12V Battery Dead

Last Saturday I experienced a completely dead 12V battery in my eGolf SEL 2019. The voltage was down to some 2V! I used a regular 12V charger to re-charge the battery (while it was plugged in, I didn't bother taking it out of the car). Next day the car worked like normal. No error indication. To ensure nothing is wrong, I took the car to its 20,000 miles service, explained the 12V battery issue, and asked to check for possible problems. VW Service determined no problem (they promised to replace the battery under warranty should this happen again). I checked the car using my OBDeleven and found 18 faults, but they were all related to the date/time when the battery went down.

A few things puzzle me:

- I tried to use the car before I had the battery fully charged. I got many error lights on the dashboard and noticed one error message reading: "12V battery low. Charge by driving". Does that mean the 12V battery gets only charged when driving? I understand that you need to run the engine in an ICE car to get the battery charged, but that should not be required in an EV where you have plenty of electric energy available without running the car.

- The OBDeleven tells me that the 12V battery drains some 300mA when the car is turned on, but not driving. Does that mean the 12V battery gets only charged when you actually drive the car?

- I would still like to know why the battery was completely drained (BTW: the HV battery was still at some 70% charged). Could it be that I forgot to turn off the car after I parked it the day before? Or what could cause this?


Regards,
Werner

It is my understanding that 12 v battery is charged when car is on (driving or not driving) and when charging car. Could be error message about driving is left over from ICE Golf. No alternator in e-Golf so DC to DC converter can work anytime car is turned on and HV battery is providing power.

Car seems to be pretty aggressive about turning off systems when you walk away and even when sitting in car for a while. Did you leave the key in the car and walk away with lights on? Maybe you have a defective 12 V battery or short somewhere?

I just bought a 2019 SEL and experienced the same thing. Dead Battery

I had the car delivered to me in Florida from Connecticut, and the dealer didn't fully charge the battery before shipping.

I received the car on a Thursday night, drove it for less than a mile and then left for the weekend. On Monday it was dead. I had to use the physical key to get into the car.

I jump started the car to be able to plug it in. I saw a video on YouTube that showed that the 12v battery charges when plugged in, but you can't start the charging process if the 12v battery is dead. I bought a portable Jump starter to keep in the car so I don't get stuck somewhere - and I keep it in the back seat, because you aren't going to open the boot if the battery is dead.

My concern is will that deep discharge damage the 12v battery? It's an AGM battery, but could it still be damaged by the deep discharge?

Does this mean that leaving the car sitting for period of time means a dead 12v battery?

I think deep discharge is bad for any rechargeable battery. I do not think you need to worry if you drive and charge your car regularly during the week and the DC DC charging system is working correctly. I have left my car sitting unplugged and locked for weeks and had no issue.

Maybe you also have a defective 12V battery?

f1geek, thanks for your reply.

I did test the battery and it tested okay. I may well have a defective battery, but more likely it just wasn't charged properly before it was shipped to me.

It's a 'new' car that was on the dealers lot for almost 2 years, but the good news is that with cars, the warranty starts at date of purchase, so I expect that if the 12v battery is bad or been damaged by deep discharges, I can get a warranty replacement.

I have my jump starter in the car and have fingers crossed. Time will tell.

Thanks again