Tesla Model S 12V Battery Analysis

It would be great to be able to start this post talking about how the Model S, much to the amazement of many owners, has a 12V lead acid battery.  Sadly, I can’t do that - because this battery is a failure-prone sore point among many owners, and far too many owners with a >2 year old car are entirely aware of it, as the battery has already failed and been replaced at least once.

This is a companion discussion topic for the original entry at https://www.sevarg.net/2016/10/30/tesla-model-s-12v-battery-analysis/

(Comments from Blogger)

2016-10-30 by Shaz-Au

Another great article, thanks for sharing.

2016-10-30 by Mike Halcrow

Not sure how the battery is doing, but what I can say is that my falcon wing door is broken.

2016-10-31 by Russell Graves

Aww… That’s the one unique feature.

I’m excited for the Model 3. It should drive used Leaf prices into the ground. I’ll probably truck out your way and pick one up in a year or so.

2016-11-01 by vanborkum

I monitor the 12 volt battery through the lighter plug. I don’t know if I could detect degradation or not. What is the lowest voltage the system will boot from? I’m getting out there on the battery so my time is coming. Also, are the charge / discharge parameters you talk about controllable by software (FW) updates?

2016-11-01 by Russell Graves

Dennis - you probably can’t detect degradation solely from voltage. You could look at the time of cycling and see if the car is cycling the battery faster, but I don’t really know what the car uses to determine there’s a problem.

As for lowest voltage? No clue. Go find someone with a bench Tesla and ask them to try.

Presumably the charge/discharge parameters are controlled by an upgradeable firmware, but I have no idea where that would be or how to mess with it. I’m just a random guy, not a Tesla engineer. I mean, I don’t even own one.

2016-12-04 by Unknown

Two years in, 28000km, and so far no battery issues. That means, having written this, I will go to my car and find a problem! September 2014 build.

2016-12-05 by Unknown

"Or maybe just leave the DC-DC converter on."
Nailed it.

They should simply drop the 12v battery.
Also remove the buggy whip holder. :slight_smile:

The Dodge TEVan went this route, and it’s obviously the superior option.

2016-12-05 by Russell Graves

Unfortunately, regulatory requirements for things like “keep the flashers working after a prime mover failure” mandate the 12V battery or a similar option. If the battery pack fails, the brake lights and such need to continue working as the car pulls off the road.

2017-01-23 by BattMobileBatteries

I am manufacturing and selling Lithium batteries for the model S 12V/accessory system! Our batteries have been working very well, although we are young, and we have done extensive testing, including cold/freezing weather utilization.

2017-01-23 by Russell Graves

I have no idea who you are, or how anyone would get in touch with you, but if you’re willing to share details of your pack, I’d be very interested. The contact form is over on the right column.

I’d assume it’s in the realm of a 4S LiFePO4 pack with some sort of internal heater?

2017-01-24 by Unknown

Excellent analysis and thank you very, very much for uncovering a lot of mysteries about the 12V-battery. As a comment, adding a 3x larger battery to get a 3x better longevity is a debatable. Also I would be cautious to draw rules from investigating the behavior of a single car, but you mentioned that, if I understood correctly.
Is there a way how we could provide additional data/readouts from our cars to help you to improve the ecologic footprint of this very specific battery?

2017-01-24 by Russell Graves

I’m happy for whatever data you can provide. I’d be very interested in seeing voltage/current/temperature readings from a fresh 12V battery as well as from your replacement. If you’d like to get me more information, there’s a contact form in the right column - let me know your email there and I’ll respond to you and we can converse that way.

2017-01-27 by dmr

Thanks for the detailed analysis. Have you send this info to Tesla? If so, have they responded?
- Dave

2017-01-27 by Russell Graves

It was posted on Reddit, but I haven’t specifically reached out to Tesla, nor would I know how to do so. I’m easy enough to contact if they want to get in touch with me, but I can’t imagine they’d reach out to some random blogger about technical issues. Learning how to treat lead acid isn’t hard.

2017-02-15 by JP

Could the voltage spikes be attempts to break up sulfation? I agree they seem odd for a sealed AGM.

2017-02-15 by Russell Graves

That was my guess, but that’s the type of thing you can get away with, sort of, with a flooded battery. They tend to take overvoltage abuse by slowly corroding and gassing aggressively. AGMs will gas and vent, but at least with a flooded battery you can add more water.

It’s just a really weird behavior.

2017-02-28 by Unknown

Maybe this?


2017-03-11 by appleguru

Where did this data come from? How can I pull similar data from my own car?

2017-03-11 by Russell Graves

From someone who has more access to their car’s data streams than the average bear - it’s not something everyone can gather this way.

2017-03-22 by Unknown

Please help me with finding a replacement for the tesla 12v battery, in my country we don’t have tesla dealer and I have 6 cars not working.
Normal 12v that fits other cars aren’t working.

2017-03-22 by Russell Graves

I have no particular knowledge of what the computer looks for. You should be OK with any AGM battery of similar size and capacity.

2017-03-31 by Unknown

tesla are using very weird standard. non of the battery on the market fit, even in amazon. i only find it at Tesla dealer

2017-04-27 by Unknown

12v battery warning came on my 2013 Model S just past it’s 3 year anniversary. Fortunately, a Tesla technician did a house call to replace the battery, covered by warranty.

2017-06-21 by myprvns?

Awesome article! Btw, any tips for users? I mean, for example i am using a dash cam which records 7/24. How much is bad for Tesla’s 12v battery? Should i leave the car in “charging” state while over nights for sure? Or should i drive my car when i want to load on my cigarette charge port? Or should i charge the car maybe?


2017-06-21 by Russell Graves

The idle draw of the computers is so huge that it dwarfs anything else connected, and there’s not a lot you can do short of hooking up a high power battery tender/charger when parked. I have no idea if that will upset the computer or not, and it probably needs to be able to source 4A to keep the battery charged with the draws on it (so your typical little battery tender won’t do it).

2017-07-04 by valor155

The dashcam uses 5W of power, tops. I think it is safe to leave on all the time if so desired. The main battery will still charge the 12V system when needed, correct? I don’t think there is any chance of damaging the 12v system by drawing 5 more Watts. I wonder if it’d impact battery longevity in any significant way. I don’t know the answer to that.

2017-07-08 by Babylonfive

It’s $400, but it’s not clear how low the temp can fall before this battery is out of spec. It’s not really well explained on that website.

2017-07-15 by Sanjeev

They are using TIs active balancing system, and it dumps the charge to li-ion batteries onto 12v to balance itself, maybe that is why you see the spikes.

2017-07-15 by Russell Graves

Huh? That statement doesn’t make particularly much sense without a lot more detail. The spikes are coming from the DC-DC converter, as I understand the system, and they always come right at the end of a 12V battery charge cycle (which has ~nothing to do with the main battery charging). I’m familiar with at least some of the TI BMS front end chips, and they normally dump into bleed resistors - the complexity required to dump charge into the 12V battery from main pack balancing is well beyond what would be reasonable. There are also some balancers that use small capacitors to shuffle charge from the high group to the low group, but I’m not sure that’s used much anymore.

2017-08-04 by Unknown

I’ve done some 12V logging on my Tesla as well, graph here:
12v constant in rear trunk | Tesla Motors Club

2017-08-19 by Unknown

Would hooking up a 12v trickle charger do any harm to the DC-DC converter while the car is parked for long periods of time?

2017-09-05 by Unknown

FWIW, I’ve put 200,000km on my 12v in four years with no failures.

2017-09-18 by Unknown

Russell. Do you know the maximum amperage of the DC-DC converter in a Tesla Model S 12v battery system? I see people have been using the cigarette lighter to run 150 watt DC-AC inverters. If the DC-DC converter is only 7 amps max, that will discharge the 12v battery. ~13v x 7A= 91 watts max.

2017-10-11 by Unknown

Still going strong at 2.75 years.

2017-11-04 by Unknown

What about hard wiring a 30W 12V solar panel to charge the battery when parked? This way the vampire can be fed when parked outside. There are flexible solar panels that can do it for $50-100 and a 3A charge controller can run $10-20. At 21 x 13 in, they can act as a dashboard shade/cover.

2017-12-10 by David Hicks

I’m still trying to understand if attaching a trickle charger to the 12v battery, as I have done with many other vehicles, isn’t a good solution pending the ultimate fix by Tesla. You need a reasonably high amp charger (4-5+ amp) for this but is there any issue with connecting a quick disconnect harness and plugging that in at night or for extended periods when the car will sit? Any issue float charging the 12x while the main charging is going on?

2017-12-27 by Dan

" I have no idea what that spike in the green chart is from, so it’s probably safe to ignore."

That is my favorite line in the article. I’m going to figure out a way to use a variation of that in my life. :slight_smile:

Enjoyed it, great article. I hope Tesla has been/is listening. Thanks.

2018-01-18 by YellowSpringer

Great article, but I have a question concerning cold-weather charging: As the owner of a 2013 Model S that recently started warning me of the need to replace the 12V battery (and as it’s out of warranty I’m considering a switch to lithium - a secondary issue), did I make a mistake by hooking up a 120V cord for ‘trickle’ charging the main HV pack on super-cold nights? January 2018 has seen 3 or 4 sub-zero F nights here in Ohio and I reasoned that letting it charge at a low current on those nights was better than going the ‘cold soak’ route. Do you think doing so was a mistake, did no harm, or given that the HV battery would have to heat itself to accept the 3 -4 mph charge, a good thing? Thanks in advance.

2018-01-18 by Russell Graves

I don’t know enough about the charging behavior to know for sure, but I can’t imagine you did any harm charging on cold nights. The pack won’t charge above what it considers a safe rate for the temperature.

I wouldn’t switch your 12V battery to lithium if you’re dealing with sub-zero-F nights. The charging system for the 12V battery isn’t going to know about that, and hard charging on cold lithium causes lithium plating, which is irreversible capacity loss.

2018-01-24 by Unknown

Excellent article; very informative. In the past year I’ve built a few Li-Ion battery packs, one 18v, one 48v, what I’ve found is that they will charge above the designed 3.7v to as high of 5.2v. I suspect the voltage spikes you are measuring could be due to this tendency.

While researching information on Li-Ion batteries I came across an article published by a University study describing the chemistry of Li-Ion batteries, which talked about runaway charging causing the batteries to overheat and catch fire. It talked about how the Li-ions acted inside the battery as opposed to conventional and lead acid batteries, which could also be a reason for the spikes.

Li-Ion is a high energy battery, as you know lead acid is a low energy battery and are not compatible. For that reason, Lithium batteries should not be used to charge lead acid batteries. My suggestion would be to add a 12v regulator circuit between the lithium batteries and the lead acid battery; (old school tech).

2018-01-24 by Russell Graves

I have no idea what you’re doing that leads to lithium cells charging to 5.2V/cell, but you might consider not doing that in the future. That’s seriously damaging to the cell, and radically increasing the risk of thermal runaway, as well as destroying cycle life. Why on earth are you letting them up that high?

Tesla isn’t just bolting the lead acid battery to the pack voltage with a big relay - the main pack is a 400V pack fully charged. There’s a DC-DC power regulator circuit in there, and you can see the behavior in the charts as it does temperature compensation, float voltages, and those spikes at the end.

So, I’m sorry, I have no real idea what your point is.

2018-01-24 by Unknown

I use an IMAX charger to charge my bats in groups of four before I build my packs. I did see the DC TO DC reg in your post, what I am referring to is a means to isolate to see if that makes a difference with the circuit by means of a different regulator circuit. I am not an Engineer, just a tech, just one method I used to trouble shoot a problem.

2018-04-11 by Unknown

hey Russell seems like Tesla should have used you as an advisor , cause I own a 15 mdl s 85d and I need a battery bad the car is completely dead I wish I had alot of money to just get it done by them but it’s just a battery change , I think at this point I’ve changed 50 in my life , My question is do you think it’s wise to put the larger battery in lead acid , of the lithium battery I live in the north east and it gets cold here sometimes -10

2018-04-11 by Unknown

hi, don’t know if the last post went through, but I had just bought a used Tesla 15 model s 85d and I need battery bad the car is completely dead I wish I had a lot of money to pay for them to do it for me , but I replaced about 50 batteries till now how hard can it be .My question is do you think it’s wise to put the larger battery in lead acid, or the lithium battery I live in the north east and it gets cold here sometimes -10 . Thanks Nick

2018-04-16 by Russell Graves

I wouldn’t put a lithium battery as a replacement 12V in a car that will get that cold. It’ll probably kill it in short order, because that’s well below the charging temperature for most lithium cells, and the Tesla system beats the 12V battery pretty hard on charge.

As far as “How hard can it be?” It’s a Tesla. You probably need the specialized shop software to replace the battery.

2018-08-28 by Jerry’s BlogSpot

First off thank you for this very informative post. I am currently considering purchasing a used Model S which would be out of warranty.

I know next to nothing about electrical wiring, but I understand that custom car audio folk use big capacitors to help prevent or mitigate damage to 12v batteries caused by high-wattage amplifiers and subwoofers. Such solutions are commercially available for ~$50.

Is there any reason to believe that getting one of those installed could lengthen the life of Model S’s 12v battery?

2018-08-28 by Russell Graves

It won’t have any impact. The car audio guys are putting large capacitor banks not to help keep the batteries alive, but to service the very high current (and short duration) needs of some of the car amplifiers in competition use. I don’t know particularly much about that world, but I do know they’re pulling insane amps, and have massive spikes on top of that. The capacitors help keep the system from browning out and shutting down, and help provide cleaner pulses to the speakers (as the amplifiers can get the amps they need).

It would have absolutely zero impact on the sort of chronic battery abuse Tesla subjects their lead acid batteries to.

2018-09-17 by Richard

How’s 3.75 years looking?

2019-01-13 by Unknown

Great analysis, great article. I think that DC2DC converter should be always on, it will put a 12V battery at eazy. Modern DC2DC converters are very effective and have very small losses. Problem can be if cooling pump starts automatically if DC2DC is on, even with very small load. I do not think that car should cover losses from outlet when it is not charging HV battery though, in the end it will not make car take more energy effective anyway. Technically it would be convenient, but some of owners have OFF PEAK TIME energy plans where device connected to the vehicle reads the time when car takes energy from the outlet.

2019-01-24 by Al Fulton

I have a Model S registered 9th Jan 2016. The 12V battery was replaced under warranty in March 2018 at 38000.
I maintain my motorhome battaries wit Battery Fighter management chargers (output 12V, 1.5 amps, on label but I suspect it charges at 13.5V then drops back to maintenance charge of 12V)
I have 2 questions -
1 Would maintenance charging when the car is parked in the garage (perhaps when not charging the main battery) be a benefit? The car is often only used every 2nd day so spends a fair time in the garage.

2 If the answer to 1 is ‘yes’, can i connect the charger via the cigarette lighter?
Many thanks

2019-01-24 by Russell Graves

If the cigarette lighter is “always live,” it should work. I don’t know if the Tesla one is or not.

However, a typical battery maintainer won’t keep up with the Tesla idle draw. 1.5A@12V is only 18W, and the car draws 30+W when “off” from the 12V system, so all you’ll do is delay the discharge.

2019-03-11 by Cyril Kalbach

I’m really glad I read this article. I was thinking about purchasing a Lithium Ion battery on Amazon. However, when I read about the cold temperature charging dangers I thought about my winter trips to Lake Tahoe and concluded I’m better off with the standard replacement from Tesla.

2019-03-13 by Russell Graves

Lead acid seems to be a dirty word among Tesla types, but they’re really quite good batteries, for certain purposes, if you treat them properly - and they are amazingly tolerant of a massive range of environmental conditions.

2019-04-10 by Unknown

WOW! That’s a great post!

I ran into some troubles with my 2013 Model S. The 12V I have now it’s the second one on this car and this last one lasted for 2 years. The problem is that the car acts very strange in it looks like the battery can’t hold the charge anymore. The computer, as a result, shuts down any system I try to use as a result so I get a full list of disabled things after a few minutes of driving ( Park Assists Unavailable, Traction Control Disabled, Stability Control Disabled, Regenerative Breaking Disabled, etc ). All of that because the 12V is acting very strange and the computer couldn’t figure that out so instead of throwing me an error to replace the 12V, keeps going and disables any consumer that uses the 12V circuit.

Thank you for this data! I now know not to buy a Li battery and I have a better understanding of why my battery failed after only 2 years.

2019-04-25 by Unknown

I have a 2015 Model S and just verified that the cigarette lighter is OFF when the car is off. It turns on just as I open the door. Very interesting information here and spot on about why the 12v battery has a short life. I will post some information later about a similar short battery life in a fleet situation I worked with.

2019-11-15 by eintstein196

How’s 4.75 years looking?