranty still, I'm just trying to see if there could be anything else causing the problems because it's a huge pain getting it out.

I have been reading elsewhere and it has also been suggested here that it could be a belt/tensioner issue. It makes sense to me (which probably means I'm wrong) that it could be an issue there since the light goes off under acceleration and the OBC shows the voltage at 13.5. At cruising speed/idle the light is on and the voltage is around 11.5. Does that seem logical?

Again, I appreciate all the advice from everybody.

It could be any of those things as the OBC voltage output should be constant, irregardless of engine rpm and your seems to be fluctuating with rpm. Slipping belts is fairly rare. I am guessing it might be your voltage regulator as that is the function of the VR. Perform the tests in BlueBee's post to determine what is failing.

And make sure your battery connections are nice and tight.