A friend of mine builds remote controlled model steamships. Real boilers, real steam engines. Four to six feet long. Seeking longer run times for the electronics, he paralleled two battery packs. A cell shorted out in one pack lowering the available voltage. That pack became a load for the other one. The end result was fire and an explosion blowing a hole in the fiberglass hull and sending the deck a few feet into the air. It will take hundreds of hours to repair. He has since learned that battery packs should not be paralleled without isolation or fusing. I have seen 3D printed battery cases for the Mavics that hold more batteries. But I do not know if they are properly fused/isolated or not. I would not want to be the person that sent a flaming drone into the forest. Bad image for the hobby......