# 12 Volt Battery in Model 3, lessons learned from Model S and X



## Mike (Apr 4, 2016)

Read a rather interesting blog on the techniques used by the Tesla software to tender the on board 12 volt battery.

Briefly, there is some history of the onboard 12 volt (lead acid) battery on each Tesla (S and X) wearing out much quicker than in a modern ICE vehicle.

The blog I read (sorry, no link) talked about how the onboard software was essentially never letting the 12 volt battery fully recover with a float charge.

The blogger recommended a larger capacity 12 volt battery, have the software rewritten to allow the 12 volt battery to get a float charge as soon as being plugged into shore power (even to the detriment of recharging the main traction battery as fast as possible) and have the 12 volt battery kept at float charge off of the main traction battery when not plugged into shore power.

Opinion: I really hope that the 12 volt battery system is less prone to maintenance in the Model 3 compared to the Models S and X. I also hope that its location is such that a swap out does not entail alot of trim disassembly . 

Thoughts?


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## MelindaV (Apr 2, 2016)

agree on expectations/hopes that changes are made to how the vehicle's systems deal with the 12v battery... however, my understanding is the 12v (at least in the current models) is not one you can pick up at auto parts stores - and - much more involved in it's connections than just a + & - connection. So most all current owners leave it to the service centers to change it out when needed.


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## Mike (Apr 4, 2016)

MelindaV said:


> agree on expectations/hopes that changes are made to how the vehicle's systems deal with the 12v battery... however, my understanding is the 12v (at least in the current models) is not one you can pick up at auto parts stores - and - much more involved in it's connections than just a + & - connection. So most all current owners leave it to the service centers to change it out when needed.


That photo alone kind of tells the story. I understand that the Model S was, from an initial design point of view, cobbled together 'no matter what'.

The battery is a glass mat style of lead acid battery . The blogger I followed was able to aquire one relatively easily via the online aftermarket .


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## AutoMcD (Nov 8, 2016)

This is disappointing to see that they are using lead-acid. That is definitely a weak link in terms of maintenance.
They make a lithium ion 12v swap-in, If the 3 comes with this I'm definitely replacing it.
Lead acid has a 5-8 year life no matter how carefully you treat it.


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## Mike (Apr 4, 2016)

AutoMcD said:


> They make a lithium ion 12v swap-in


I am not a subject matter expert, I'm just remembering what the original blogger said about this type of solution.

Apparently, L-Ion 12 volt battery cannot accept a charge until it is at certain minimum temperature. The whole concept of "cold soaked" 12 volt battery seemed to really spook the original blogger ref "solutions" to the 12 volt battery problem.

Like you, my impulse was, why not just use a L-Ion 12 volt battery?

There must be some reason why Tesla is not using it (yet).....


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## AutoMcD (Nov 8, 2016)

Mike said:


> I am not a subject matter expert, I'm just remembering what the original blogger said about this type of solution.
> 
> Apparently, L-Ion 12 volt battery cannot accept a charge until it is at certain minimum temperature. The whole concept of "cold soaked" 12 volt battery seemed to really spook the original blogger ref "solutions" to the 12 volt battery problem.
> 
> ...


Honestly my expectation is a 12V buck converter to make a constant 12V straight off the main battery pack, with one of those 0.5F audio capacitors for good measure. That there is another battery system at all seems a bit odd to me.


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## garsh (Apr 4, 2016)

AutoMcD said:


> Honestly my expectation is a 12V buck converter to make a constant 12V straight off the main battery pack, with one of those 0.5F audio capacitors for good measure. That there is another battery system at all seems a bit odd to me.


I think you want a separate electrical system. Something that can run some tests to make sure that the high voltage battery pack is in good shape and won't explode or catch on fire when you turn it on. I know this is the approach that Nissan took with the Leaf. I assume Tesla did something similar, but I haven't looked into it.


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## garsh (Apr 4, 2016)

garsh said:


> I know this is the approach that Nissan took with the Leaf. I assume Tesla did something similar, but I haven't looked into it.


Oh, and since an electric car doesn't require "cranking power" from the 12v battery, it's been hypothesized (but not tested, to my knowledge) that you could "start" a Nissan Leaf in a pinch (a dead 12v battery) by using 8 D-cell batteries in series.


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## Badback (Apr 7, 2016)

Has anyone asked Tesla why they did it this way?


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## Mike (Apr 4, 2016)

Badback said:


> Has anyone asked Tesla why they did it this way?


I was hoping that by bringing up this issue here, someone listening in who has a communications link with Tesla would ask that very question.


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## @gravityrydr (Apr 12, 2016)

From 
http://www.teslarati.com/understanding-tesla-12v-battery-service-warning/

"The 12V battery maintains power for critical systems when the main battery pack is damaged or disabled. It powers the hazard lights, airbags, door locking and unlocking operations, as well as other critical components of the Model S. The 12V battery also ensures that electronics are "awake" and listening to the key FOB in order to automatically lock and unlock the vehicle based on proximity. It also allows the car to maintain its 3G connection for remote access when the rest of the vehicle is powered off. If the 12V battery happens to fail, it will isolate the main battery pack from the car and prevent charging. This is a safety feature of the Model S designed to help protect first reponders in the event of an accident."
​Makes sense, but still seems odd to me. Curious how Tesla will handle this on the Model ☰


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## Michael Russo (Oct 15, 2016)

garsh said:


> Oh, and since an electric car doesn't require "cranking power" from the 12v battery, it's been hypothesized (but not tested, to my knowledge) that you could "start" a Nissan Leaf in a pinch (a dead 12v battery) by using 8 D-cell batteries in series.


@garsh , I bet you'll be happy to turn (in...) that Leaf...


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## AutoMcD (Nov 8, 2016)

I suppose that makes sense, I still don't have to like it! lol


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## teslaliving (Apr 2, 2016)

I've got over 70K miles on my Model S in just over 2.5 years and have only had one battery replacement. The car gave sufficient warnings (low charge) and I was never out of service. In the early days there were more reports of issues with the 12V battery but recently there haven't been many complaints about it. My feeling from my own experience and in reading lots of Tesla news is that the 12V battery issue is mostly under control now.


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## Mike (Apr 4, 2016)

teslaliving said:


> I've got over 70K miles on my Model S in just over 2.5 years and have only had one battery replacement. The car gave sufficient warnings (low charge) and I was never out of service. In the early days there were more reports of issues with the 12V battery but recently there haven't been many complaints about it. My feeling from my own experience and in reading lots of Tesla news is that the 12V battery issue is mostly under control now.


Hopefully the trend you speak of continues.

For s***s and giggles, I'd love to see the actual failure rates on a chart.


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## Guest (Feb 22, 2017)

My 50 cents:

Li-ion is not that beneficial for 12V battery. 
At -25C it should enter hibernation/protection mode (not going to start). As it has very small thermal capacity
Li-ion 12V battery might get cold soaked within one night at -27C to freeze it. 
And it will definitely not charge at that temperature. Will require heating elements, complexity for management system.

If I had to choose something, I would use sealed AGM battery. Not large. 30Ah. 
And definitely add independent monitoring sensor and add software that controls charging independently.
Idea is to keep SOC above 60% at all times. Charge it to 90% and stop. Even if car is not used for long periods.


It's possible to add secondary mini DC-DC converter into the main pack (12V output only, so no orange cables).
With maximum output power of 5-10A. This inverter should have it's own firmware and logics. This would allow 12V battery
size to be reduced in size considerably (50-70%?) and would keep SOC of AGM battery in ideal range easily (60-80%).
With correct state of charge AGM battery lasts at least 8 years. This is how long BMW can keep OEM battery even if
start-stop cranking is used tens of thousands of times. 
This mini DC DC converter could, potentially, be used with solar roof option to send excessive electricity into HV pack without
enabling contactors using 12V line.


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## Francois C (Sep 11, 2017)

arnis said:


> My 50 cents:
> 
> Li-ion is not that beneficial for 12V battery.
> At -25C it should enter hibernation/protection mode (not going to start). As it has very small thermal capacity
> ...


I have been told that Tesla did exactly that on facelift Model S. Small cd-dc in the HV battery pack, about 100w. Takes care of the main contactors and the sensors looking for key fob presence. That in turn enables less usage of the 12V battery.

Should have a lot less cycle on the main contactors as well on it's AGM battery.

As a side note, Tesla just introduced a 3rd gen 12V battery. Now says DCS "+" . Wonder what has been improved.


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## garsh (Apr 4, 2016)

Francois C said:


> As a side note, Tesla just introduced a 3rd gen 12V battery. Now says DCS "+" . Wonder what has been improved.


Not sure. C&D Technologies doesn't mention this particular model on their website. The DCS series are their deep-cycle batteries - intended to survive being drained pretty low on a regular basis. That is generally something you should try _not_ to do with lead-acid batteries, as they generally prefer to remain at 100% charge. Deep-cycle lead-acid batteries are typically used for things like motorized scooters for the elderly.

http://www.cdtechno.com/pdf/lit/12_106110.pdf


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## SSonnentag (Mar 30, 2017)

teslaliving said:


> I've got over 70K miles on my Model S in just over 2.5 years and have only had one battery replacement. The car gave sufficient warnings (low charge) and I was never out of service. In the early days there were more reports of issues with the 12V battery but recently there haven't been many complaints about it. My feeling from my own experience and in reading lots of Tesla news is that the 12V battery issue is mostly under control now.


I wouldn't consider 2 batteries in 2 1/2 years as being under control yet. My ICE vehicles go 5-7 years before needing replacement batteries.


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## Thomas Mikl (Jun 26, 2017)

And you live in AZ?
With our temperatures in winter I am lucky to get 2-3 years per battery in the ICE car.


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## Guest (Sep 12, 2017)

Lead acid battery is a totally different beast. Cold is not a problem. Neither is heat (in case of sealed lead acid, AGM for example).
The most important factor is management. And Tesla, and Nissan Leaf, totally fail at that.
BMW worked it out around 10-13 years ago. Their 12V batteries last for 5-8 years with extremely heavy use (start-stop, webasto).

How? SOC management. Charging voltage fluctuates between 12.5V and 15.5V. And intelligent drain control.

Thomas, likely your vehicles is bad at managing 12V battery and your driving habits are not sufficient on their own.


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## TesLou (Aug 20, 2016)

arnis said:


> Lead acid battery is a totally different beast. Cold is not a problem. Neither is heat (in case of sealed lead acid, AGM for example).
> The most important factor is management. And Tesla, and Nissan Leaf, totally fail at that.
> BMW worked it out around 10-13 years ago. Their 12V batteries last for 5-8 years with extremely heavy use (start-stop, webasto).
> 
> ...


I wonder if it helps that BMW (or at least the last two I owned) put the battery in the trunk? Seems like a better insulator from temp extremes; compared to other ICE cars.


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## JWardell (May 9, 2016)

TesLou said:


> I wonder if it helps that BMW (or at least the last two I owned) put the battery in the trunk? Seems like a better insulator from temp extremes; compared to other ICE cars.


Oh yes, smart choice for weight distribution...DUMB choice when it turns into a swimming pool. Note my special battery water extraction tool, which stayed in my car for two years that it took me to give up and drill a damn hole:


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## BILJ (Aug 4, 2017)

Forgive me if these are newbie questions, but here it goes:
1) When does the 12v get charged, and from where? Only while car is plugged in? While driving? From the main battery pack?
2) How low can the voltage go before it causes operational problems? ICE cars often won't start when voltage is below 12.0. Say you left a hard-wired dash cam running for a couple days and voltage dropped to 11.5. What would happen to a Tesla?


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## Thomas Mikl (Jun 26, 2017)

arnis said:


> Lead acid battery is a totally different beast. Cold is not a problem. Neither is heat (in case of sealed lead acid, AGM for example).
> The most important factor is management. And Tesla, and Nissan Leaf, totally fail at that.
> BMW worked it out around 10-13 years ago. Their 12V batteries last for 5-8 years with extremely heavy use (start-stop, webasto).
> 
> ...


Sorry but this is not true 

As a physicist I can tell you that heat-cold is a problem for lead batteries, even if they are sealed. However it is not the heat or the cold alone that kills it. It is the whole climate and while it is fun here in Austria to have very hot summers for bathing season in our many lakes and very cold winters for our ski team  it is deadly to lead acid batteries (yes when they are sealed).
Lead acid batteries have something called "water creep". Which basically means the electrolyte looses water over time (now I am not native in English, but I hope I get the point through). Usually with sealed batteries this is a long process, especially when outside temperatures stay at the same level and discharging the unit is not so bad (the battery has good management as you say).
However with extreme stress on the battery due to quick changing (and in battery terms this can be 1-2 months) the stress on the electrolyte increases and the water creep is higher. Very bad is actually very hot weather, but when the weather does not get cold again due to electrolyte expansion this is not so much an issue. However if you had a very hot time (100°F for example) and then go down to below 0 in a matter of 1-2 months the battery will be hard taxed to compensatem especially with high drain of added heaters (which most of our EU diesels have, because the engine is so damn efficient and does not warm up quickly enough) and start/stop systems, the battery will fail in winter due to stuff that happened in summer.

Now yes there are technologies and batteries that are better at handling this, but they also cost a lot more, hence no automaker uses them and I would loose warranty if I installed one...


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## Petra (Sep 12, 2017)

BILJ said:


> Forgive me if these are newbie questions, but here it goes:
> 1) When does the 12v get charged, and from where? Only while car is plugged in? While driving? From the main battery pack?


In the Model S/X, the 12V battery is charged via DC/DC converter fed from the main pack whenever the car's systems deem it necessary (doesn't matter if the car is parked, charging, or whatever). When parked, you'll occasionally hear the main pack's contactors close and it's probably a safe bet that the 12V is being charged (though, other things can trigger bringing the main pack into the loop as well).



> 2) How low can the voltage go before it causes operational problems? ICE cars often won't start when voltage is below 12.0. Say you left a hard-wired dash cam running for a couple days and voltage dropped to 11.5. What would happen to a Tesla?


Off hand, I don't know the minimum voltage for operation.


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## garsh (Apr 4, 2016)

BILJ said:


> 2) How low can the voltage go before it causes operational problems? ICE cars often won't start when voltage is below 12.0. Say you left a hard-wired dash cam running for a couple days and voltage dropped to 11.5. What would happen to a Tesla?


If the 12v battery gets too low (not sure on the exact voltage/current limits), then it disconnects (i.e. electrically isolates) the main pack from the rest of the car and the car will no longer run.

You can read more here:
Understanding the Tesla 12V Battery Service Warning


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## Guest (Sep 13, 2017)

TesLou said:


> I wonder if it helps that BMW (or at least the last two I owned) put the battery in the trunk? Seems like a better insulator from temp extremes; compared to other ICE cars.


It does. Though some BMW's fit battery under the hood but not in the engine bay compartment.
Another extremely important reason is weight distribution.

".DUMB choice when it turns into a swimming pool. "
Water in the trunk is not acceptable in the first place. Fix the leak, not consequences.

Pour water with garden hose and observe. Inspect sunroof drains if applicable. Inspect taillight seals. Inspect trunk lid adjustment.


BILJ said:


> 1) When does the 12v get charged, and from where? Only while car is plugged in? While driving? From the main battery pack?
> 2) How low can the voltage go before it causes operational problems? ICE cars often won't start when voltage is below 12.0. Say you left a hard-wired dash cam running for a couple days and voltage dropped to 11.5. What would happen to a Tesla?


1) it gets charged during vehicle charging, vehicle driving. In case of long period of nothing happening, vehicle wakes up and charges 12V battery occasionally (depends on EV, software settings)
2) Voltage can go down to at least 11.5V on most cases. Though this voltage already degrades battery itself (measured in days at that voltage, not in minutes). At around 10.5V most modules are not stable anymore. ICE vehicle starting depends on battery capacity, degradation, CCA value, engine itself (friction to overcome). My BMW can start at 11.2V. It is vehicle specific.
Voltage itself is not that important. Problem is in voltage drop in case of whatever action. For example, discharged 12V battery can be at 11.5V but if Tesla tries to wake up, voltage might drop significantly.



Thomas Mikl said:


> As a physicist I can tell you that heat-cold is a problem for lead batteries, even if they are sealed. However it is not the heat or the cold alone that kills it. It is the whole climate and while it is fun here in Austria to have very hot summers for bathing season in our many lakes and very cold winters for our ski team  it is deadly to lead acid batteries (yes when they are sealed).


Still. Loss of water only applies to flooded batteries. And this problem is heavily reduced with VRLA flooded batteries. *AGM and gel batteries do not lose meaningful amount of electrolyte during the whole service life* (but only if handled properly).
If battery is charged and kept at cold temperature, it will last for decades. If battery is charged above 13.0-13.2V (depends on temperature) for weeks, it will lose capacity, and electrolyte. This voltage results in about 80% SOC. If battery is charged at 13.6-14V (normal vehicles) at high temperatures, it will lose electrolyte with hours. If battery is charged at 13.6-14V in cold winter for an hour each day, it might not get charged enough. Again, it will degrade (sulfate).
And yes, this is the problem with 12V batteries. Below 60% SOC it sulfates (degradation), above 80% they start flaking (degradation). And to be between those, charging voltage fluctuates a lot. Dumb alternator or DC-DC converter will NOT keep that range. That includes Nissan Leaf and Tesla.
Keeping any (except regular flooded) battery between that range will give it a decade of service life. Flooded batteries (non valve regulated) will still lose water. Though these can easily be refilled. Though these are rare now in automotive industry.


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## JWardell (May 9, 2016)

arnis said:


> Water in the trunk is not acceptable in the first place. Fix the leak, not consequences.
> 
> Pour water with garden hose and observe. Inspect sunroof drains if applicable. Inspect taillight seals. Inspect trunk lid adjustment.


Bwahahaa you think I didn't try those, over and over, for years? Don't buy a car assembled in Britain.


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## Guest (Sep 14, 2017)

Too late. Bought a Leaf. SUNDERLAND. Catastrophy. Build quality was... Rude. Yuck.. Had like 15-20 things fixed or replaced.

You should be able to find the leak though... not nuclear science.


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