The Prince of Darkness (Battery Light)

I've had my E39 one month. 2000 528i with 146k on the clock.

On the second day I had the car, I had to replace the battery. Since then it has run well and started easily every time.

Today after work I started it, and the battery light stayed on. I thought it was a glitch but after a restart, the light remained on. I checked the battery voltage with a voltmeter and it was 11.7 V with the engine off and with the engine running voltage was the same.

I'm assuming this means the battery is not being charged. I left the car parked overnight where I work. It would still start but I knew it should not be driven. Tomorrow I will return with a battery charger.

I'm hoping to charge the battery up to about 13.2 V and then drive it to my independent mechanic (a former BMW dealer tech, located less than 2 miles away), and have it diagnosed/repaired.

I'm assuming it's the alternator, but could there be another (and hopefully less costly) explanation?

I know the Prince of Darkness for old British sports cars is called "Mr. Lucas" but what about for Bavarian cars?

Any advice would be greatly appreciated.

If the alternator is only putting out 11.7 and the car has 146k I would definitely suspect the alternator.

The battery voltage was 11.7 with the engine off AND with the engine on. So it's possible that the alternator was putting out NO current at all. I'll be sorting that out today.

I've noticed that voltmeters have been deleted as an instrument from all modern cars. I have always thought that a voltmeter was one of the most important instruments to have. I'll be looking for a way to add one to the car. I could have noticed this problem before the "idiot light" (which to it's credit functioned as it should) lit up.

It sounds like you need a new alternator based on your testing.

You can buy a charging system tester from Radio Shack with LED lights and cut off the test leads. Replace the leads with a cigarette plug and you'll get a transportable system tester. The nicer but permanent option is a hidden gauge in the glove box.

You should make it to the mechanic fine with a full charge. Just don't run any electrical accessories unless needed.

Prinz der Schwärzung (Prince of Darkness Part II)

The problem as I had described it above turned out to be a worn alternator. It was only putting out about 11.5V charge. I had it replaced it with a new Bosch 120A alternator.

That was Thursday. Friday all is OK. Over the weekend I did not drive the car, except to move it from one parking space to another on Sunday. When I started it Sunday, I noticed that it seemed to crank at a lower RPM. I checked the battery voltage and it was above 12V.

Monday morning: Battery voltage is just above 7V. Car will not start. Had to wait 30 minutes for the guy from AAA with a "Booster Pack" (they seem to work quite well, may have to pick one up as this is the second time in a month I've needed a boost).

Today I replaced the battery with a new one under warranty (100 Ah / 860 CCA battery was one month old, but ran for that month with the weak alternator). I also charged the battery at work with a Schumacher battery charge with LED readout. At the end of the charge it was above 13V and read 95% charged.

So now... the moment of truth - tomorrow morning. My car will have been sitting outside at night and temps will hit a low of about 47ºF.

I tried checking the battery drain with a multi-tester in DC Amp mode (pass-through), between the negative battery terminal and the negative battery strap (in series with the battery). I got a reading of 12 mV, which obviously would not drain a 100 Ah battery overnight.

As a precaution, I pulled most of the accessory fuses in the trunk. Leaving just the fuel pump and alarm connected. I unintentionally pulled the central locking fuse, but I'll put that back tomorrow in the daylight. (BTW, why can't the Germans make a nice fuse diagram instead of just a list and you have to count the fuse positions)?

If anyone has any ideas or experience with this, I would really appreciate hearing from you. As enjoyable as my E39 is to drive, I also need it to start up every time.

Forgot...

I suppose I could pull the negative terminal on the battery, but then how do I get the trunk open in the morning with no power to the locking system?

Couldn't think of a solution for that one, so I left the battery connected.

i locked my trunk with the battery disconnected today after that flaming LKM incident and only way i was able to open it was to attach my booster pack to the + block terminal on the engine and popped the trunk.

That's great to know about the Booster Pak.

In the end, it looks like I had a bad battery due to a weak alternator. I changed the dead battery and the weak alternator killed the new battery in 30 days. Fortunately it was still under warranty. Installed a new alternator and all is well so far. But I still want to add a voltage meter to my car somewhere so I can spot the problems early.

Wow, Jorge; you haven't had very good luck with the "new" E39........

Keeping that battery charged

On the E39- and most of the cars since the late 90's there is always a small drain on the battery. On the E39 it should be less than 40ma when the car is asleep (16 minutes after last contact and the trunk lights have gone off.

If you are not using your car on a regular basis (driven at least every few days for 1/2 hour or more then you will have to monitor the battery voltage and apply a charge as appropriate.

If a battery gets a significant discharge it can be damaged, and will certainly require a longer charge> I put my charger (automatic voltage and current control unit) on at least once a month for about 12-24 hours.
It should charge the battery to the same voltage limit as does the car (14.3)

Cheers
Jim Cash

Alternator just broke on mine last week. I thought I had enough battery charge to make it home, but I was wrong! Left stranded 4 miles from the nearest exit and no cell reception.

I'm going to be storing my car for 18 days while I am out of the country. Are there any precautions I should take? I'd disconnect the battery, but then there would be no way to open the trunk to connect it again. My car will be parked in the underground parking garage of my building.

Also, I will be in Germany for two weeks. Any suggestions on where to rent an Autobahn worthy car? I will only need the car on weekends.

BTW, no more electrical problems since I changed the Final Stage Resistor. It's been 5 months now. Knock on wood/steel/aluminum.

Jorge, your key will still work in the trunk lock regardless of the battery. If you have a good car cover, I'd use it. The place tends to get really dusty. I'd also make sure the gas tank is topped off and toss some mothballs under the hood/around the car to keep the rodents away.