At 5 degrees C I flew the aircraft down to 18% battery capacity. The last 15% at low hover. I landed, but my intention was to drain the battery down to 7 %. The wind was picking up a bit, so I brought the Phantom close to my house in shield by the entrance door, surrounded by obstacles. This was approximately 35 m from my home point where I landed. My next step and plan was to start it up and run on idle until I reached 7 %, propellers on. Everything looked good for about 15 sec. Then the motors increased in RPM and the Phantom took off. I grabbed the controller and tried to get control, but did not. Control inputs did not give any results. I pressed the RTH-button and then l regained control. Height was about 16 m. Landed immediately (safe, 52 sec/14% left), knowing I was low on power. This sequence took about 15 sec. After the incident, I think I realized what happened (replayed the flight). When the battery capacity hit the H-icon on the timeline, the Phantom decided to go home. 3 sec after start: Tips, RC assisted takeoff (visible for 5 sec). Then, after a total of 13 sec: Tips, low power, returning to the home point (35 m away). It took off. According to the manual, you do not have any control until it reaches 20m height. I have learned a lesson, but have not seen any warning about this in the manual. The software logic should prevent the Phantom to take off when low on power and idling on ground.