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Original question:

BQ25756: Driving both the system and the battery with the BQ25756

BQ25750: Driving system and/or charge multi-pack smart batteries

Loïc Andrieu
Prodigy 10 points
Part Number: BQ25750
Other Parts Discussed in Thread: TPS2121, BQ25756, BQ25756E

Hi all,

I need to build a system than can be powered by 4 smart batteries or from a DC power supply if the batteries are not present.

When both the DC supply and the batteries are present, the DC supply should charge the battery as well as power the system.

My initial idea would be to go for 4x BQ25750, one for each battery, the output of the 4 charging circuit would be tied together with ideal diodes (probably TPS2121, or LM73100) to power up the system load.

Would this kind of circuit work?

According to the question linked to this one, it looks feasible to do this also with a BQ25756 however in this case I'm not quite sure the system can be powered when the batteries are not present.

Would you recommend some other circuit to achieve my goals?

Thanks for your help.

Cheers,

Loïc

  • 0
    TI__Genius 12950 points

    Hello Loïc,

    I will get back to you tomorrow on this.

    Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Genius 12950 points

    Hello Loïc,

    Thanks for being patient with the wait. Just to clarify, the 4 smart batteries are charging separately and then will be tied together if external ideal diodes? This circuit will work and you can use the BQ25756/BQ25756E IC for this.

    Also when the batteries are not present, you can set EN_TERM=0 and EN_PRECHG=0. Depending on your system voltage and current load, this might allow the BQ25750 to supply the system without a battery attached.

    Let me know if you have any other questions.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Hi Ethan,

    Thank you for your answer.

    In my understanding I can only connect one battery to the BQ25750, hence I intend to make 4 charging circuitry based on BQ25750 one for each battery. See block diagram below.

    To clarify, in my ideal scenario the BQ25750 based charging circuit would detect automatically if a battery is present and if the DC supply is present, in case both are present it would charge the battery and provide current to the load, if no battery is present it would provide current to the load from the DC input, if 1 or more batteries are present with no DC input it would provide current to the load from the batteries.

    I do not want to remove the batteries to charge them externally.

    So from your answer I understand that for each charging circuit I would need to detect if the battery is present and set EN_TERM and EN_PRECHG accordingly. Is that correct?

    Best regards,

    Loïc

  • 0 in reply to Loïc Andrieu
    TI__Genius 12950 points

    Hello Loïc,

    Thanks for the clarification.

    What's the maximum current load for the system? You might be able to just program 1 BQ25750 or BQ25756 to power the load.

    I will try to get back to you with more information before the end of the week regarding this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Hi Ethan,

    The system requires about 60W total, the batteries providing 12-16V this would make 5A max.

    How would the schematics look like with only one BQ25750? Would all batteries be connected in parallel? If so, then what happens if the batteries are not at the same voltage when plugged in? The higher voltage battery would recharge the other ones which does not seem like a good idea.

    Looking forward to the additional information you can provide.

    Best regards,

    Loïc

  • +1 in reply to Loïc Andrieu
    TI__Intellectual 2520 points

    Hi Loic, Although based on the voltage and current ratings, one BQ25750 would do the job, as you said, it would be difficult to manage the charging and discharging levels of the batteries without an MCU or something that is programmed to provide this information to the system. Hence, it would be preferable to use a different BQ25750 for each battery. This kind of circuit would work and in our experiments and discussions with other customers, we have seen them make such parallel operation work successfully.

  • 0 in reply to Utsav Gupta
    Prodigy 10 points

    Hi Utsav,

    Thank your for your confirmation, I'll try this route then.

    Best regards,

    Loïc