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PTC in the oinment - another fine mess?

Michael Banak
Intellectual 915 points
Other Parts Discussed in Thread: BQ20Z90

I inherited a design in which a PTC is placed in series with the lowest battery in the stack. The bq20z90 system thinks it is monitoring that lower battery, but in fact, it is monitoring the cell voltage plus any drop across that 40mOhm PTC  At Chg/Dsg current levels, it provides a noticeable offset, mid-tens of mV, and I see the CB bit flaring away, indicating a continuing, fruitless attempt at cell-balancing.

I think this is bad. Would anyone smarter than me happen to know what to expect with this dumb 40mOhm resistor sitting in series with my lower cell. i think this is going to skew the z-track, the cell-balancing algorithms and thus my capacity. What can I expect long term?

Golly, I wish they had put that PTC elswhere!

If I am fretting over nothing, I wish someone would tell me.

Regards,

Michael A. Banak

  • 0
    TI__Intellectual 825 points

    Michael,

    The PTC may seem to impact the gauging operation in three aspects: Impedance measurement (Ra table), capacity measurement (FCC and RemCap), and cell balancing. So let's take a look at each of them.

    The added resistance to the bottom cell is true. The gauge would in no way tell the difference between the PTC and the cell impedance, so it would actually learn a higher Ra values than the other cells. It is as if the bottom cell has more resistive electrodes.

    The FCC and remaining capacity estimates are dependent on the cell impedance. With a increased bottom cell resistance, at given temperature and current, the predicted FCC and RemCap are going to be less than normal. But this is a good judgement because any series resistance will increase the voltage drop that appears at Pack+, which then decreases the energy output.

    The impedance track cell balancing algorithm is capacity and SOC-based. In other words, it depends on how far each cell is from the full-charge state. The Z-track algorithm does not use impedance information to calculate the capacity and SOC, which means that cell balancing has nothing to do with the impedance. (SOC is tracked by coulomb counting, and when battery is at full relaxation (= no current load), the cell voltage is used to correlate SOC. The full chemical capacity, Qmax, is learned when battery collects two OCV readings under full relaxation with a charge or discharge between them, and the charge/discharge is gauged by the couloumb counter. )

    So in summary, the big picture now is that you have a more resistive bottom cell. The higher resistance in this cell will decrease FCC. In addition, in both charging and discharging, the voltage drop on this cell is higher than those from the others. You may observe that right before charge termination this cell has higher voltage, but as the current tapers the voltage difference is diminishing. Similarly, when approaching full-discharge, the bottom cell may have the lowest voltage among them, but once current is cut-off, all three cells should have less difference in voltage.