I was flying my Mavic Pro this AM in a very large soccer park (legal fly zone). At the normal low battery setting of 30%, the controller notified me as always. I set the low battery warning at 30% to give me plenty of time to RTH should I be out further than 1500 ft. However, at the 30% warning I was directly overhead. When I confirmed RTH it asked me if I wanted to return to HOME POINT or CURRENT LOCATION. Never did that before. It normally would beep and do it's return to home sequence. I hit RTH and the Mavic began to move away from me. I did not panic. I canceled the RTH. It then warned me CRITICAL BATTERY although it had 27% remaining. It began to land. I could not exit, I could not go forward or backward, up or down. Lucky for me the Mavic landed in an open field about 400 feet away. I search via the map and luckily I was able to find it with no damage. I ended my flying for the day and retuned home to check it all out. As an added note: I always do a preflight check of all settings and the aircraft to be sure there are no issues including any and all updates to firmware. It flew one full battery with no issues then on the second battery the issue happened as described. Any idea's?