"One other thought. If your 3S LIPO is your main pack (supplies motor current too) and it has a charge balancing connector, you can tap the 2S LIPO voltage and use that to power your board. Of course, that will unbalance your pack during use, but if…"

"My APM+IMU draws about 210 ma.  With a 6.5 volt drop across the regulator, nearly 1.4 watts will be dissipated in the regulator.
 
So, how hot is it getting?   I'm guessing pretty hot.  Maybe hot enough that the regulator is shutting itself down to…"

"What Vcc voltages are you seeing when running with the 3S LIPOs?"

"The problem you're experiencing stems from the fact that the reference for the A/D converter is Vcc. The calculation of battery_voltage assumes that Vcc = 5.0 volts. But it is not. On my board, when powered from the ESC/BEC, Vcc is actually 4.58…"

"With the standard code, the current sense output is supposed to be connected to AN1.  Have you done that?"

"Sorry folks.  The "problem" was caused by my testing in front of a big iMac display.  It apparently has a strong magnetic field.  When I walked the test rig to a neutral magnetic location, the yaw readings are what I expected.
 
Newbie error.  Darn.…"

"Addendum:

The auto nulling of magnetometer offsets appears to be working well. After I pitch and roll the board for a minute or so, the yaw reading becomes almost completely insensitive to pith and roll attitudes of about 30 degrees.

In addition,…"