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BQ27520-G4: Unable to set a design capacity above 15400 mAh

Ross Johnson
Prodigy 230 points
Part Number: BQ27520-G4
Other Parts Discussed in Thread: BQ34Z100-G1

Hello,

We have recently decided to increase the battery pack size of one of our products, using 21700 cells. The new design capacity should be 19600 mAh. I am not able to get Gauge Studio to accept a capacity above 15400 mAh. I notice that when the datasheet was revised to B, one of the changes was to reduce the capacity to 14500 mAh. However, 32Ah still appears widely in the datasheet and indeed on the product page of TI's web site. So I'm not clear if this is a bug in Gauge Studio, or am I hitting a poorly documented limit of the part.

Any advice would be greatly appreciated.

Just out of curiosity, if I were to set the design capacity to 15400, how would that affect the behavior of the gauging? Would it even pass a learn cycle?

Thanks, Ross

  • 0
    TI__Mastermind 24870 points

    Hello Ross,

    I don't recommend going above 15400 mAh, that will cause the energy calculations to over flow potentially.

    I would say you can try to calibrate the sense resistor such that 2 A = 1 A reported. that will apply 1/2 scale. You will need to scale taper current, qmax, design capacity, any mAh measures to 1/2 as well.

    Thanks

  • 0 in reply to Kang Kang
    Prodigy 230 points

    Thanks Kang for the suggestion. Is this something you are aware has been done by someone else previously?

    I assume when you list those parameters above you simply mean that ALL current (mA) settings and results will need to be scaled?

    What about power (mW/cW) settings and results? I operate the gauge in 'current load' mode and don't use any of the 'power' related results. Can I safely ignore these?

    Cheers, Ross

  • 0 in reply to Ross Johnson
    Prodigy 230 points

    Hi Kang,

    I found the Application Report (SLUA760) "bq34z100-G1 High Cell Count and High Capacity Applications" which gives a pretty good run down on the subject of scaling for large packs.

    I also found Application Report (SLUA792) "Enhanced Resolution Gauging for Low Current Applications Using Scaling" which addresses the same thing but for the opposite reason (i.e. small packs).

    I'm running a learning cycle now so fingers crossed!

    /Ross