# the 12V battery and why/how when it dies



## Jay79 (Aug 18, 2018)

garsh said:


> Sounds like a dead 12v battery.


Any thoughts on why the 12v dies so early? I've heard this on far too many other peoples cars and one would think with it being managed by the BMS it should last much longer than a traditional car would.


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## Nom (Oct 30, 2018)

And to add another q - shouldn’t there be a warning telling you that you need to replace it soon? Is there no way to tell it is failing?


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## garsh (Apr 4, 2016)

Jay79 said:


> Any thoughts on why the 12v dies so early?


I don't really know.


Nom said:


> And to add another q - shouldn't there be a warning telling you that you need to replace it soon? Is there no way to tell it is failing?


Well, when the car is "off", the HV battery is disconnected, and all the computers run off the 12v. So if those computers aren't getting adequate power, all kinds of weird things can start happening. I would think that at some point Tesla would update the software to recommend checking the 12v battery when some of these common symptoms occur, but I guess they'd rather you contact service to diagnose the problem.


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## Nom (Oct 30, 2018)

@garsh - just seems like Tesla can have some quality check that would alert them and you that the 12V needs some TLC. Just like any other QC they do on the rest of the car. Having it fail as a surprise is ... well ... a surprising outcome given how well Tesla does in monitoring the car.

Perhaps you are saying the same thing. But I wouldn't think you would need funky things happening in other systems to be the symptom that trips the flag. Monitoring of the actual battery, which shows a problem, should be what trips the flag.

Perhaps monitoring the 12V battery itself is hard?


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## garsh (Apr 4, 2016)

Nom said:


> Perhaps monitoring the 12V battery itself is hard?


Tesla doesn't like adding additional equipment when the existing hardware can do the job adequately. Examples:

Using the drive units as heaters for the battery instead of adding a heater.
Using the cameras to control the automatic wipers instead of adding a rain sensor.
Using the cameras to control the automatic high beams instead of adding a light sensor.
Adding unnecessary hardware increases parts costs, production costs, and production time.


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## Kizzy (Jul 25, 2016)

garsh said:


> Tesla doesn't like adding additional equipment when the existing hardware can do the job adequately. Examples:
> 
> Using the drive units as heaters for the battery instead of adding a heater.
> Using the cameras to control the automatic wipers instead of adding a rain sensor.
> ...


The car needs to know when to recharge the 12v battery. Surely there's something testing it. And with the way it logs data, it must have a method of tracking.



Nom said:


> And to add another q - shouldn't there be a warning telling you that you need to replace it soon? Is there no way to tell it is failing?


There's a warning (some folks get it). But often it's very little notice. I'm left to wonder if the nature of the battery use just makes it difficult to detect an issue well ahead of time.

Thinking about recent iPhone battery issues. For iPhone 6/6S, in cold temperatures with older batteries, the phone would often shut off during sudden processor loads. Apple's solution was to throttle performance once a battery reaches a certain number of charge cycles to keep this from happening.

This reminds me about the behavior of 12v batteries. I recall watching a video on the measurement of their voltage. It can drop under load. So, while it might seem to be fine with the computer running from the battery, perhaps when it comes time to engage the contactors, the voltage - which was fine just before - is suddenly too low when trying to engage the contactor. This is just speculation and pondering on this issue.


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## garsh (Apr 4, 2016)

Kizzy said:


> The car needs to know when to recharge the 12v battery. Surely there's something testing it.


Yes, but probably only while the car is active. My guess is that even then, it's only looking for a slightly low voltage on the battery, which is what happens when the battery dies due to old age & too many charging cycles. If the battery fails for some other reason, they probably don't have a good way to detect that, other than various systems going haywire.

I found this informative article that discusses Tesla's lead acid battery.
https://teslatap.com/articles/12-volt-battery-compendium/


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## Ed Woodrick (May 26, 2018)

Jay79 said:


> Any thoughts on why the 12v dies so early? I've heard this on far too many other peoples cars and one would think with it being managed by the BMS it should last much longer than a traditional car would.


Look at the car for which this was reported. Did it happen to be an X? If so, there's a known issue with the system marking these batteries bad a little too early. Problem is when they are marked bad, the car just doesn't work.

In reality, the 12V battery on a Tesla (not EVs in general) works a lot harder, as the computers, especially with Sentry mode turned on, draw a lot of current from the battery and even when fully asleep are often more than most other vehicles. In today's ICE, starting the car is a really easy thing, the started and engines are significantly more efficient than 30 years ago. You could probably even hand crank today's engines.


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## Jim H (Feb 11, 2017)

Jay79 said:


> Any thoughts on why the 12v dies so early? I've heard this on far too many other peoples cars and one would think with it being managed by the BMS it should last much longer than a traditional car would.


The 12v battery charge rate was a concern for me when I posted my testing on this almost a year ago. I had a reading of 14.98v at the 12v when the M3 was charging. I know in the past that a charge rate that high will kill a battery early. I know with lead acid batteries that are filable, that a high charge uses up the solution in the battery, and exposes the cells. That will lead to death of the batteries. Not sure if this happens in a sealed battery like we have in the M3, but I suspect it is. This topic was debated and I contacted the SC to inquire about this, and most felt this high charge rate was not a problem. 
My 12v is still good after 15k, but if I could reduce the charge rate for 12v battery to a more common rate, I would.


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## MelindaV (Apr 2, 2016)

Jim H said:


> The 12v battery charge rate was a concern for me when I posted my testing on this almost a year ago. I had a reading of 14.98v at the 12v when the M3 was charging. I know in the past that a charge rate that high will kill a battery early. I know with lead acid batteries that are filable, that a high charge uses up the solution in the battery, and exposes the cells. That will lead to death of the batteries. Not sure if this happens in a sealed battery like we have in the M3, but I suspect it is. This topic was debated and I contacted the SC to inquire about this, and most felt this high charge rate was not a problem.
> My 12v is still good after 15k, but if I could reduce the charge rate for 12v battery to a more common rate, I would.


not sure about the 12v in the 3, but coming from having Optima batteries (also sealed), their recommended recharge is 10 amps, 13.8-15.0 volts - so the numbers you listed are right in that range (not knowing the amps).


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## garsh (Apr 4, 2016)

MelindaV said:


> not sure about the 12v in the 3, but coming from having Optima batteries (also sealed), their recommended recharge is 10 amps, 13.8-15.0 volts - so the numbers you listed are right in that range (not knowing the amps).


Found a reference for that.
They even say to go up to 15.6v for rapid charging.

https://www.optimabatteries.com/en-us/support/charging/charging-tips

OPTIMA YELLOWTOP® CHARGING INFORMATION
Recommended charging information:

Alternator:
13.65 to 15.0 volts, no amperage limit.

Battery Charger:
13.8 to 15.0 volts, 10 amps maximum, approximately for six to twelve hours.

Cyclic Applications:
14.7 volts, no current limit as long as battery temperature remains below 125°F (51.7°C). When current falls below one amp, finish with two-amp constant current for one hour.

Rapid Recharge:
Maximum voltage 15.6 volts (regulated), no current limit as long as battery temperature remains below 125°F (51.7°C). Charge until current drops below one amp.

Float Charge:
13.2 to 13.8 volts, one amp maximum current, time indefinite (at lower voltage).
Strictly adhere to all limits.


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## msjulie (Feb 6, 2018)

@garsh Both the Optimal site and it's "fallback" site failed to believe they had the info to suggest a battery for my 3 ... I was just curious but that's a little funny too


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## chiapet15 (Nov 15, 2018)

Are the 12V lead-acid batteries starting to fail en masse? Would switching to a lithium 12V battery be the long-term solution?

For reference: https://teslaownersonline.com/threa...ry-replacement-options-costs-downsides.12344/


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## Ed Woodrick (May 26, 2018)

chiapet15 said:


> Are the 12V lead-acid batteries starting to fail en masse? Would switching to a lithium 12V battery be the long-term solution?
> 
> For reference: https://teslaownersonline.com/threa...ry-replacement-options-costs-downsides.12344/


Not for the Model 3. Tesla changed from the Model X which has significant 12V battery issues.


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