Compass Cal required before every flight


I am running AC3.5.7 on a Pix32 Pixhawk PX4 Quadrotor.

I do the compass cal and everything is fine, auto flight is great, loiter is perfect. Flying around manually I get no errors what so ever. If I unplug the battery quickly and plug back in, everything is still fine and I can continue flying.

As soon as I unplug for longer than about 5 minutes, I get the “Pre-arm: compasses inconsistent” warning and have to do another compass cal for it to go away. Then it flies perfectly and does not come back during the flight.

I am using a M8N GPS and compass mounted externally with no rotation. Ive also got compass 2 selected to use which I believe is the internal one.

Ive also tried accel cal and also checked the compass 2 and the external one individually to make sure they where both pointing in the correct direction.

Just not sure why its so intermittent and needs a re cal every time. Could this be a memory thing? Mem batts flat in FC or GPS unit?

The perfect flights and compass cals were also done in the middle of the field away from vehicles or metal.

Other messages are:
Barometer calibration complete
EKF2 IMU0 initial yaw alignment complete
EKF2 IMU1 initial yaw alignment complete
GPS 1: detected as u-blox at 115200 baud
EKF2 IMU0 initial tilt alignment complete
EKF2 IMU1 initial tilt alignment complete
EKF2 IMU0 origin set to GPS
EKF2 IMU1 origin set to GPS
u-blox 1 HW: 00080000 SW: 2.01 (75350)
EKF2 IMU0 is using GPS
EKF2 IMU1 is using GPS

Thanks for your help.

Maybe disable the internal compass and if possible do the Compass/Motor calibration.

If I disable the internal compass, does this have and negative side affects? As in now the FC can only rely on one compass?

Usually the in-consistent compasses message is due to the internal compass having a much different level of magnetic interference. It shouldn’t be dependent on a swap of batteries - so I am confused as to why this error is randomly popping up. Only using the external compass should fix this. Failure rate for the compass modules are very low (never had one fail before) so you should be completely fine.