I first filed a report in the pixhawk family board because this happens to be on a pixfalcon and some testing shows that it's unable to read a voltage below 0.1V on the current sensing wire of the power module connector.
Because my Amps per volt is around 21.4, basically it sees no current at all until I'm using about 2.5A
That post is there: http://discuss.ardupilot.org/t/pixfalcon-doesnt-read-any-amps-until-about-3a-and-then-power-module-is-not-linear-beyond-that/17248
But then, a current to volt table shows me that even beyond 3A, there is an issue with power modules not giving linear values for current.
At 3A, amps per volt is about 25, and it slowly goes down to 21 as I reach 10A+, so even putting the pixfalcon sensor issue aside, measuring straight off the power board shows that AP needs some kind of non linear table to get proper amps from at least some sensors since their amps per volt is not linear.
I have 3 power modules, one from 3DR and 2 clones, they all look like this
I've also seen that with other power modules.
I've definitely see the non linear option with sensors plugged into a pixhawk too.
For now, I set 21 Amps per volt, and about 11.5V per volt with that sensor, and all those sensors send data via power module port in pixhawk or pixfalcon
The table below shows the pixfalcon seems to have a flaw where it cannot reliably read the voltage from the power module when it's too small (0.037V for 0.45A, 0.12V for 3A), but beyond that the sensor is also not linear:
A was measured with an amp meter off the battery
V was measured on the power module between Gnd and I
V A A/V
0.037 0.48 13.0
0.040 0.96 24.0
0.059 1.50 25.4
0.068 1.75 25.7
0.086 2.15 25.0
0.105 2.60 24.8
0.120 3.00 25.0
0.200 4.50 22.5
0.370 8.00 21.6
0.530 11.0 20.8
0.670 14.0 20.9
0.800 16.6 20.8