I had a browse and found many conflicting information on how to power the APM board from the external external power connectors while unsoldering the jumper.
MY ESC's are only rated to 500mA and I am afraid to fry them as they do get very hot just from powering the APM. Given the conflicting messages online, I had a look at the Schematic of the APM and Looked at the circuit. Looking at the LM317 specs, powering the board with max 12-13V from the Lipo should not be an issue as its input and input/output delta voltage is well rated above.
I tried to power the board from the 3S flight Lipo, however the the board starts behaving weird and keeps either rebooting or never comes out of calibration.
I am at my wits end. Is it the IMU that has additonal circuitry that is interfering ? Could not find PDF schematic of the IMU to trace the power management. The other thing I was thinking about is the resetable fuse playing up ? However, its after the LM317 so, the current draw should be any different between a 2S or 3S LIPO.