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Original question:

BQ25185: NTC temperature threshold

BQ25185: TS/MR pin

Lucas Bagrow
Prodigy 10 points
Part Number: BQ25185
Other Parts Discussed in Thread: BQ25186

Tool/software:

Hello TI experts,

I am currently working on a battery powered system and I need help with the NTC.

For the context, I want to use a single 18650 Li-Ion cell and I want to manage its charging temperature. The manufacturer's specifications indicates that it must charge only between +10°C and +45°C and in the BQ25185's datasheet I can't find a clear way to select the good NTC. I saw on this forum that you advise to use a 103AT-2 10K NTC, but first what will be the cold and hot temperatures with it ?

And I saw an another topic :  https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1472003/bq25185-ntc-temperature-threshold?tisearch=e2e-sitesearch&keymatch=BQ25185%20NTC#

Where the owner couldn't use the chip with his NTC characteristics, am I in the same case ?

Thanks

Lucas

  • 0
    TI__Expert 8505 points

    Hi Lucas,

    Thanks for reaching out.

    The TS voltage thresholds of the BQ25185 give COLD=0°C and HOT=60°C when using an NTC with a resistance of 10k at 25°C and a beta value of β=3435K (such as a 103AT-2). This is a very common and widely available type of NTC.

    The BQ25185 uses a current-based TS implementation rather than voltage-based. This means it uses an internal current source on the TS pin to bias the NTC, and the resulting TS voltage is compared against internal references to determine which temperature range the battery is in. A voltage-based TS, like that's used by the BQ2407xT, uses an external voltage divider circuit on the TS pin. This approach allows for much more flexibility in the configuration of the desired battery temperature range than the current-based TS, which can be used with external resistors in series and parallel with the NTC, as shown below:

    Since the BQ25185 has fixed HOT and COLD voltage thresholds, the challenge when configuring the TS pin for a "narrow" temperature range, like 10°C to 45°C, is finding an NTC with a large enough beta value to horizontally compress the VTS vs. temperature curve. Common beta values for NTCs are in the range of 3000-5000K, so this usually constrains how much the curve can be compressed. I'd also recommend selecting an NTC with a 1% β tolerance and a 1% R at 25°C tolerance, which can further limit NTC selection but is ideal for safety.

    I understand the BQ25185 is a standalone device, but the BQ25186 (I2C-controlled) provides configurable TS voltage thresholds via I2C, which makes it easier to configure the TS pin for a 10°C to 45°C range. For example, the COLD threshold can be adjusted from 1.0075V (fixed on the BQ25185) to 0.8200V, and the HOT threshold can be adjusted from 0.1150V (fixed on the BQ25185) to 0.1850V.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 10 points

    Thanks for your answer.

    I made some calculations and I find out that I need an NTC with a β = 5830K for 10k @ 25°C, and that's impossible to source...

    So it looks like I have 3 choices: 

    - I can change the IC for a BQ25186 and thus complicate my battery management.

    - I can use an NTC with a β = 4500K that would give me Tcold = 5.6°C and Thot = 51°C. Which would not really comply with the manufacturers recommendations.

    - Or I can change the 18650 cell.

    Am I right ?

    Lucas

  • +1 in reply to Lucas Bagrow
    TI__Expert 8505 points

    Hi Lucas,

    Yes, you are correct. I also found that an NTC with a β = ~6000K would be needed, which is definitely not something that can be easily sourced.

    Like you mentioned, β = 4500K is around the highest beta value for a 10k @ 25°C NTC that is widely available.

    I agree those are the 3 options that make sense.

    If possible, I think your best option would be to change the 18650 cell for one with a larger temperature range. Even a temperature range of 0°C to 45°C would be easier to achieve.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 10 points

    Thanks for your help.

    After looking for an other 18650 cell, I found that the majority of them have the sames manufacturer's requirements and I have taken note of the plating effect under +10°C thus it seems to be more related to chemistry. So, I think the best option is to modify my schematic for a BQ25186, being able to modify the differents thresholds seems to be a good features too (I have a host µC on my board so it is not an issue).

    Thanks a lot, have a great day

    Lucas