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Any chance we can get a sticky thread for the 12V battery?

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M

maximizese

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Jan 16, 2018
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California
#1
I've got a 11/13 built S85 with the MCU2 upgrade and have finally had the "12V battery low" message pop up after 112K miles. Tesla has preemptively replaced the 12V battery twice before, one when the HV contactors were replaced under warranty and the other time rear window defroster was fixed under warranty. I plugged in my Anker battery monitor into the 12V socket over the last week and saw the voltage fluctuate between 12.7-13.4V. I sent in a request via mobile app for Tesla to remotely reset the battery warning because I'm not convinced it needs to be replaced. Instead of doing that, Tesla set up mobile service to replace the battery for ~$265 ($159 for the battery, $89 labor, plus tax). I've searched threads to learn what I could about the 12V battery and thought instead of having threads about the 12V pop-up every now and again that perhaps there should a be a sticky with how the Tesla Model S uses the 12V battery, how it charges, and how to prolong the life. Personally, I'm surprised Tesla hasn't engineered their own auxiliary battery system that to be better than the traditional system.

BTW, does anyone know the threshold voltage that the Tesla needs to power and start the car? I believe 10.8V was necessary for both my old 2007 Lexus Rx400h and our 2013 Fiat 500e. Our S typically sits for days on end in our garage and with the MCU2 and newer software I've noticed that the pumps run and the contactors audibly open and close at least a few times a day. In hindsight, I regret not placing a 2A trickle charger on the battery posts (behind the nose cone) as that could have prolonged the life of the 12V...it's about 2.4 yrs old at this point. We've gotten more than 6 yrs out of the tiny battery in the 500e, but that car does not have all the connectivity and vampire drain that the Tesla come to know.
 
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#2
the 12v battery isn't like a normal car battery it is a deep cycle battery and has a different point at which it fails. if you get the replace 12v warning just go buy the battery from tesla and replace it in your driveway its not rocket surgery its a 10-20 min job.
Pull out the frunk liner (2 bolts and 2 nuts) then the 12v is right there in plain view. Unless you have the old old location in which its by the passenger wheel well in which case its like an extra 5-10 min job but again still super easy.

You can go to fixyourtesla.com to see the instructions on how to do it but since that site has been taken down you need to use the wayback machine and find one from early last year. its blocked on my work computer after i post this i will edit it with a link from my phone.


 
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#3
I got tired of seeing the low 12V battery warning when I'm consistently getting 12.8V in the morning so I did the ole hard reboot by changing the wheel configuration under the "Service" menu on the MCU. Annoying message gone but I'll keep an eye on the battery health. I'd love to know the minimum voltage needed to start the car. Upon researching the 12V battery, it seems like Tesla has had issues with them dating back to 2014 and I'm not sure if they ever came up with a remedy.
 
#4
I've got a 2017 MS 75D and have had the 12v battery warning on for over a year now. There's nothing wrong with it at all, I monitor the voltage on a monthly basis and it's always over 12.4v.

I occassionally do the 'wheel configuration' reset and this clears the warning for a while (maybe a couple of days) but it always comes back. I asked a mobile technician why it keeps coming back and he said the battery has some kind of 'trip switch' in it, which means if it ever shows a low voltage or anything like that, then it's ALWAYS going to error and the only thing to do is replace it. Very poor from Tesla,
 
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#5
Just left this on a different thread:

Ohmmu 12v battery is excellent choice. Installed this myself in a 2016 Model S 90D. Works so well! No error messages and afterwards I happen to notice that the lights on the car we’re slightly more responsive (activation of headlights, turn-signals). It is so much lighter, and going strong after three years.
 
#10
The reason Tesla can warn you of the 12v battery's expected demise is they have some additional electronics to monitor the battery's health. It typically provides about 3-4 weeks of warning. Honda uses the same system in their ICE cars, but few other manufacturers do as it adds cost. When you reset the car, it resets the monitor, but it will pop up again fairly quickly. Don't ignore it, but it's also not super urgent.

If you do ignore it within 4 weeks, in almost all cases, the battery will fail and you'll be stranded. It will not fail while driving, as the DC-DC inverter will keep things powered while driving. This is similar to how an alternator keeps an ICE car powered while driving when the 12v battery dies.
 
#11
vcor said:
The reason Tesla can warn you of the 12v battery's expected demise is they have some additional electronics to monitor the battery's health. It typically provides about 3-4 weeks of warning. Honda uses the same system in their ICE cars, but few other manufacturers do as it adds cost. When you reset the car, it resets the monitor, but it will pop up again fairly quickly. Don't ignore it, but it's also not super urgent.

If you do ignore it within 4 weeks, in almost all cases, the battery will fail and you'll be stranded. It will not fail while driving, as the DC-DC inverter will keep things powered while driving. This is similar to how an alternator keeps an ICE car powered while driving when the 12v battery dies.
Click to expand...
I'm always curious to know how things work, but this one makes me even more curious. 7.5 months/8K miles later, I'm still getting the warning but my 12V battery still fluctuates between 12.7V-13.7V. I'll replace it at some point but I'm not sure why it's warning and if the battery even needs to be changed. I've got over 9 years on the original Mopar 12V battery in our 2013 Fiat 500e. That car also has a 12V warning indicator that I've yet to see triggered; so far that one registered between 12.7-13.4V

I'm just wondering if my Tesla is giving a false warning and it seems the only way I know how to verity is to drive it until the 12V battery drops below 12V.
 
#12
Here's more info on one of the types of battery sensors Tesla uses: Intelligent battery sensor (IBS) | HELLA It's not just voltage that is monitored.

Keep in mind the Tesla 12V battery is charged/discharged about 4 times a day. This is far more than a typical ICE battery and is the reason they use an AGM battery (prior to 2021). So an EV's 12v battery is not an apples-to-apples comparison to an ICE 12v battery. Quite a different application and use case even though both are cars.

Here's a lot more about the 12v battery I wrote earlier this year: 12 Volt Battery Compendium – TeslaTap
 
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#13
vcor said:
Here's more info on one of the types of battery sensors Tesla uses: Intelligent battery sensor (IBS) | HELLA It's not just voltage that is monitored.

Keep in mind the Tesla 12V battery is charged/discharged about 4 times a day. This is far more than a typical ICE battery and is the reason they use an AGM battery (prior to 2021). So an EV's 12v battery is not an apples-to-apples comparison to an ICE 12v battery. Quite a different application and use case even though both are cars.

Here's a lot more about the 12v battery I wrote earlier this year: 12 Volt Battery Compendium – TeslaTap
Click to expand...
The Fiat 500e also has a battery sensor...always wondered what that was. I plan to take the frunk tub out and inspect the battery grounds for corrosion while I inspect the HVAC drain hose as well. Any way to test the battery sensor itself other than swapping a known properly functioning sensor in and checking for different results?
 
#17
One important info regarding 12V battery in Tesla - it's voltage has NOTHING to do with it's health. It's right that Tesla cycles it like 4-5x a day, therefore there's used deep cycle type. Another thing is, those cycles are made in approx 100 - 50% SOC, not to zero to ensure all systems will work. If car would let go the voltage down to 0%, all systems might die.
The warning message is more likely connected with hou much capacity does the battery have between those 100-50% or so SOC and also maybe with it's internal resistance I believe. And this is countinuously monitored. So monitoring the voltage will tell you nothing, just you can monitor the charging...
 
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#18
ResHacker said:
One important info regarding 12V battery in Tesla - it's voltage has NOTHING to do with it's health. It's right that Tesla cycles it like 4-5x a day, therefore there's used deep cycle type. Another thing is, those cycles are made in approx 100 - 50% SOC, not to zero to ensure all systems will work. If car would let go the voltage down to 0%, all systems might die.
The warning message is more likely connected with hou much capacity does the battery have between those 100-50% or so SOC and also maybe with it's internal resistance I believe. And this is countinuously monitored. So monitoring the voltage will tell you nothing, just you can monitor the charging...
Click to expand...

After the low voltage 12v batery message appears, how long do you have till the 12v battery dies?
 
#19
Droschke said:
After the low voltage 12v batery message appears, how long do you have till the 12v battery dies?
Click to expand...
Mine survived approx 3years. Yes, if the voltage drops, it will also warn you, but that will be most likely problem of DC/DC converter, not the battery itself. When my battery died, the voltage was absolutely normal but it had only a fraction of the residual capacity and also very high internal resistance.
 
#20
ResHacker said:
Mine survived approx 3years. Yes, if the voltage drops, it will also warn you, but that will be most likely problem of DC/DC converter, not the battery itself. When my battery died, the voltage was absolutely normal but it had only a fraction of the residual capacity and also very high internal resistance.
Click to expand...

Were you driving your car for 3 years with the message on? Or, your 12v lasted 3 years before the messages popped up?

Let me rephrase my original question:

I'm wondering after the 12v low voltage warning message appears on the car's screens, how long do we have before the 12v battery dies?
 
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