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BQ24210: Connecting two devices in series

Jyrki Koljonen
Prodigy 50 points
Part Number: BQ24210

Hi,

I'd like understand if there's some technical reason why BQ24210 couldn't be connected in series like illustrated in the attached picture. The idea would be to get "two cell" system voltage (7.4V)  without a need to directly charge a series connection of two lithium cells. Thereby the need for a separate cell balancing circuitry would be avoided.

I know by experience that this works in lab/test board level at least with some devices, but is there something in the BQ24210 design which should be taken into account in this kind of circuit topology?

Jyrki 

  • 0
    TI__Guru 71366 points

    Hi Jyrki

    We have not evaluated this configuration and cannot recommend or endorse it.  

    My concern would be the balance of charge on the two cells due to performance difference of the solar cell.  Expect that both cells can be full charged and then support load.

    Could have problems if the cell is only partially charged then discharged.  This will depend on your use case and requirements.

    Try to match solar panel and battery, keep thermal of the BQ24210 the same.  

  • 0 in reply to Bill Johns
    Prodigy 50 points

    Hi,

    Thank you for the comments.

    Now, I have a further question concerning the BQ24210:

    The specification sheet defines that the programmed fast charge current (Iout) shall be 50mA...800mA. The Iout is set with resistor connected to the Iset pin of the IC. 

    Now, if one would use the IC with a small solar panel capable of supplying only, say, 100mW, the maximum available current for charging a lithium battery cell  would be only in the ballpark of 20mA. Is this still a legit operation condition from the BQ24210 behavior point of view? Does it still perform as intended although the programmed max output current (50mA) wont ever be reached?

    Regards,

    Jyrki

  • 0 in reply to Jyrki Koljonen
    TI__Guru 71366 points

    Hi Jyrki

    Jyrki Koljonen said:
    Is this still a legit operation condition from the BQ24210 behavior point of view? Does it still perform as intended although the programmed max output current (50mA) wont ever be reached?

    (Update) Yes will still work.  If the solar panel can not provide 50mA or ISET current.  But as input voltage drops the VBUS_DPM will reduce output current to prevent Vin from crashing. Charge will continue but at lower current.

  • 0 in reply to Bill Johns
    Prodigy 50 points

    Hi,

    Just to be sure, I suppose you meant:

    "..If the solar panel can’t provide 50mA or ISET current..."?

    Regards,

    Jyrki