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BQ25172: Inquiry about its minimum charging time, NTC detection & design note for 3 nimh batteries in serial

Max Zhang
Prodigy 10 points
Part Number: BQ25172

Tool/software:

Hi, Team,

(1) Is the available minimum charging time 4 hours ?  (can it be 2 hours only?)  So, if the battery with 1100mah capacity, I can just use 275mA charging current.

(2) If I put the 3 batteries in a plastic box and just use the metal pin/spring to contact with the batteries, can I just put the NTC nearby the soldering point of the metal pin/spring for the temperature detection? or do you have any more appropriate suggestion?

(3) For using BQ25172 for the charging of 3pcs nimh batteries in serial, are there somewhat important design notes we need to know ?

Thanks & Regards,

Max

  • 0
    TI__Guru* 75806 points

    Hi Max

    Max Zhang said:
    Is the available minimum charging time 4 hours ?  (can it be 2 hours only?)

    (UPDATE) Sorry but cannot change the timer, 2 hours is not possible, minimum charge time is 4 hours.

    Max Zhang said:
    can I just put the NTC nearby the soldering point of the metal pin/spring for the temperature detection?

    This should work

    Max Zhang said:
    For using BQ25172 for the charging of 3pcs nimh batteries in serial, are there somewhat important design notes we need to know ?

    The BQ25172 is a linear charger and have a high power dissipation, good thermal layout is important.  Large copper area on the top level and thermal vias connecting to copper layers.

  • 0 in reply to Bill Johns
    Prodigy 10 points

    Hi, Bill,

    Thanks for your information.

    But I would like to re-clarify what is the minimum charging time for this IC ?    

    According to the above table in the datasheet, the safety time is 4 hours, so, I can't use C/2 for 2 hours charging (use Vbat_ov to stop the charging),  and can just use C/4 for 4 hours only, right?

  • 0 in reply to Max Zhang
    TI__Guru* 75806 points

    Hi Max

    Updated the first post to correct the 2hr / 4 hr question.

    The BQ25172 is timer-based charger and charge termination when timer expires.  Not a safety timer that will set a fault.  Also it is not expected Vbat_ov will trip.

    Yes can charge at C/4 for 4 hours.

  • 0 in reply to Bill Johns
    Prodigy 10 points

    Hi, Bill,

    Thanks for your information.

    Let me ask one question further; because it is timer-based, for example, if I use 250mA to charge the empty battery, it will be about 1000maH (capacity of the battery) in it after 4 hours.

    However, if the input power is disconnected (due to some issues) from the charger when the battery has been charged with 750maH. And then I re-provide the input power to the charger again, ie, the timer will re-count again, theoretically, the battery will be kept charging to the target of (750 + 1000)maH after 4 hours, so it is over-charged,  what will happen ?  how will the battery be protected? will the life/quality of battery be decreased ?

  • +1 in reply to Max Zhang
    TI__Guru* 75806 points

    Hi Max

    Max Zhang said:
    how will the battery be protected? will the life/quality of battery be decreased ?

    NiMH battery is very tolerant of over charge and this condition will not be a problem.  In designing the timer-based charging system we worked with a large NiMH developer and manufacture.  They have approved and recommend the device.

    UPDATE - Also the BQ25172 will check battery voltage before starting charge.  If the battery voltage is above 1.33V / Cell charge will not start, considered full.

  • 0 in reply to Bill Johns
    Prodigy 10 points

    OK, Bill. Noted and thanks.