Hello,
How does bq27421 learn the true capacity of a battery if a wrong initial value is entered?
For impedance track algorithm to work it must know true SOC. If the true capacity is 1400mAh, and one incorrectly enters 2000mAh in the beginning, how does the device correct this value?
The difficulty exists because the gauge is NOT an “active” component (by the analogy with a transistor) in the sense that it could stop the current of the entire PCB board in order to measure OCV. If it could, then it might conveniently cut off the current at one instant, measure the OCV to corresponds to, say, 85% of SOC, and charge for a while, then cut off the PCB board current again, measure another OCB corresponding to say 92% of SOC. Then the charge Q during the interval dive 7% would give the true capacity.
But this is impossible since this would disrupt the normal operation of the entire device. A compensation for this seems to be the existence of “Quit current, Discharge Relax Time, Charge Relax Time”, in which the gauge attempts to take OCV readings, because at such low current (40mA default) the voltage drop due to internal resistance becomes more or less ignorable. Therefore “relax mode” readings are essentially substitute for true OCV reading which might never be obtained in real running. Is this what “relax mode” is for?
However, such a quit current might not be realizable. For example, if the device draws 1A constant current (like a resistive load, this is what the title suggests) throughout its operation, and charges exactly 1A back to battery before tapering in charging mode, then one never runs into discharge relax mode. Then the gauge would never have chance to read what is close to the true OCV, and will remain throughout the entire lifetime of the whole device.
This seems to imply such a necessity chain:
- in order for bq27421 to work, we must be able to enter relax mode;
- but in order to enter discharge relax mode, the whole device cannot draw constant current. It must be designed to appropriately switch to low current (<=quit current) by itself to enable gauge’s accurate OCV (although not exactly “open”) reading.
And this additional overhead on the software end seems to be a requirement of all impedance-track technology gauges?
Hui