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BQ20Z80 & BQ29312 BMS systems wired in series in a larger battery pack

Jim Woodward32
Prodigy 20 points
Other Parts Discussed in Thread: BQ20Z80, BQ29312, BQ34Z100-G1, BQ76930, BQ77908A, BQ78350-R1

We manufacture battery packs and have been using a BMS that utilizes the BQ20Z80 and BQ29312 chip set for a number of years.  This BMS can be set up for 2 to 4 series cell connections by setting solder jumpers properly on the printed circuit assembly.  Our circuit design is pretty darn close to the reference design in the evaluation module for this chip set - there are differences in part selection and such but for the purposes of this question you can reference TI's reference design.

What we want to know is for a pack design that we need to have 6 cells in parallel can we take two 3 series (3S) BMS's and wire them in series.  The pack positive would go into the POS input of the top (positive most 3 series) 3S BMS and the NEG output of that top most BMS would go into the POS input of the bottom (negative most 3 series) 3S BMS with the pack negative coming out of the NEG of the bottom 3S BMS.

Is it possible to do this without problems?

Are there any issues or things to be concerned about with doing this?

We know there would be a balancing issue that would develop between the top most BMS module and the bottom most BMS module of the pack.  The only way to clear that imbalance issue is discharge the pack until both BMS cut off and then charge up from there.  But over a number of CHG and DSG cycles the top and bottom modules will get to a charge capacity imbalance that can only be corrected again by doing a full DSG of both top and bottom modules and then charging.

What else should concern us?

Thanks in advance for your help.

  • 0
    TI__Guru 73830 points
    Jim
    If I understand correctly, you are trying to use a 6S configuration with 3 series cells connected to one bq29312 and the other 3 series cells connected to another bq29312. These two packs will be connected in series to form the 6S pack. Is this correct? If so, will each pack also contain a set of FETs and a sense resistor? If this is the case, then this could be possible, although I have not seen anyone try it. IF you only have one set of FETs controlled by one bq29312 and the sense resistor monitored by the other, then I do not think that it will work. It will be better to use the bq34z100-G1 and bq77908a chipset or the bq78350-R1 and bq76930 chipset.

    Tom
  • 0 in reply to ThomasCosby
    Prodigy 20 points

    Each BMS has its own FETs and sense resistor.  Essentially take the reference design schematic of your evaluation module for this chip set and wire each up as a 3s pack then wire those tow reference schematics in series using the POS & NEG outputs of the BMS's to form a 6s battery pack.  

    Based on the answer you did provide it sounds like you don't see any real issue to doing this.  We have seen that the two 3s sections can get out of balance with each other and the only way to bring them back into balance is to do a full discharge where both BMSs cut off their discharge FETs and then charge back up from that point.