I am using a 3DR robotics X8 model 2013, with 2 batteries in parallel( each one: 4200 mAh, 3 cells. I get a flight time of 7 minutes (voltage drops to 11.1 V). I always have an additional weight of 300 grams. I need more flight time. Do I have to change the batteries? buy a new one with 10000mAh.