Hi. It is not a pixhaw or software problem, is a power lay-out problem. What happens, especially in big frames like hexacopter F550, with long power wiring, is that the return of part of the currents from the ESCs, they do it by the thin cables that come from the Power Module to the pixhaw. This generates an inverse voltage on the negative cable, which is subtracted from the voltage measurement, which is 1/10 of the actual voltage, so if the battery has 11.3 volts, the sensed voltage will be 1.13 volts, per This is that a voltage of 0.1 or even 0.2 volts generated in this return, produce the effect that the pixhawk detects 1 to 2 volts less of the real, The effect is proportional to the total drone current
This I solved by modifying the place where the sensed voltage is generated, "translating" the output resistance of the voltage divider of the Power Module. The small 1.5Kohm resistor in the Power Module, which is connected in parallel between the V sense pin and earth, must be removed. Then connect a 1.5 khom resistor between the same wires, but very close to the pixhaw connector. (You must be very careful in this operation). With this the voltage generated in the cable, due to the return of the ESC current, is subtracted from the voltage of the battery and not of the sensed voltage, becoming negligible its effect. The same can be done with the current sensor, but in this case the resistance is usually 110khom or 72khom, depending on the size of the PM. (The value is the one that is measured with a multitester, between Isense and Earth.
After this modification, the voltage and current readings proved to be very reliable and accurate