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BQ25720: How to charge NiMH

Michael Burton
Prodigy 30 points
Part Number: BQ25720
Other Parts Discussed in Thread: TPS25751, BQ24401, BQ25792

Tool/software:

Hi, how can we configure the BQ25720 to charge NiMH cells?

The product spec says it supports NiMH chemistry among others, but the data sheet doesn't mention NiMH even once.

I also can't find any app notes on charging NiMh cells with this chip.

1. Does this chip support NiMh or is there an error in the product spec?
2. If NiMH is supported, how does it terminate charge cycle? Does it use Delta V? Temperature rise rate detection? Constant voltage? Trickle charge with a timer?
3. Can it support more than 4S since NiMh has lower voltage than Li+?



  • 0
    TI__Mastermind 33625 points

    Hi, Michael,

    It looks you are looking for a dedicated NiMH charger, try BQ24401

    BQ25720 is a smart charger, however, it does not provide termination or delta V detection. 

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 30 points

    Thanks Tiger, I need to support both NiMH (5S) and Li+(3S) packs in my design and was hoping to handle both from the same charge IC if possible. The dedicated NiMH chargers like BQ24401 don’t support Li+ nor are they supported by  power delivery controller ICs like TPS25751 and they don’t support buck-boost in order to charge from 5v when BC1.2 is all that is available.

    I think even if the BQ25720 can only handle slow-charge algorithm it will still be acceptable for our design. It says it can charge NiMH on the product spec. Can you tell me how to do this?

  • 0 in reply to Michael Burton
    TI__Mastermind 33625 points

    Hi, Michael,

    BQ25720 provides telemetry registers to monitor the battery voltage and battery current, also an independent comparator. This should allow the user to add additional circuitry to monitor the battery voltage and termination the charger by the user. Sorry, we don't have a reference design on this. 

    If your charge current is less than 4A,  you may consider BQ25792, an integrated buck-boost charger with similar architecture. 

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 30 points

    Thanks Tiger, my current reference actually uses the BQ25792 which looks like a great chip, but the specs don't mention NiMH on this chip which is why I was considering swapping to the BQ25720 which does supposedly support NiMH. Can you advise which features between the two ICs differ which makes one more suited to NiMH than the other?

  • 0 in reply to Michael Burton
    TI__Mastermind 33625 points

    Hi, Michael,

    No need to switch. They have similar functions, both need extra components to support NiMH. 

    Regards

    Tiger