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Near Total Electrical Failure

Posted: Wed Feb 26, 2020 7:18 am
by JeffD
Yesterday afternoon I noticed the Check Engine light come on when I started on a short trip of about 4 miles. There was about 18 miles of range remaining on the battery and about 65 miles on the Rex, and the Rex hasn't activated since the last maintenance run about two weeks ago, and didn't activate on this trip.

After I parked my car and had been at my destination for about an hour, the car suddenly started blaring the alarm and my key fob wouldn't shut it off. It finally went off on its own.

I went to drive back home and the car wouldn't activate. A couple of interior lights would come on when I opened the door, but neither the shifter lights nor the instrument panel lights would come on. My key fob would not lock or unlock the door, nor would the door lock/unlock button on the door activate the door locks. Nothing worked except for a few small lights, like the glove box light or the door handle lights. I though the key fob may have lost its battery, so I got the second key fob from my wife and it wouldn't activate anything either.

I put the car charger on (after having to pull the emergency charging port door release), but the charging port light didn't come on even though the charger lights indicated a good charging status.

After being on the charger overnight, there is no change in status. Does anyone have any ideas what may be the cause of this situation?


Re: Near Total Electrical Failure

Posted: Wed Feb 26, 2020 10:09 am
by jadnashuanh
Your 12vdc battery may be toast. Normally, it is recharged by the main batteries via a DC-DC converter. But, if it is way gone, when you plug it into an EVSE, there is an emergency 12vdc trickle charger. That may or may not charge the 12vdc battery enough to make things return to normal. THat whole procedure can take many hours, but overnight, if it is the battery, it would have likely worked.

So, you could pull the HV disconnect, remove the 12vdc battery and take it somewhere to be checked, or, if you have a fancier battery charger, put that on the now disconnected battery and see if it can revive your battery. If it's bad, replace it. If it's good, you'll probably have to have it towed to a dealer for them to diagnose your problem.

All of the computers essentially run off of 12vdc, so if that battery is bad, you won't be able to start things up.

Re: Near Total Electrical Failure

Posted: Wed Feb 26, 2020 10:10 am
by vreihen
When in doubt, the 12V aux battery failing (3-4 year typical lifetime) causes all kinds of weird electronic gremlins.....

Re: Near Total Electrical Failure

Posted: Wed Feb 26, 2020 11:04 am
by alohart
You almost certainly need to replace the 12 V battery. If you are able to do this yourself, you'd save a couple hundred dollars over what a BMW dealer would charge. You could buy a replacement battery at a BMW dealer, but you'd pay over $50 more than if you bought the same battery, an AUX18L, from a local East Penn Manufacturing dealer.

Re: Near Total Electrical Failure

Posted: Wed Feb 26, 2020 5:36 pm
by JeffD
Thanks, a 12V system failure does sound like that would result in the behavior I see. I actually am still under BMW Roadside Assist and warranty, so I was able to get it towed to the dealership and they are on it now. I'm assured that everything, except for a possible diagnostic charge, will be covered by my warranty. The intake person at the dealer had never seen the behavior the car exhibits, but I'm hoping it's actually something as simple as the 12V battery.

Forgot o post this earlier. Just got a call from the BMW dealer and it is the 12V batter. My question is why did this fail so catastrophically? The car behaved fine on the way to my 4 mile trip and then was totally dead when I was ready to go two hours later. And the car's main battery wouldn't provide any level of charging. Seems like with all the indicators and sensors, there should be one for this.

Re: Near Total Electrical Failure

Posted: Thu Feb 27, 2020 10:36 am
by eNate
I wouldn't call it catastrophic failure, just a routine failure with catastrophic (yet predictable, known!) results. Crazy, ain't it?

And with all the discussion of the necessity to "register" fresh 12v batteries so the i3's super-intelligent brain can systematically track the performance trends of this oh-so-critical component to the car's life force -- like an overweight pacemaker -- extracting peak performance from it while new yet treating it gently like a fragile eggshell when it's aged, what do we get when it comes time to put it to pasture? An alert from our Connected app? A warning chime and a pop up message on the dash? A call from BMW to schedule service immediately? Nope, we come back to a car that's experiencing a grand mal seizure.

Thanks, BMW.

Re: Near Total Electrical Failure

Posted: Thu Feb 27, 2020 11:55 am
by MKH
Seems like with all the indicators and sensors, there should be one for this.
You'd think. But since the 12 volt battery is what turns on/powers up all the ECU's in the car, ie. all the computers that control everything - when the 12 v
dies, no power to the car's 'brains'. The car can draw power from the HV traction battery, but only if the 12v battery powers up/turns on the DC/DC converter first. :ugeek:

Re: Near Total Electrical Failurer

Posted: Fri Feb 28, 2020 4:14 am
by Bertone
What's the price at the dealer of the 12v battery?

Is it mandatory to reprogram it?
Or is it sufficient just to swap in the new one?

Re: Near Total Electrical Failure

Posted: Fri Feb 28, 2020 10:22 am
by JeffD
Yes, the battery must be registered with the car's ECU, which monitors and regulates current and charging. I don't have the equipment for that, but for the price the dealer is charging, I could buy a tool, like a Foxwell scanner, and still come out better.

I understand that the 12V battery is needed to operate the diagnostics, but I still think they should provide an indicator that tells me that I'd better replace the 12V battery before the thing fails this catastrophically.

And yes, I consider this a catastrophic failure, which is defined as: A catastrophic failure is a sudden and total failure from which recovery is impossible.

If I have to get the thing towed, that's pretty catastrophic. In the end it's a minor component failure and relatively low cost to recover, but not field recoverable.

Re: Near Total Electrical Failure

Posted: Fri Feb 28, 2020 12:09 pm
by jadnashuanh
Around here, PepBoys will register the battery for about $35.