12V Battery drained

[MX2017-75]

One morning about two weeks ago, I found my car completely dead. I found out that the charge level on the 12v battery was about 4volts, so I figured that the battery was shot. Seemingly reasonable at 3 years and 100.000km. The battery was replaced with a new one, and the car worked normally again, so far so good.
But, only 5 days later, the car was dead once again. The new, now flat, battery also measured in at roughly 4V. I now got hold of a proper battery charger, and recharged both the new and old battery, and both of them charged right back up to full voltage - both of them seems to be in good working order.
I thought that maybe the onboard 12v charger has gone bad, so I checked the voltage with the car running, and it seems to be at 14.9v - which I guess is normal.

Today I found out that our (missing) spare key fob has been lying in the center console the whole time - could this have caused the 12v to drain completely? The car was plugged in to the wall charger both times it died, if that matters.

 

[Socalcommuter]

Not sure about the key fob and whether it caused the dead 12v but I did find a similar thing that your main battery doesn’t do a great job of charging your 12v. I had my main battery near full and still my 12v died.

 

[mbp11]

This is so weird. I think you need to visit the Service Center again. Where are you located in Europe?

 

[DCGOO]

One morning about two weeks ago, I found my car completely dead. I found out that the charge level on the 12v battery was about 4volts, so I figured that the battery was shot. Seemingly reasonable at 3 years and 100.000km. The battery was replaced with a new one, and the car worked normally again, so far so good.
But, only 5 days later, the car was dead once again. The new, now flat, battery also measured in at roughly 4V. I now got hold of a proper battery charger, and recharged both the new and old battery, and both of them charged right back up to full voltage - both of them seems to be in good working order.
I thought that maybe the onboard 12v charger has gone bad, so I checked the voltage with the car running, and it seems to be at 14.9v - which I guess is normal.

Today I found out that our (missing) spare key fob has been lying in the center console the whole time - could this have caused the 12v to drain completely? The car was plugged in to the wall charger both times it died, if that matters.

If the measured 12V is actually 14.9, that is far too high. You have a problem that needs to be fixed immediately. Normal voltage from a 12V battery, not under charge is 12.6. Under charge it should be 13.4 to 13.6, maybe as high as 13.8. Prolonged operation at 14.9, will kill it in short order.

 

[Camelroel]

If the measured 12V is actually 14.9, that is far too high. You have a problem that needs to be fixed immediately. Normal voltage from a 12V battery, not under charge is 12.6. Under charge it should be 13.4 to 13.6, maybe as high as 13.8. Prolonged operation at 14.9, will kill it in short order.

How to Test Your Auto Battery | Battery Testing & Maintenance | Autobatteries.com.

14.9V is a bit high but 14.7V in a normal car is OK. I don't know where tesla sets the limit.

 

[brkaus]

Our model 3 goes to 14.77 or slightly higher for very short times.

 

[DCGOO]

Our model 3 goes to 14.77 or slightly higher for very short times.
View attachment 638551

Correct. Sustained operation at 14.77 will cook any lead acid (including AGM) battery probably within a couple of months. The charger in my RV normally floats at 13.2, going up to 14.5 for 15 minutes every 20 hours. I don't check it often, but I have always found my X to be roughly 13.6. Occasional trips north of 14.5 are fine, just as long as it doesn't stay there too long.