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li-ion battery parallel cell fuel gauge

bumkyu koh
Prodigy 110 points
Other Parts Discussed in Thread: BQ27621-G1, BQ27742-G1, BQ27541-G1, BQ27510-G3, BQ27741-G1

Hi, I'm designing a Li-ion battery charger and its fuel gauge for robots in a company.

The Li-ion battery consists of 4 of 2600mAh Li-ion battery cells. I mean 4 in parallel.

I found a charger for it and wanted to use Bq27621-G1 for its fuel gauge. However, an FAE in my local distributor said that it is not recommended to use this solution to gauge 4-p li-ion battery because impedance difference between each battery cells causes incorrect gauging.

Is there any other recommended solution in this case? I'd like to make the development period faster, price is less important. 

Very thanks.

  • 0
    Genius 3970 points

    Yes, the bq27621-G1 isn't recommended for your use because it uses the cells' internal impedance as a "current-sense resistor" and it also only supports up to 6000 mAh. If you intend to use the gauge inside the battery pack (that is, behind the Li-Ion protection circuit) I'd recommend the bq27541-G1; if you want to have a combo fuel gauge and protector, the bq28z560-R1 (DFN package), bq27741-G1/bq27742-G1 (BGA package) are good choices. If you want your gauge on your system's mainboard, you can go with the bq27510-G3 (also a DFN package). However, unlike the bq27621, the previously mentioned chips will need a current sense resistor and a few resistors/caps for filtering.

    What cell make and model are you planning to use? Chances are it'll have already been characterized and have a Chemistry ID assigned by TI. If this is the case, then configuration of the gauge is simplified significantly since much of the 'learning' of the cells would be done for you already.

  • 0 in reply to en-US
    TI__Genius 14985 points

    Thanks for the reply, Jason.  I just have one minor clarification.  The bq27621-G1 will probably be fine for this application or any 1SxP application.  It will still treat the parallel cells just like one big cell.  Assuming they are all the same cell models and are reasonable matches to one of the pre-loaded chemIDs in the bq27621-G1, the fact that there are multiple in parallel won't really have any effect.  You do want to make sure you have a good Kelvin connection to as close as possible to the cell tabs.  You also do need to be mindful of the capacity limitations of the gauge, but very tiny or very large cells are actually easier to work with using bq27621 than some of the other gauges that use sense resistors since you can just scale everything up or down by factors of 10.  See the Quick Start Guide for bq27621-G1 for more info on that.

    http://www.ti.com/lit/pdf/sluuap5

    Also see the section about "Zero Configuration" in there.  We know some customers just want to buy a gauge and slap it on their PCB without messing with characterizing batteries or configuring gauges, and in some cases the bq27261 might work just fine for them in just this way.  For multi-cell (ySxP) packs, it might even work for some with zero configuration if you just divide the voltage down by "y" times using an op-amp voltage divider as shown in SLUA496.