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12V battery question

Sorry if this has been asked elsewhere.

Does the main battery pack keep the 12V battery charged for as long as there is power in the main pack?

I'm planning to install at least one dashboard camera into my Model S. In "normal" cars there is always the risk that the 12V battery will be drained if the camera runs continuously and draws current for an extended period (days) when the engine isn't running. It is possible to use a device like this to avoid the problem, but on the Model S does the problem essentially go away?

I assumed there was not a separate 12V battery in the Model S, just an outlet configured to deliver 12V from the regular battery. That would make a lot more sense than having a separate one.

There absolutely is a 12V battery - it works the lights, instruments, etc.

I found this note on page 25 of the owner's guide which seems to imply that there is a 12 volt auxiliary battery:

"CAUTION: If the Battery’s charge level falls to 0%, you must plug it in. If you fail to do so within a month, you can permanently damage the Battery. This damage is not covered by the warranty. If the Battery falls to a critically low level for an extended period of time, it is not be possible to charge or use the vehicle without jump starting or replacing the 12V battery. If you are unable to charge the vehicle, contact Tesla."
(emphasis mine)

Another paragraph on the same page:

"NOTE:When the low-power consumption mode is active, the auxiliary 12V battery is no longer powered and may go flat within 12 hours. In the unlikely event this occurs, you may need to “jump start” or replace the 12V battery before you can charge. To do so, contact Tesla."
(emphasis mine)

These are the only two mentions of the 12V auxiliary battery throughout the entire owner's guide. No hint on where the auxiliary battery is located to how it can be accessed to "jump start or replace" it.

Thanks Volker. I saw those too. I inferred from the words that the battery will draw down from the main power pack until the main power pack depletes, but I wanted to check to see if anyone could confirm.

Ok, fair enough. I guess there must be good engineering reasons to have a separate auxiliary battery.

WOW. I wonder if the 12 volt battery helps to prevent the main battery from catastrophic failure. That would be good to know that if for whatever reason (jail time :-) you can't charge your Model S that all you may have to buy is a 12 volt battery and labor.

Either way. If installing a 12 volt electronic device it would be highly recommended to use a cut off device for when the voltage gets low. That would be true in an ICE too. I ruined a car battery long ago and stranded myself because I didn't use one. Never again.

I didn't look closely at the one you linked but they make them with timers and/or low voltage cut off. I recommend both.

The 12v battery runs the onboard electronics and lights. The battery must supply the voltage for the handshaking with the charger. So, if it goes flat, you can't charge the pack.

Which brings up the question (also asked in a Model X forum thread) if the 12V battery can be used to jump start your friend's (or wife's) ICE. If so, how? And if not, why not?

To clarify: "How?" as in "Where is it located and how can it be accessed to apply the jump-start cable clamps?"

If you are looking at the car from the front, the 12V battery is located to the left rear of the frunk and is inaccessible. Here is a link to a thread with pictures:

ddruz, thank you for posting those pics! Very interesting. Are you positive that we are looking at a Model S there? As far as I can tell, it might as well be a Model X. Not sure if that makes any difference wrt my question, though.

Volker.Berlin, I don't check the Model X forums either here or at TMC and I originally downloaded the pics from one of the forums.


The 12V battery is way to small to power a starter motor. If it's a real emergency, you can do it the Prius way:

1. Take the keys from the other vehicle's driver and keep them with you.

2. Hook up the 12V batteries in the normal way being very sure to get the polarity correct (otherwise you're buying at least a 100 amp fuse.

3. Leave the batteries connected for 15 to 30 minutes depending upon how dead you think the other battery is. Be sure the donor car is in READY mode.

4. Disconnect the batteries.

5. Give the keys back to the other car's driver.

Step one is the most important one here because if the other driver has the keys there's a very good chance he will try to start his car while the two are connected. That's basically Russian roulette with five loaded chambers.

The 12V battery is way to small to power a starter motor.

Size doesn't tell everything. A motorcycle battery can jump-start a car, and those are a lot smaller than car batteries. Unless a car is a truck of some kind that requires 24volt system.


Small in terms of amp/hours, not small in dimensions.

jerry3, You have specs for that battery? All I have is few pictures and a lot of assumptions.


Nope. All I have are assumptions as well.

Also based on the posts about the Roadster 12V battery.

That makes them "educated guesses" which is better than what I have.

Heck, 8 1½V D cells in series gives 12V. The battery could be really puny.

While the Li ion batteries reportedly only lose 1% of their charge per month, what about the 12V battery? If I don't drive my ICE for a couple of months, the battery may go dead because of the clock and other stuff that is on all the time. What happens in the model S?

The large battery charges the small one.

Well, there is no "starter motor", but I'd guess the 12 vdc battery is there to open the doors, allow you to light the panel, and power the electronics so you can charge the main battery. I was honestly hoping to never see a 12 volt battery. But a big 12 volt makes sense in that it can still unlock the car and power the systems in an emergency.


Me too. The 12V battery seems to be the bane of EVs and hybrids. However, I hear that Telsa produced the first Roadsters without a 12V battery and had to change back for reasons I'm not certain of. So apparently the problems with them are less than the problems without them.

For the curious, I just found out that the battery on the S is a 12V, absorbed glass matt35Ah battery that is slightly (20%?) smaller than a 'normal' car battery.

There is a separate 12V battery used for powering all the standard 12 accessories used in standard cars. Nothing new. But the reason you can't get anything going if the main pack dies is because it powers all the contactors and relays that engage the 12v battery. So without the main pack you lose connection to the 12v batt as well, even if it is healthy.

@CnJsSigP - the issue is the other way around - if the 12v dies you can't charge the car. The 12v (not the main pack) holds the contactors open. If the 12v dies it isolates the main battery pack, and you are SOL.

D'oh! You're right, Nick. I was just uh, ahem, testing you... and you passed!