I took the alternator to a place which re -cons them, they checked every thing out and said the regulator was faulty. I have run carsoft and my bmw scanner and have no fault codes.I cleaned the wires which go to the alternator aswell.

OK i just got back from the dealer... took it for a test drive and the light came on a few times. While driving, the technician entered the hidden diagnostic menus (by pressing and holding the right button that produces "CONTROL CHECK OK"), and he went into one setting that monitors the voltage. The voltage was steady between 12.8 and 13.1 which is normal, and even when the battery light came on, the voltage held steady. So the battery is OK even after 5 days since the problem started. I need to take it back for a more thorough check of the alternator. But on the 45 minute drive home, the light didn't come on at all, and the only difference was that I had the headlight switch on 0 instead of A (i always had it on A)... coincidence? Strange.

Comments?

Hi i dont think that a voltage of 12.8 to 13.1 is normal the float voltage for a lead acid battery should be at least 13.4 your voltage regulator is not correct to me.If you add more loads ie. foglights high beam etc.what does your charging voltage read?

12.8 is normal UNLESS the engine is running. alternator should produce 13.x - low 14V.

Well, it was between 12.8 and 13.1 most of the time, and when I turned on the xenon headlights, the voltage stayed at the same levels. The service advisor told me that the normal range is between 12 and 14. It shouldn't be under or over.

That does seem low to me... with your 170 amp alternator you should load test it with a carbon pile tester (puts physical load on the system, test in live conditions) If the local dealer only has the new "smart" electronic impedance testers it may be giving inaccurate readings by not putting a real load on the battery. I'd go to a shop with the old school VAT 40 load tester and have them test the system. Load test the battery at the battery posts at 1/2 the cca reading for 15 seconds... the voltage shouldn't drop below about 10.2 volts. Load test the alternator from the jumper points under the hood at 170 to 180 amps with the engine running at about 2000 rpms and you should get at least 180 amps. I've had cars in the past that tested great when cold, but after driving it for an hour the alternator wasn't charging for shit... could get weaker as you drive as well...

Im having the same issue now with my X. Battery light flashes briefly for a period of 5 minutes or so. Doesnt stay lit. No trouble starting. No dim dash lights or anything.

Just had my alternator replaced about two months ago. When I realized it was dying (all dash lights came on) and I was trying to make it to the shop, the battery died all the way when i stopped for gas and shut off my X like a dummy (thought i'd be saving battery energy). I had to get a jump in order to drive the final two blocks to the garage.

Did this "kill" the battery? Like I said, it still starts like normal, no extra straining or anything.

I went to RS Strauss and had the battery tested. It was 10.78 volts with the car off and 13.8v with the car running. From what I'm reading, I need a new battery. Right?

Battery Voltage
 

If the car is not running, the battery should read a minimum of 12.6. Otherwise, it means that one of the cells is dead. There are 6 cells, minimum voltage should 2.1V per cell * 6 = 12.6V.

If the car is running, you'll see voltage from the alternator, which should be putting out a healthy 13.7ish Volts. That just means your alternator is working fine. It doesn't mean the battery is still good.

If the battery won't accept the higher voltage, then one or more of the cells is bad and it's time to get a new battery.

Match group for your new battery, or alternatively, match Cold Cranking Amps (CCA), but make sure your battery hold down system will fit properly, and don't cheat and not hook up the battery fluid overflow hose.

Thanks for the wealth of information and :welcome: to Xoutpost!!
I didnt know what you meant by matching the group and I'm not sure what the CCA's should be although I'm sure they can tell me in the store.

I promise to hook the hose up and ensure proper fitment that it will be held down correctly.