E46 M3 Losing All Electrical Power / No Start

[Marc E46M3]

Update from today:

I left the battery connected to my Type S smart charger/maintainer while I was out of town for a few days. It was set to 12V/AGM/5A and had plenty of time to fully charge.

Got back today, disconnected the charger and let the battery sit for about 30 minutes before testing.

Battery with engine off: 12.56V

Started the car and let it idle for about a minute with everything off.

Idle, no accessories: 14.10V

Then turned on the headlights, HVAC blower on max and rear defroster, with the stereo off. Waited about a minute and tested again.

Idle under full accessory load: 13.84V

So this was a much different result from the previous test when the battery had started around 12.1–12.2V and voltage eventually dropped into the 12.5–12.7V range under load.

Looks like the advice to fully charge the battery before testing the alternator was spot on. With a properly charged battery, the alternator seems to be charging normally and handling the load just fine.

At this point I don't think there's enough evidence to justify replacing the alternator.

Still need to figure out what caused the original incident where the car shut off and went completely electrically dead until it was jumped. Next step will probably be a proper battery load/CCA test, checking the main power and ground connections/voltage drop, and possibly checking for a parasitic draw.

For now I put it back on the maintainer.

Thanks again for all the advice. Definitely helped narrow this down.

[sapote]

At this point I don't think there's enough evidence to justify replacing the alternator.

Still need to figure out what caused the original incident where the car shut off and went completely electrically dead until it was jumped.

Alt is fine. But what is the mileage on it or the car? These alt will fail to charge any moment from 165K miles which needs to have the carbon brushes replaced.

On the original incident, it could be just a driving with a badly discharged batt which caused crazy things can happen with the electronics modules.

[sapote]

I had no idea there was a hidden dash display lol, and I have yet to try it. I don't drive the car much - have put on roughly 5K miles over the past 5 years. During this period, I have noticed the DSC and brake lights come on and off intermittently. I suspect that it's related to the recent issue, but I'm not certain.

The batt needs to be measured and charge up periodically. I would hook up the 5A charger overnight every month if batt cable disconnected, or every 2 weeks if the cables connected.

[Marc E46M3]

The batt needs to be measured and charge up periodically. I would hook up the 5A charger overnight every month if batt cable disconnected, or every 2 weeks if the cables connected.

Is it safe for the battery if I keep it connected to the trickler?

[Marc E46M3]

Alt is fine. But what is the mileage on it or the car? These alt will fail to charge any moment from 165K miles which needs to have the carbon brushes replaced.

On the original incident, it could be just a driving with a badly discharged batt which caused crazy things can happen with the electronics modules.

The car has 37K on it - not much.

[sapote]

Is it safe for the battery if I keep it connected to the trickler?

for during the time car is parked, or to charge up the batt to full charged? It cannot charge up a discharge batt.

[Marc E46M3]

for during the time car is parked, or to charge up the batt to full charged? It cannot charge up a discharge batt.

Only when the car is parked and not in use.

[sapote]

Only when the car is parked and not in use.

that's its job to keep batt full charge w/o drying up the acid.

[Marc E46M3]

that's its job to keep batt full charge w/o drying up the acid.

Makes sense. I just wasn't sure if it would damage the battery long-term or not. Thanks for all your help!