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BQ24074: Is device entering thermal regulation?

Kevin Honeyager1
Prodigy 30 points
Part Number: BQ24074

I have a BQ24074 set to charge a 1.75Ah lithium battery at its recommended charge current of 875mA.  When performing a battery charge test at 40C, it appears the fast charge portion of the cycle ends after about 32 minutes, and the measured battery voltage at that time is 4.02V.  I am seeing very similar behavior when charging the battery at  room temperature (~21C), where the voltage at which the charge begins to taper is 4.03V.  Is this a curve of a device entering thermal regulation? Considering the input voltage is from a 5V USB power adapter, I find it hard to believe that a charge current of 60% of part's maximum designed current could be entering thermal regulation, especially after 30 minutes. Board has the appropriate layout with thermal pad and thermal vias, and is enclosed in a plastic housing with the battery.  Should I see a constant current phase until I get into the Vbatreg range of 4.16-4.23V?

I'm thinking this is a normal charge curve, but would like a 2nd opinion from TI or other users of this device.

Entire charge curve:

Detail view:     

  • 0
    TI__Mastermind 21590 points

    Hello,

    I don't think this is thermal regulation because i would expect this to occur at lower VBAT voltage with same battery charge current and system load where the delta between VOUT and VBAT will be higher.

    Are you able to measure the battery voltage at the BAT pin on the device instead of on the pack? At such high currents, impedance between BAT pin and battery can cause the charger to see a higher battery voltage and terminate earlier than expected.

  • 0 in reply to Raheem
    Prodigy 30 points

    Raheem,

    Thanks you for the feedback.  The issue is likely my instrumentation setup. I have some 5' coax cables going into the chamber because the measurement device is outside the chamber.

    I do have one other question for you. It is being suggested that we derate this IC to 50% of its normal junction temperature range, which would be 75C.  This would limit our charging current to ~600mA off of our 5V supply rather than the battery manufacturer's recommended charge current of 875mA.  Our charging system is spec'ed to operate at ambient temperatures up to 40C. Derating 50% seems awfully conservative, given that the datasheet states the device is able to operate for 20000 hours at 1.5A charge rate at 125C.   I am also thinking that the device spends 90% of its charge time at a battery voltage of 4V or above, so only looking at ~.9W most of the time.  Also, we would probably charge this battery at most once per week, so roughly 200 hours of charge time per year.

    Given that this device has built-in thermal regulation, what is your opinion of charging at 875mA? I don't really want to give up shorter charge time unless it is necessary from a reliability perspective. 

  • 0 in reply to Kevin Honeyager1
    TI__Mastermind 21590 points

    Hello Kevin,

    If you don't mind me asking, where did this suggestion for temperature derating com from?

  • 0 in reply to Kevin Honeyager1
    TI__Mastermind 21590 points

    Kevin,

    TI stands by the ratings we have in our datasheets. The device has thermal regulation so can work at high temperatures as long as it's within the datasheet limits