High Capacity Backup Battery Will Keep You Powered For a Week Let's assume you need to drive around 300miles straight, for some reason. However, for some reason you only get 270-290miles per charge. Would it be possible for something like the above device to give you enough juice for say an extra 100-300miles, assuming there isn't a charging station near by where you run out of juice? In theory, this might only cost an extra $500-1000, if that. Would this further help with range anxiety? Does something like this exist?