# Tesla S Battery Thermistor Spec



## boekel (Nov 10, 2010)

*Re: Tesla 3 Battery Thermistor Spec*

Are you asking about the ones on the BMS board? or somewhere else in the battery?


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## duncgyro (Aug 22, 2017)

*Re: Tesla 3 Battery Thermistor Spec*

I believe they are inside the battery and used to monitor the inflow and outflow liquid temperatures and thereby control charging rate. Am I mistaken?


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## boekel (Nov 10, 2010)

*Re: Tesla 3 Battery Thermistor Spec*

I don't recall seeing sensors on the Coolant lines (but I might be wrong)

I did find them on the BMS boards, making contact with the cells underneath (see photo's)


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## boekel (Nov 10, 2010)

*Re: Tesla 3 Battery Thermistor Spec*

btw, those measure about 12,5 Kohm at about 15 degrees C


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## duncgyro (Aug 22, 2017)

*Re: Tesla 3 Battery Thermistor Spec*

OK. Do you think a second thermistor is just for backup or does it have a specific use? I did not notice exact locations when I removed the OEM BMS card. I do not see a thermistor on my replacement BMS card, so I assumed it was inside 

My intention is to use one to disable charging in the event of low battery temperature, and I'd like to know the "B" values (ohms per degree). MY BMS comes calibrated for a 10K B3950. If the Tesla thermistor is indeed 10K at 25C, and 12.5K at 15C, perhaps this is sufficient to determine the slope? 

Perhaps full project disclosure would help. 

The OEM BMS boards were replaced with Stealth EV boards in preparation for use with the Electrodacus.com "SBMS0" model. 

This BMS is generally used for off-grid homes, but my application is to upgrade the solar (800W) and energy storage (10 kWh)in my RV with minimal impact on current traveling equipment storage. Thus the Tesla batteries are mounted below the floor in a newly fabricated insulated enclosure where the original AGM batteries were. An aquarium heater/thermostat and pump will be used to keep the batteries above 5C. 

The new BMS will disable charging should the temperature falls below, or overvoltage occurs. The new BMS will also display the temperature and can be adjusted once I know thermistor parameters.


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## boekel (Nov 10, 2010)

*Re: Tesla 3 Battery Thermistor Spec*

Ahhh your talking about Model S/X battery modules 

by 'Tesla 3' I assumed Model 3...


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## duncgyro (Aug 22, 2017)

I am using Tesla Model "S" batteries.


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## duncgyro (Aug 22, 2017)

Sorry for confusion from the start!


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## kennybobby (Aug 10, 2012)

*Re: Tesla 3 Battery Thermistor Spec*

The older model S temperature sensors were located on the input and the exit sides of the coolant ports. i reckon they could measure the delta-T to determine the heat load being generated and carried out by the coolant.

Forum member tomdb reversed the tesla cmu boards such that they could be commanded to report voltage and engage balancing.

i assume you have a tesla module that you removed the CMU board and it would have the temperature sensors embedded in the module with some sort of clear rtv compound. There is pictures of them somewhere on the forum.

p.s. you can edit the first post to change the title


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## duncgyro (Aug 22, 2017)

*Re: Tesla 3 Battery Thermistor Spec*

The original post "edit" button does not appear 

Attached image of the removed Tesla card. Nothing was embedded with RTV that I can tell. I measured the two "T" pin pairs as previously noted with the new (connections only) Stealth EV card. At 50F ambient, the resistance measures about 21 K ohm. So I "think" the thermistors are within the battery itself.

So I still have the two questions:

1. Which are the "inlet" and "outlet" thermistor connections? I'd also like to know which way the coolant should flow!

2. What is the ohms/degree spec?


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## kennybobby (Aug 10, 2012)

*Re: Tesla 3 Battery Thermistor Spec*

Yes that is the board that has been reverse engineered, i think the schematic has been posted on this site. i know i traced it out and maybe others have also.

The coolant inlet and outlet are going to be whichever side you make them to be. There is a 4-wire connector on the back side of the board, what was plugged into that? Should be 2 temperature sensors potted in some sort of hemispherical plastic plug that is stuck into a pocket in the module and held with some goo. Take a picture and post it up--it's worth a thousand words. 

don't know why you can't edit your first post--you are the author


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## Tomdb (Jan 28, 2013)

*Re: Tesla 3 Battery Thermistor Spec*

The thermistors work with the Orion BMS and Dilithium BMS without an issue.

These are a 10K thermistors B 3435K. The Orion 2 Software says10K B 3380K 

Inlet and outlet you can work out yourself, do you even have a module in front of you? 

As the coil loops just under the HV connection points, also this is where the thermistors are.


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## duncgyro (Aug 22, 2017)

Thanks Tomdb!

Learning these are connected via the 4 pin connector, it was easy to find them. Per a previous request, I have attached pictures of the two thermistor locations. I still can not tell which one is the coolant "inlet" vs "outlet" sides as I can't determine coolant flow direction. What am I missing? Firstly, which fluid connector is the "inlet"? But I can now trace the wires to the 4 pin connector, and then back to the Stealth EV connector.

You said _These are a 10K thermistors B 3435K. The Orion 2 Software says 10K B 3380K_ Not sure how to interpret this. Are you saying although the thermistors are 3435 spec, the Orion works "ok" with a 3380 setting?

My BMS is pre-calibrated for a 10K B3950, but I can change parameters.


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## duncgyro (Aug 22, 2017)

duncgyro said:


> You said _These are a 10K thermistors B 3435K. The Orion 2 Software says 10K B 3380K_ .


Re-reading this, perhaps the Orion 2 software specs says the Tesla battery uses B3380 thermistors? Which is correct?


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## Tomdb (Jan 28, 2013)

duncgyro said:


> Re-reading this, perhaps the Orion 2 software specs says the Tesla battery uses B3380 thermistors? Which is correct?



I would trust the Orion Software, as their engineers had the modules in for testing. 

Worst case just hook it up and the batteries should be at room temperature, so you can see how far off it looks.


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## duncgyro (Aug 22, 2017)

Tomdb said:


> I would trust the Orion Software, as their engineers had the modules in for testing.


I agree that a corporation's suggestion is more likely correct than any word-of-mouth. However I could not find or verify this B spec mentioned in their online BMS manuals? Is that where you found it?



Tomdb said:


> Worst case just hook it up and the batteries should be at room temperature, so you can see how far off it looks.


The value for 5C is critical, as that is where charging should be disabled. Not "room temperature". An over-temp limit is also desired, although not very likely in my operation. Reaching both limits while monitoring coolant temps and thermistor values indicates some pretty extensive testing.


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## kennybobby (Aug 10, 2012)

What tolerance range of electronics are you building or going to use? If the value at 5C is critical then you will need to use very tight tolerance circuit components (better than 1%). What sort of tolerance do the thermistors have?

The difference in resistance at 5C using 3435K (22.896k) or 3380K (22.594k) is only 391 Ohms, or 1.7%.

There is an online Beta calculator at https://www.ametherm.com/thermistor/ntc-thermistor-beta

i would recommend a more robust control system assuming 10% tolerance parts and not try to cut the slice so thin that a slight error puts you into undesirable territory or trouble. 

Don't rely on an automated temperature measuring circuit of unknown tolerance to control your charger--put yourself as the final man in charge of deciding if it is too cold to charge, or add heaters such that it is never a question.


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## duncgyro (Aug 22, 2017)

*Re: Tesla 3 Battery Thermistor Spec*

Do you believe the Tesla thermistors have that loose of tolerance?

The Electrodacus.com SBMS0 can program the cutoff limit as desired, so 7 or 10C would give more margin. Batteries are mounted within an insulated and sealed box, and a 100 W aquarium heater will initially provide heating. This should be more than adequate unless OAT goes to -20F, and I really don't plan on camping at that extreme, and/or would have shore or generator power. I can easily upgrade the liquid heating system or add more insulation outside the box if it appears to be required.

http://www.thermomart.com/download/ntc resistance value 10K-3380-.pdf

..... shows 3380K and 3435K equivalent values - assuming one of these are the Tesla equivalent.

Still don't know whether coolant flow direction is important.


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## kennybobby (Aug 10, 2012)

*Re: Tesla 3 Battery Thermistor Spec*

See the heading of that table shows the R(25C) to be 10k +/- 3%

R25=10000Ω:±3% B25/50=3380K (B25/85=3435K): ±1%

The 3380 was calculated using the span from 25 to 50C.

The 3435 from 25 to 85C.

So both values are correct depending over the span you want to use them.

The graphs at the bottom show how the error gets worse as you go below or above 25 C by both percentage and degrees C. 

Nothing is perfect, they are just resistors. You can buy them pre-tested and sorted into groups of tolerance and equal values. If you buy a lot of them you would certainly specify these things.

You are the plumber of your own cooling system; you know which side is the inlet and the outlet; The outlet will always be warmer than the inlet.


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## duncgyro (Aug 22, 2017)

*Re: Tesla 3 Battery Thermistor Spec*

I know the output of my heating system will be warmer. What I still need to know is: 

a) which is the better battery fluid line to use for inlet - or does it even make a difference? It certainly does for automotive radiators, but that is because they are mounted vertically. My batteries are horizontal.

b) which thermistor is closest to which inlet? I need this info because I want to use the exit temperature. The thermistor with the yellow wires is closest to the negative battery terminal, but this does not necessarily means it is closest within internal fluid flow to the opposite fluid inlet tube. I don't know the battery plumbing.

In my RV coach application, there will be negligible battery heating. I am more concerned about very low ambient temperature. No mechanism is provided for battery cooling.


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## boekel (Nov 10, 2010)

*Re: Tesla 3 Battery Thermistor Spec*

not that it matters much but: the outside wires on the plug are TS1 on the tesla BMS, the inside wires are TS2


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## kennybobby (Aug 10, 2012)

*Re: Tesla 3 Battery Thermistor Spec*

Here is a picture showing the coolant loop as it snakes thru the module. It is aluminum flat tubing formed in a serpentine loop to "touch" each cell covered with a yellow kapton insulating tape and a grey thermal pad. i don't think coolant direction matters.


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## duncgyro (Aug 22, 2017)

*Re: Tesla 3 Battery Thermistor Spec*

THANK YOU KennyBobby! This picture makes perfect sense. I now know the "lower" thermistor will be nearest the "lower" fluid inlet tube. I will feed the heated fluid into the lower tube, and measure the exiting fluid temperature from the upper.


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## duncgyro (Aug 22, 2017)

The T1 measures the inlet closest to the plus battery terminal. Thank you to all that helped.


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## superzanti (May 22, 2020)

TLDR: The B value is about 4300, A better value is likely a 4365.

This thread is long dead, but considering this is the first google search result for "Tesla Battery Thermistor Parameters", I figured this would be the ideal place for this.

I have the 'Tesla Smart Battery' used in the Mercedes Benz B Class.
They're different from what most people here are using, but I don't see why Tesla would have used different thermistors, especially considering the design of the batteries are nearly identical.

While not super robust, I used a calibrated temperature probe, and measured the thermistor temperature over a range of temperatures. I weighted the values based on the difference between both thermistors on the module (2 values close together indicates that the system is at a temperature equilibrium and my probe is probably measuring very accurately).
There were some temperature values, such as while heating up quickly, or cooling down quickly, that are consistently moving with the same slope of the line, but are off because of that 'temperature equilibrium' I referred to.










After exploring various thermistors online, I found this standard NTC thermistor that appears to match all the characteristics the best:
Thermistor 4365K on Mouser
Meaning the value would be 4365.

I wrote a Matlab script to do the curve-fitting, if you want it feel free to PM me, but if you want to do a real basic version of this, this SRS Thermistor Calculator is very good:
SRS Thermistor Calculator


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## Sandler (Sep 15, 2020)

Does anyone know what type of 4 pin connector/plug is needed to connect to the thermistors?


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## superzanti (May 22, 2020)

Are you looking for a wiring harness? If so I found the atx power supply plugs work pretty well.
But if you're assuming it's something special, it's not, just 2 wires for one thermistor (positive and negative) and 2 for the other.


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## oudevolvo (Mar 10, 2015)

I have these connectors in stock, found here Tesla battery module thermistor connector kit - EVcreate


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## Sandler (Sep 15, 2020)

I did find some cheapos on amazon that seem to work fine. Jst-xh-2.0-4 pin connector. But I also ordered the after market boards from StealthEV and intend to use these to clean up my wire harnesses. Here's a photo of my 7x 5.6 kwh tesla module battery pack


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