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BQ2947: Combining two battery packs both with BMS

Tim Clavette
Prodigy 10 points
Part Number: BQ2947
Other Parts Discussed in Thread: BQ40Z50,

I would like to combine two 3s2p Li-ion packs to make one 3s4p pack.  Each of the 3s2p packs utilize the BQ2947 for over-voltage protection and both use the BQ40Z50 to balance cells.  If I start with two fully charged cells they may slowly drift apart from each other.   However, it seems if one pack is above the other, it will try to charge the other and trigger the over-charge/over-discharge current protection.  It doesn't seem like good practice to anticipate a case where the protection be tripped, but it seems that  the part is designed to handle the potential mismatch until they are brought back into synch.

  • 0
    TI__Guru 54715 points
    Hi Tim,
    Most likely the bq2947 will never trip. The more common challenge when connecting 2 batteries in parallel is as you indicate, the current between them when the voltages equalize. For example if you install a 9V battery in a system which already has a 12V battery, the higher voltage battery may over-current discharge and the lower voltage battery may over-current charge. Your system should have some mechanism to use one battery until they match or equalize them before connecting them together.
    If your bq2947 is a 4.25V device it would not fault unless the system was charged above 12.75V or the cells in one of the packs became imbalanced and it was charged to a more normal voltage. Of course the bq40z50 should balance cells and this should not occur as long as the cells remain well behaved.
  • 0 in reply to WM5295
    Prodigy 60 points

    Hello WM5295,

    as we are talking about 3S2P Li-ion battery packs, 9V would be a completely depleted pack, wereas 12V would be an almost full pack.  It seems rather obvious not to connect those together... so your answer does not really say that much.  What if both packs are near 12V...  Does the BQ provide any protection at all? Should we use another BMS to balance the charges of the two packs?  Is there a suggested maximum voltage difference that could be tolerated? What would be the way to go about and still use the two packs if they are to a similar voltage? Or, is your suggestion still to empty one pack and switch to the other?

    Thanks

  • 0 in reply to Manuel Robert4
    TI__Guru 54715 points
    Hi Manuel,
    A trained user may know not to plug a depleted pack into a system with a fully charged one, and the example is dramatic. If you connect 2 packs together the current between the packs will be determined by V = IxR. If you don't control the path the current could be high. The bq2947 or bq40z50 do not adjust the current path other than to protect if the monitored conditions exceed the set limits. You could build a system which could determine when to allow the batteries to be connected perhaps through a diode, a resistive path, or directly. You can read the pack voltage from the bq40z50s for use in decision making. A FET has a built in diode. You could also build a dc-dc converter to equalize the packs, or as you propose use only 1 at a time or use them until they match before connecting together. It is not something the battery can provide beyond the voltage data and protection if something exceeds the limits set.
  • 0 in reply to WM5295
    Prodigy 60 points

    Thanks, I have also been guided towards the Power ORing solution (which is just a bit more complex than what you are proposing). Power ORing can be implemented using TI's TPS241x family.  That will be my solution.  Thanks for your help.