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BQ76920 or BQ40Z50 parallel LIFePO4 battery packs

ClaudeArpin
Intellectual 320 points
Other Parts Discussed in Thread: BQ76920, BQ40Z50, BQ78350

I am designing a system that will have up to 5 parallel battery packs. Each pack consists of 4 cells in series with a bq76920 or bq40z50 controller for balancing and control. From what I have read, paralleling these battery packs if their SOCs are close is straight forward, the pack with the higher SOC will send current to the one with the lower SOC and the pair will reach equilibrium. Of course I will include protection to avoid a 80% pack connected to a 10% pack does not fry the PCB while pumping too much current into the lower pack. The two BQs I am considering have charge and discharge FETs , so for this to work, I would have to put the pack with the higher SOC in discharge mode and the one with the lower SOC in charge mode. For two packs, that is simple but for 5 sets I will need a pretty complex algorithm. Can I turn-on both FETs simultaniously to speed up the equilibrium?  Also the two FETs are in series, the current flows through one diode drop in both directions, turning both on would increase efficiency if the FET's resistance is low enough.

Thanks

  • 0
    TI__Guru 73830 points
    The guage will turn both FETs on when the pack is charging or discharging. The only event where one of the FETs will be turned of is a protection event or at charge complete. e.g. You can setup the device to turn the CHG FET turns off at charge complete. The CHG FET will turn back on when it detects a discharge current. The same applies to charging. The device also has a feature called Emergency Shutdown. This feature will allow you to turn the CHG and DSG FETs off to isolate the cells from the other packs. You can control this feature with an external GPIO pin or an SMBus command.
  • 0 in reply to ThomasCosby
    Intellectual 320 points

    Great thanks for the quick answer.

  • 0 in reply to ThomasCosby
    Intellectual 355 points
    Using two battery packs in series should also be straight forward, right? Each one is including a bq40z50. I have tested this configuration while turning off the FETs of one pack with a SBS command. Unfortunatelly it does not interrupt the current. I have checked the Gate Voltage and it looks okay.
  • 0 in reply to Roman Buchholz
    TI__Guru 73830 points
    That should work as long as the upper pack is completely isolated from ground and referenced to the top of the battery stack on the lower pack. SMBus communication will also have to be isolated.
  • 0 in reply to ThomasCosby
    Intellectual 355 points

    I am talking about two independent battery packs, so there is a direct connection between pack1+ and pack2-, because just the pack connections are visible to the outside!

  • 0 in reply to Roman Buchholz
    TI__Guru 73830 points

    Roman,

    I bench tested the stacked pack configuration and did find an issue. I used the lower pack to stop the discharge path and that work. The problem is that the load pulls P+ of the bottom pack below its ground and the FETs cannot be turned back on. You will need to add a series diode to top pack to isolate the load when the discharge path is turned off, but I did not have a chance to test this solution.
    Tom

  • 0 in reply to ThomasCosby
    Intellectual 355 points
    Thomas,

    in order to change pack1 and 2 whenever the customer wantsto do so I would need to add the diode to both packs. I would like to test this, but I do not get where exactly to put the series diode. Can you refer to the EVM design or the datasheet?
  • 0 in reply to Roman Buchholz
    TI__Guru 73830 points
    Roman,
    It looks like this will be a challenge. The problem is that the P+ pin gets pull below ground when either of the packs is turned off. This triggers the reverse protection circuit that forces the DSG FET off and it cannot be turned on until the reverse voltage is removed. You can remove it by turning the other DSG FET off, but it returns with either pack is enabled again. It think that the only way to overcome this is to remove the load. Another alternative may be to move to a different device that support low-side CHG / DSG FETs. You could consider the bq76920 and bq78350 as the chipset for you pack.
    Tom
  • 0 in reply to ThomasCosby
    Intellectual 355 points
    I already removed the reverse protection. Using another device is no longer an option for me, because I have invested too much time into this gauge. Where should the series diode be placed? Removing the load is not a good option for a UPS battery pack, because sometimes the protected system is not nearby. Would it help to clamps the pack connections with a big condensator?