• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 69 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ76920: limit charge current on LifePo4 battery pack.

FJ
Intellectual 340 points
Part Number: BQ76920

Tool/software:

I have an application where I need to charge a small LifePO4 12 volt pack (2Ah)  (0.5C max charge) with a charger that can exceed the maximum charge rate.  I cannot use separate charge/discharge terminals.   I currently use BQ76920 in my BMS's for large packs where the charge current is less than the max charge.

How might I limit the charge current and/or separate the charge from discharge current in a battery pack?

  • 0
    TI__Mastermind 37360 points

    Hello FJ,

    You may want to add a pre-charge MOSFET. This could be a MOSFET in parallel with the CHG MOSFET, however there could be a resistor in this path that can be used to limit the current. This would have to be controlled by a MCU.

    The BQ769x2 family actually implements a high-side P-Channel MOSFET used for this exact purpose. It also supports separate charge/discharge paths.

    If you want to separate charge/discharge path, you could also do so by separating the CHG/DSG MOSFET paths, however, load detection would not work in this case.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 340 points

    Do you have an application note on this?  This sounds like a good option.  But would I have to control from the MCU when to enable charge vs discharge?   

    I was thinking of this circuit, not sure if this would work as well.  Where

    a power resistor would limit charge current and a mosfet would allow high current output.  Might this work too?

  • 0 in reply to FJ
    TI__Mastermind 37360 points

    Hello FJ,

    FJ said:
    But would I have to control from the MCU when to enable charge vs discharge?   

    Yeah, it would be pretty manual. 

    FJ said:
    Do you have an application note on this? 

    For low-side, we have the following app-note from a different device family that may be useful (Section 8 Precharge and Predischarge for Low-Side FETs):

    However, the BQ769x2 family is high side, so the implementation would be different here. In your case, if you were to follow something like Figure 8-1. Precharge and Predischarge Test Circuit Schematic, you would need to have something pulling the PCHG node to Vss, when you want to enable the pre-charge FET. This may be using an N-Channel MOSFET. 

    FJ said:
    a power resistor would limit charge current and a mosfet would allow high current output.  Might this work too?

    Maybe so, but I'd suggest to run some simulations to see how well it would do. We have not tested a circuit like this before.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 340 points

    I currently use a low side configuration in my other applications.

    Is this what you are referring to as far as implementing a pre charge mosfet?  Q13

  • 0 in reply to FJ
    TI__Mastermind 37360 points

    Hello FJ,

    Yes, something similar to that. Although this seems to be using REGOUT to drive Q13. So the drive voltage probably be ~5-V or so, I think it should work.

    The example from the app note uses BAT and assumes the Zener would protect the gate-source of the FET.

    Best Regards,

    Luis Hernandez Salomon