Update power docs with info on how to calibrate

[hamishwillee]

This is supposed to address #119.

@DonLakeFlyer @sanderux I have not actually done battery setup before so this needs a review. As I understand it (and what this says) is that you enter battery cells, and high/low voltage values and the other values are auto-generated. However you can do further calibration by measuring voltage/current and entering these values to recalculate the voltage divider and volts/amp.

Is this correct?

[sanderux]

The other values are not auto generated. You always need to do a calibration of both the voltage and the current divider. Sometimes these are provided with the power module or vehicle.

[hamishwillee]

Sometimes these are provided with the power module or vehicle.

Thanks @sanderux -what do you mean "provided by the vehicle"? Do you mean that the power module will supply all the values?

[sanderux]

I mean the supplier sometimes provides the numbers. Or if you buy a vehicle that is running something like APM you could get the values from that system.

[hamishwillee]

Thanks. I've updated accordingly. It does not talk about the ardupilot "automated" update of values, but could do so if @DonLakeFlyer wants to comment on how "exactly" that works.

OK for this to be merged?

[DonLakeFlyer]

It's Calculate not Calibrate. Same below

[hamishwillee]

Thanks. Fixed

[DonLakeFlyer]

Or if you buy a vehicle that is running something like APM you could get the values from that system.

Can someone point me to how that works? Wondering if there is some missing QGC feature there.

[sanderux]

Suppose you buy a Solo, it comes with APM and already has the calibration values. no need to get fancy current measurement tools to calibrate if you want to switch to px4, just copy the dividers.

[DonLakeFlyer]

Oh. I thought you were saying that there was some ArduPilot coded feature which you could call to automatically set these or something.

[sanderux]

No, but i do have a much better way to calibrate the current sensor.
It uses 2 readings, a low and a high current reading.
Based on that it can determine not only the scaling but also the offset.

cscalib.py.zip

[hamishwillee]

Oh. I thought you were saying that there was some ArduPilot coded feature which you could call to automatically set these or something.

Me too!

No, but i do have a much better way to calibrate the current sensor. It uses 2 readings, a low and a high current reading. Based on that it can determine not only the scaling but also the offset.

I looked at that and it made sense. Worth getting into QGC @DonLakeFlyer

[hamishwillee]

@DonLakeFlyer I fixed that calibrate/calculate typo. Can this be merged now?

[DonLakeFlyer]

yup

[hamishwillee]

Thanks, merged.
FYI, I also created mavlink/qgroundcontrol#5877 to track @sanderux current sensor calibration approach.