The barometric altitude decreases the longer I fly even though I am maintaining an altitude in stabilize mode. The logs are attached and I would just like some input as to what might cause this behavior. I suspect I will have major issues in altitude hold as I think the quad may keep increasing altitude to try to maintain an altitude.

Baro drift is caused by atmospheric conditions out of our control.

This log was the same day with about the same conditions but with a different quad/apm. The baro seems to hold a pretty steady altitude through out the flight with no decrease showing. Is there a way to install an external barometer on the apm2.5?

There isn’t an easy way that I know of. You could possibly have a faulty baro. There could be variation caused by temperature drift. We do correct for that, but our correction isn’t perfect.

1-mid throttle could be set incorrect, you should calibrate the transmitter

2- check your IMU acc Z vibration, under 3 m/s*s

2- if you are moving through air, dynamic air pressure increase witch cause to decrease the pressure sensed by baro (physics lesson). So autopilot thinks it is raising so copter would slightly descend. (2-3 m)