I have to replace the NTC droplet resistor of some 6S BMS, but how do I select the right NTC resistor value ? I use li-ion 21700 batteries.

 

Thermistors have a characteristic resistance at 25°C.
You can measure the resistance when the thermistor is removed from the circuit. While each thermistor has a given resistance vs temperature curve, this will give you a good starting point to select a new thermistor.

 

Thermistors have a characteristic resistance at 25°C.
You can measure the resistance when the thermistor is removed from the circuit. While each thermistor has a given resistance vs temperature curve, this will give you a good starting point to select a new thermistor.

Big board is 100k and small board is 10k @25c. Next I guess I have to heat it up and measure ?

 

hi oslosl,
This may help, do a resistance measurement ever 10 degrees Centigrade, post your results.
E
Ideally, have the NTC disconnected from the PCB.

 

hi oslosl,
This may help, do a resistance measurement ever 10 degrees Centigrade, post your results.
E
Ideally, have the NTC disconnected from the PCB.

I unsoldered one leg.

 

Hope this helps.

Big board:
25c - 100k
30c - 85k
40c - 64k
50c - 46k
60c - 36k
70c - 26k
80c - 17k
90c - 10k

Small board:
25c - 10k
30c - 9k
40c - 8k
50c - 6k
60c - 5k
70c - 4k
80c - 3k
90c - 1k

 

hi oslosl,
This PDF should help you determine the response curves and give an idea of the NTC types,

E

 

Hope this helps.

Big board:
25c - 100k
30c - 85k
40c - 64k
50c - 46k
60c - 36k
70c - 26k
80c - 17k
90c - 10k

Small board:
25c - 10k
30c - 9k
40c - 8k
50c - 6k
60c - 5k
70c - 4k
80c - 3k
90c - 1k

It sure does. Now just find a part that comes close. It doesn’t have to be exact but close enough in the temperature range of interest.

 

It sure does. Now just find a part that comes close. It doesn’t have to be exact but close enough in the temperature range of interest.

Thanks. If I want to use two NTCs (because I have two seperate 3S compartments) in parallel, I guess I have to double, but also think about the threshold value too ? If that normally is max 45c it means 55k (big board) and 7k (small board), but with 2 NTCs ?

 

Thanks. If I want to use two NTCs (because I have two seperate 3S compartments) in parallel, I guess I have to double, but also think about the threshold value too ? If that normally is max 45c it means 55k (big board) and 7k (small board), but with 2 NTCs ?

No.
The thermistor and supporting hardware is designed to operate over a given temperature range. If you need to replace the thermistor for whatever reason, then replace it with one that gives the same resistance at the threshold temperature. Don't mess around with the thermistor.

If you have two separate 3S compartments in parallel I would have to assume that they are charged with a single BMS charger. You will have to rely on the single thermistor getting a temperature reading wherever it is located.

 

No.
The thermistor and supporting hardware is designed to operate over a given temperature range. If you need to replace the thermistor for whatever reason, then replace it with one that gives the same resistance at the threshold temperature. Don't mess around with the thermistor.

If you have two separate 3S compartments in parallel I would have to assume that they are charged with a single BMS charger. You will have to rely on the single thermistor getting a temperature reading wherever it is located.

Ok. If battery temperature gets too high, is that normally all the batteries ?