Dear friends, I have a flying wing Phantom FX61 equiped with a Pixhawk, power module and a MinimOSD. The problem is, when you connect the battery , the MinimOSD and the APM Planner shows good voltage, lets say 12,5 volts on a 3 cell LiPo Battery. Then you arm your plane (Pixhawk armed), the voltaje remains ok, around 12,4 V. The problem begins when you put WOT and maintain it, the WattMeter connected to the battery shows a stable 11,8 V, whit a current drain of 12,5 Ah, but suddenly the OSD and Mission Planner begin to show a steeply down to 10,5 and shortly to 9,5 and less Volts, but in the Watt Meter steal you can see a solid 11,4 V and good power, but the Pixhawk enters in Voltage Fail Safe and RTL mode. I tryed with the callibrating mode of the Mission Planner, but the Voltage only operates well when you have no consume, once you put WOT shows the same behavior. What can I do?
The Pixhawk/Power Module combo senses voltage by using a simple two resistor voltage divider. It could be that heating of the nearby current sensing resistor is warming the voltage sense circuit and changing the divider resistance values.
You didn’t mention if it’s a real 3DR Power Module. If not, that could be an issue if quality control wasn’t adequate. Swapping it out would make your problems go away.
It also could be a poor surface mount solder bond on those resistors. Were it me I’d probably have a close look at the board and maybe try and remelt and reflow the soldering to R1 and R1, the voltage sense resistors.
You also didn’t mention how big a current draw you typically run. If you are operating at the higher end of the Power Module’s current range, you might see issues there, especially with heating. An alternative would be to use the Attopilot current/voltage sense board with much higher current capacity.
You can always turn off the voltage failsafe so you don’t get the RTL action (I keep mine turned off). You’ll still see the voltage on your Minim, and if you want to know true voltage, just cut back the throttle for a second. In any case it’s almost certainly an issue with your Power Module and not your Pixhawk.