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Ran out of power… could it be the 12V battery?

BrettS

Active Member
Mar 28, 2017
2,110
2,513
Orlando, FL
So I just had an interesting experience in my 2015 Model S. I was on my way to a supercharger and I knew I was cutting it a little close… it showed that I would reach the supercharger with 1% remaining. I have pulled up at superchargers with only a couple of percent remaining before, and even 0% remaining once, so I wasn’t too worried. However, this time, a little less than a mile from the supercharger while the car still showed 1% remaining, it started beeping and telling me that it was out of power and going to shut down. As I was pulling over the instrument cluster screen shut off. Shortly after that the MCU shut off and at that point the car was completely dead. I know that the remaining battery power is just an estimate and given that it was at 1% I figured that while it was inconvenient it was likely that the BMS system had just miscalculated by a bit and I wasn’t even going to post about it.

However, what really confused me was that $130 later, after a short ride on a flatbed and a 12V jump the car came back to life and still reported 1% remaining. And surprisingly (to me, at least), it also allowed me to put it into gear and I was able to back into the supercharger stall under the car’s own power. I‘ve seen other posts that suggest that the car stops charging the 12V battery when the main battery is very low and I‘m wondering that if it’s possible that it wasn’t the main battery pack that ran out of power and shut off the car, but the 12V battery instead.

This is a March 2015 model S that I got used from a private party in March of 2017. I know that I have never changed the 12V battery and I don’t know for sure if the original owner ever did, but at best the 12V battery is 4+ years old and at worst it’s the original 6+ year old battery. On a possibly related note, I have also noticed that lately vampire power drain seems to be increasing with the car losing as much as 7 or 8% a day. I’m wondering if this could be a symptom of the 12V battery not holding a charge and the car needing to use the main battery pack to charge it much more than normal.

I haven’t seen any alerts or warnings about the 12V battery, so maybe I‘m just reading too much into this, but do you think it’s possible that the 12V battery is on it‘s way out?
 

sorka

Well-Known Member
Feb 28, 2015
8,058
5,876
Merced, CA
Your 12v battery probably needs replacing but your car would have still turned off the main battery at the same point. The battery also likely recovered just enough to allow you to move the car after sitting for a while with no load.
 

Lasttoy

Active Member
Mar 24, 2017
1,595
850
St Augustine, Fl
I was on 95 southbound, in NC. All of a sudden the car shutoff going 75. No warning before the emergency flash on screen. After 8 hours a female at Draper told me the 12v battery was only at 12v. It has to be at 14v. """U need a super jump of at least 14 volts""". Tow truck came back. Jumped me with his big truck, 14.5 volts. In about 15 minutes my car lit up. He stayed till my battery was at 14.5 volts. I super charged and came home to Florida. They were never saying 12v battery was bad. Six months later it wouldn't charge. No message. I called Draper. He said 12v battery was bad. I changed it. Whipped no more problem. I had purchased it the day I got home. Oh changing it was fun. Took longer to figure out how to lift it out that anything else. But it is easy. Follow you tube videos. Roll down windows. Leave doors open . I forgot, mine looks original, very old.
 

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