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Discussion Starter #1 (Edited)
Does anybody know the specs for the two internal battery thermistors? I know it is an NTC 10K series, but what is the "B" (range/resistance) specification?

Also, is the "T1" thermistor across from the plus terminal or the negative terminal side?
 

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Discussion Starter #3
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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Discussion Starter #6
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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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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Discussion Starter #11
Re: Tesla 3 Battery Thermistor Spec

The original post "edit" button does not appear :mad:

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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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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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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Discussion Starter #14
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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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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Discussion Starter #17
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?

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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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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Discussion Starter #19
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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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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