I had a fairly scary incident today... I was out on a property and did a compass calibration, took off and flew for the duration of the battery with no problems, then landed at 25% to change batteries. I shut down the aircraft, but not the controller or iPad, changed the battery then switched the aircraft back on, then started the motors and took off again and that's when the trouble started. The aircraft was almost uncontrollable. I say "almost uncontrollable" because the sticks did actually make the aircraft move around - it made it respond, but very erattic and it flew almost at full speed most of the time. I had it going back and forth in a criss-cross pattern for a while as I tried to work out a pattern whilst making sure it didn't get too far away from me. I realized I could keep it in a basic pattern with this back and forth motion. Incidentally, ATTI mode made no real difference. I decided to lower the altitude from where I had kept it (about 100ft) and managed to bring it down to about 20 ft and brought it basically over the dirt road where I brought it down and landed it on the road safely, and shut the motors down, but left the aircraft switched on. I then checked the sensors and calibration readouts and the compass was down at about 1300, and not around 1500 where it normally is. the Accelerometer and Gyro readouts were about normal (between -1 and +1) . I then decided to do a compass calibration as the aircraft was still switched on, and the battery at 50%. After this calibration, the compass readout was back at around 1500. I decided I needed to know if this was the problem and it would be much more conclusive if I took off again whilst it was still the same continuous session, so I started the motors again, set the home point and took off and it flew beautifully again. I flew around with full control for the remainder of the battery and brought it back in and landed with no problems. So I guess the confusion in my mind is the experts who have very definitively stated that you do not have to calibrate the compass every time you fly, and some say it can be harmful to do so. I'm now thinking that I will do a compass calibration even after a battery change and even if it is in the same position as the last battery. Do any of you have opinions on this - had similar experiences etc?