I would love for Tesla to offer a portable battery that can be carried in the car to extend the range. (Think iPhone battery case.) Perhaps a power wall that fits in the storage space under the rear deck or lays flat in the cargo space. It should plug into the main battery system. Charge the accessory battery when charging the main battery. Even if only 15-20 kWh would add significant range for a road trip. Any reason this is technically not feasible?