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12v battery died while main battery was fully charged and car plugged in to charger

AaronD

Member
Feb 15, 2020
19
11
Salt Lake City
Drive my 2018 Model S on Jan 2nd. Plugged it back in to charge that same day with main battery down to about 50% and target charge level set to 70%. Went to drive it last night and the 12v battery was dead. Jumped the 12v battery and when the computer powered on it showed the main battery at 70%.

thoughts? When does that 12v battery charge? Why would it ever die when the car is plugged in to an electrical outlet the whole time and the main battery is fully charged?
 

ShawnA

Supporting Member
Nov 13, 2017
925
674
Edwardsburg, MI
Hi @AaronD,

This is the season that batteries fail...
I just replaced the 12v battery in my 2018 Model X from March of 2018...

The cold season aggravates any possible faults or weaknesses in the systems.

They die routinely in a few years...

Shawn
 
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AaronD

Member
Feb 15, 2020
19
11
Salt Lake City
Sure, batteries fail. Not really the point of the question.

but the battery is still working, a day later, after being jumped and presumably charging back up after I drove the car and had it on for a while and charged the main battery again.

the 12v battery can obviously hold enough charge to run the onboard electronics. the issue is the 12v battery was allowed to lose all its charge when the main battery was fully charged and the car was plugged in. The question is, why would the car let that happen? By design? Design flaw? Big? At what events/times should the 12v battery be charging?
 

LoudMusic

Member
Jul 21, 2020
331
430
Arkansas
Tesla uses "old tech" in the 12v battery. I'm sure they have reasons that are documented on the internet, but Shawn is right, the age of your 12v battery and the time of year are pretty hard on that chemistry.

Some people have replaced the OEM battery with a lithium battery. I'm told that's perfectly fine and may go that route when my 12v battery goes out.
 

AaronD

Member
Feb 15, 2020
19
11
Salt Lake City
I understand, winter is hard, batteries die.

Ive had to replace 12v batteries in cars before. Had to replace one on my last Honda Civic in the dead of winter, mainly because it only got charged when the alternator ran and never charged when just sitting there turned off

the difference, and hence the question, is this car, the Model S, has a massive main battery that, my understanding, is supposed to be used to charge the 12v, and that this car was plugged in to a source of electricity the entire time.

so, under those conditions, why would there ever be absolutely 0 energy left in the 12v battery when it’s clear that 12v battery is not entirely done since it has since recharged and stayed usable after being jumped. That’s the question. It is ok to not have the answer.
 

DerbyDave

Member
Jul 2, 2020
609
316
Kentucky
When the 12V battery gets old, sometimes it can't be recharged and can't hold a charge for long even if it recharges, or can't hold a charge at all. My Lexus Hybrid OEM battery finally died after 7 years. While "dead", it ran the headlights, the interior accessories, and everything, but was not good enough to start the car. The 12V battery was charged everytime I drove the car I never had but this one problem with it. It was old. Replaced the battery and everything is fine. Replace your battery, and everything will be fine. It is time.
 
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AaronD

Member
Feb 15, 2020
19
11
Salt Lake City
Seems the actual question is still not clear and perhaps my fault for phrasing the topic the way I did.

What are the conditions under which a charge is applied to the 12V battery on a 2018 Model S and what are the subsequent conditions under which the charge is stopped? That’s all I’m asking.
 

tes-s

Active Member
Oct 6, 2013
2,471
2,690
CT
You may have a DC-DC problem. The voltage of the 12v battery is monitored, and when it is low the contactors engage and the DC-DC is turned on charging the battery. When it gets up to voltage, the DC-DC is turned off.

If the 12v battery is not functioning within spec, you get a warning to replace the 12v battery. You usually have a few weeks before it stops functioning.

The ICE equivalent is the alternator - your battery may be fine, but if the alternator is not working the battery will not charge and will eventually be dead. Not need to replace the battery - just the alternator.
 
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AaronD

Member
Feb 15, 2020
19
11
Salt Lake City
You may have a DC-DC problem. The voltage of the 12v battery is monitored, and when it is low the contactors engage and the DC-DC is turned on charging the battery. When it gets up to voltage, the DC-DC is turned off.

If the 12v battery is not functioning within spec, you get a warning to replace the 12v battery. You usually have a few weeks before it stops functioning.

The ICE equivalent is the alternator - your battery may be fine, but if the alternator is not working the battery will not charge and will eventually be dead. Not need to replace the battery - just the alternator.

Thanks... and ugh... I was hoping for an unfortunate answer like the 12v only charges when the car is on and hence since I left it sitting there for 6 days without waking it up or anything that it was going to just steadily lose charge. But if it’s constantly monitoring even when the car is not woken up for days then yea, sounds a lot like what you’re describing.

I was hoping I’d replace the battery and switch habits to turn the car on for a bit every day or two, but sounds more like it may need a trip to the service center...

anyway, thanks again
 
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Rocky_H

Well-Known Member
Feb 19, 2015
6,108
7,096
Boise, ID
@AaronD yes, sounds like what @tes-s was saying. Sorry people had some trouble understanding you. Yes, the 12V is monitored ALL the time, and there should be no reason (if the car is functioning normally) for the car to let it run out of energy when it's plugged in.

For people who were blowing it off with "batteries just die": No, that has a different symptom. Because the car always keeps trying to monitor and refill the 12V, if that 12V is going bad and can't take a charge, the car keeps trying and trying and trying to refill it, and it just sucks down energy off the main battery endlessly. We've seen that symptom here on the forums before where the car starts losing 20-30 rated miles per day because it's trying to put energy into a 12V that just won't take it--like pouring water into a bucket with a big hole in the bottom.

So I agree that it sounds like the DC to DC conversion system failed.
 
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ucmndd

Well-Known Member
Mar 10, 2016
6,399
11,918
California
I disagree that this is anything other than a failing 12v battery.

After the car sits for a while and sleeps, it does not “constantly monitor the 12v battery”. It wakes up every so often and polls it to determine if it needs a charge.

A failing battery can discharge rapidly and fail to provide the needed current, especially in the cold. I suspect in this case the failing battery lost enough charge/current between the polling intervals that by the time the car noticed, the battery was unable to provide enough current to close the HV contractors, so it was unable to charge and completely died.

Start with the simple solution first. You’re certainly within the battery replacement window... particularly for somewhere cold like SLC. Replace your 12v battery.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
6,108
7,096
Boise, ID
After the car sits for a while and sleeps, it does not “constantly monitor the 12v battery”. It wakes up every so often and polls it to determine if it needs a charge.
I don't think that's true. I'm pretty sure that is one of the things it keeps active all the time, even when the car is "sleeping". People gripe about Teslas having pretty high constant idle vampire drain versus other makers' electric cars, which shut off more completely, but this is one of the things you get from keeping some computer activity on all the time.
 

ucmndd

Well-Known Member
Mar 10, 2016
6,399
11,918
California
I don't think that's true. I'm pretty sure that is one of the things it keeps active all the time, even when the car is "sleeping". People gripe about Teslas having pretty high constant idle vampire drain versus other makers' electric cars, which shut off more completely, but this is one of the things you get from keeping some computer activity on all the time.

Regardless, the idea that it should be difficult/impossible for the 12v to completely die doesn't pass muster with me. If the battery is failing and can't close the contactors to power the DC-DC converter and charge itself, it's gonna die.
 

ShawnA

Supporting Member
Nov 13, 2017
925
674
Edwardsburg, MI
Hi @AaronD,

You started the thread saying that you had a Model S 2018.
To make a more complete determination a few more details are necessary...
Is the car MCU1 or MCU2?
If it is MCU2 was it delivered that way or has it been upgraded?

One other member had a car upgraded to MCU2 but the software
was not compatible to check the 12v battery.
He had a few batteries die before the software problem was identified and corrected...

Shawn
 

TMeister

Gearhead
Oct 6, 2016
271
365
Boise, ID
There are actually two ways a Model S (at least the 100 kWh ones) keeps the 12v battery charged. The one everyone knows about is the "close the contacts", get the big DC-DC converter spun up, and dump whatever 12v is needed for running the car electronics and charging the 12v battery. The other is the small DC-DC converter inside the main pack that provides enough power to keep the battery charged and the car running in sleep or standby. Early cars had to "close the contacts" to send HV to the DC to DC converter to do anything. Those cars watched the 12V bus and did this process periodically. This cycled the 12v battery a lot hence many have found periodic 12v battery replacement a necessity. Newer cars do not need to operate the contactor and so preserve them and the 12v battery a little better.

You could have a failure in the smaller DC-DC converter. Unfortunately, it is inside the main pack but you are clearly under warranty if that has failed. Time for the service call to diagnose.
 

Rocky_H

Well-Known Member
Feb 19, 2015
6,108
7,096
Boise, ID
Regardless, the idea that it should be difficult/impossible for the 12v to completely die doesn't pass muster with me. If the battery is failing and can't close the contactors to power the DC-DC converter and charge itself, it's gonna die.
Except for:
but the battery is still working, a day later, after being jumped and presumably charging back up after I drove the car and had it on for a while and charged the main battery again. The 12v battery can obviously hold enough charge to run the onboard electronics.
If the 12V battery were as totally trashed as @ucmndd is saying and couldn't be charged and plummeted off a cliff and died in a few minutes in between these supposed polling intervals, it would not have been able to be charged externally and hold that charge for the following day. So that doesn't seem to fit the behavior.
 

AaronD

Member
Feb 15, 2020
19
11
Salt Lake City
Thanks for the responses. It's a 2018 100D. The manufacture date was Octoberish I think. I don't know about MCU1 or MCU2. Since jumping the battery last weekend, it's been able to wake up every time since. So the 12v battery is not trashed. All signs point to DC to DC problem.
 

ucmndd

Well-Known Member
Mar 10, 2016
6,399
11,918
California
Except for:

If the 12V battery were as totally trashed as @ucmndd is saying and couldn't be charged and plummeted off a cliff and died in a few minutes in between these supposed polling intervals, it would not have been able to be charged externally and hold that charge for the following day. So that doesn't seem to fit the behavior.

Not necessarily. Lots of variability in how the charging cycles line up, weather, etc. If the car detected the 12v was low early in the morning at the coldest point in the day, it's completely unsurprising that a failing battery wouldn't be able to close the contactors and actually charge. Then it sat for most of a week per OP's story, draining completely until he found it dead.

Charge it back up and it will probably be fine again for a while, right up until it isn't again. This is just how it works.

Anwyay. I've said my piece. Sounds exactly like a failing 12v. Speculating about more extraordinary failures is silly without first investigating the obvious.
 

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