BQ27750: Adding a capacitor to the temperature sensor circuit

Mihon Lipa

Part Number: BQ27750

Hello. In my circuit, I added a 0.1 uF capacitor in parallel with the NTC-thermistor (B57861S0103F040), wrote the coefficients in the temperature model (they work fine on bq4050). The internal temperature is measured correctly and shows -40, but the cell temperature is 10 degrees higher, that is -30 C. I think that this is the case in the capacitor, which I installed additionally. Please tell me, is it possible to put a capacitor in the measuring circuit of the NTC-thermistor? And is it possible to somehow compensate for the presence of a capacitor so as not to evaporate the capacitor?

over 5 years ago

0 Andy Liu (BMS) over 5 years ago

TI__Guru 52115 points

Without this 0.1uF capacitor, does the temperature reading become normal?

Andy

0 Mihon Lipa over 5 years ago in reply to Andy Liu (BMS)

Intellectual 390 points

I dont know. To check this, I need to completely disassemble the entire battery pack. 
I would like to know from you - can this be due to a capacitor?

0 Andy Liu (BMS) over 5 years ago in reply to Mihon Lipa

TI__Guru 52115 points

I don't think a 0.1uF capacitor will lead to this issue. As I mentioned before, I would suggest you remove the 0.1uF cap and check if the same issue still occurs.

Andy

0 Mihon Lipa over 5 years ago in reply to Andy Liu (BMS)

Intellectual 390 points

The +55 C test was successful. Both sensors showed +55 C. I will try to remove the capacitor and repeat the 
test at -40 C. Please see the attached log file at -40 C. Note the Temperature column. 
Before the start of the discharge, 
the temperature sensor shows -16.4 C. -30.4 C the temperature sensor began to show only after the 
discharge current started. If you disable the SLEEP and IN SYSTEM SLEEP bits in the DA Configuration, 
then the temperature sensor immediately displays -30.4. Question: why is -16.4 C exactly, 
and does this mean that BQ in SLEEP mode correctly measures the temperature? 
By the way, if you look at LifeTime before the discharge, 
then in the column of the minimum temperature the value -16.4 is written, and not -30.4 C ... why?

test_DSG_0.2C_m40.xlsx

0 Kang Kang over 5 years ago in reply to Mihon Lipa

TI__Mastermind 24870 points

Hello Mihon,

You should not have a capacitor on the TS circuit. We need to charge up the network quickly as we only engage the pull up for the TS network briefly in FW.

Adding a capacitor even larger than 1 pF will cause measurement accuracy issues.

0 Mihon Lipa over 5 years ago in reply to Kang Kang

Intellectual 390 points

Thank you so much. Removing the capacitor solved the problem. But I still don't understand why with a capacitor the temperature is

measured normally at +55 C, but not at -40 C.

0 Kang Kang over 5 years ago in reply to Mihon Lipa

TI__Mastermind 24870 points

The ADC takes readings on TS pin. If the capacitor is there, we measure a lower voltage due to a slower rise time and the internal conversions get messed up.

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BQ27750: Adding a capacitor to the temperature sensor circuit