• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 65 subscribers
  • Views 788 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ24600 Thermistor calculations

Neil Moore
Prodigy 40 points
Other Parts Discussed in Thread: BQ24600

Hi,

I am using a BQ24600 device to charge Li-ion battery. However, when I do the calculations for the thermistor that is fitted to the pack, I get negative resistances for one or other of RT1 and RT2. The thermistor in question has a cold resistance of 25.3k and a hot resistance of 5.3k. The calculations come back as RT1 being 10.3k with RT2 being -210k.

I have checked this with the spreadsheet tool mentioned in other posts and get the same values.  As I can not change the thermisor, my only option appears to be allowing the pack to charge at -5 degrees as the cold resistance (32k) gives positive results.

Any help would be greatly appreciated.

Thanks

Neil

 

  • 0
    TI__Mastermind 31120 points

    As one narrows the window of operation the R2 value (bottom resistor) becomes larger.  The smallest temperature window is when R2 goes to infinity (open) any further reduction (mathmatically) requires a negative resistor.  Increase your window size to get a valid bias.  Try 28k and 4k and start moving in the desired threshold until one gets a high resistance for RT2 and then this is the best you can do.

  • 0 in reply to Charles Mauney
    Prodigy 40 points

    That being the case, is there any reason why I could not operate it with RT2 open circuit?

  • 0 in reply to Neil Moore
    TI__Guru 56650 points

    I think you can set RT2 open to maximize your temperature window.

  • 0
    Intellectual 620 points

    Hi Neil,

    I just went through that whole thing too and confirmed the equations in the spreadsheet. I plugged in your numbers and got the same -210k you did.

    Honestly, the R values you give for hot and cold don't look anything like the thermistors values I've seen. Can you give me the manufacturer's part number for it? I'd like to check it out.

    Andy

  • 0 in reply to Andy10159
    Prodigy 40 points

    Andy,

    I don't have the actual part number of the thermistor as it is built inside a battery pack. From measurements it seems to have B of about 3400K. Calculating B between 0 and 50 gives a value of 3324K.

    The resistances I have given were for +5 degrees and +40. I've never designed for a Li-ion pack before and I know that there are problems if you charge it below freezing, so I'm giving myself some headroom. At the moment it looks as if I will end up with a low cut off about 1 degree and have to accept a high cut off of 45 degrees.

    Neil

  • 0 in reply to Neil Moore
    Intellectual 620 points

    All I can say is they must have had a single HUGE customer with a weird thermistor ...

    You can always put in your own window comparator and drive the TS pin through a voltage divider that has 4 discrete voltage states.

    0 < V1 < VCOLD,

    VCOLD < V2 < VHOT,

    VHOT < V3 < VCUTOFF,

    VCUTOFF < V4

    V1 can be ground and V4 is can be Vref if that simplifies things any.

    I'll kick it around a little bit this weekend. Doesn't seem overly complicated.

    Andy