When you buy an electric car, you can often choose among various battery sizes. The larger your battery, the more range you have -- and you may get some extra performance -- and the longer your pack lasts, but the extra capacity is very expensive and adds weight to the car. The truth is, most people only need the extra capacity of a long range car when doing road trips. A modest 150 to 200 mile range car is sufficient for driving around a town, depending on the town.

A good answer would be to design the car to allow an additional rental battery pack. This pack could go in a special mounting rack in the existing battery rack, or it could have a place to mount in the rear or front trunk. It would need wiring and cooling tubes.

A 15kwh pack -- good for about 60 miles of extra range -- would weigh about 80 kilograms (175lb) so this has to be a deliberately designed system. Connection might need an authorized service team if you want to be sure it's right. The pack would cost around $3K at today's prices. But you could rent it for a more reasonable rate only when you want to do a road trip. If it's $100 to rent it takes 30 road trips before renting cost you any money, and the value of not carrying that weight the rest of time.

It turns out that getting closer to 300 miles of range does make a difference on long road trips, especially in the west. It opens up more places you can go, more ability to do diversionary side trips and the ability to explore more beyond the established charging networks.

For the shorter range cars -- the ones with 70-150 miles of range, this is also possible, but the pack would be quite large and heavy, and would take more significant space in the trunk, if that's where it goes. With that size and weight, a service center install/remove are likely, and the rental fee would be higher. However, if you consider how much cheaper these short range cars are, it's likely that rental fees would still be quite a savings compared to buying a longer range car as long as road trips only take place rarely. In addition, the car without the pack will be much lighter, more efficient and have extra space.

Even people with long range (300 mile) cars might even consider this option. Adding 60-100 miles of range would offer tremendous flexibility on road trips, and access to roads that simply are not practical today, such as U.S. 50 across Nevada. There, the distance between Tesla superchargers, at 550 miles, is too large even with an extra pack, but a large pack and a stop of a few hours at an RV park could make it doable.

Generators

The main option that people have explored in this direction has been generators. Most plug-in hybrid cars are "parallel" hybrids where the gasoline motor drives the wheels directly. The BMW i3 REX is an exception -- it has a small generator with a motorcycle engine built in, and a 2 gallon gasoline tank. The generator doesn't drive the wheels, but it provides enough electricity to barely drive. In the USA, the car is programmed to only use the generator once the battery is low, leading to anemic driving and very poor hill climbing. For the driver, it's much nicer to offer the European option, where you can set the generator to run at the start of a long trip to keep the battery topped up, and then switch it off when you are in range of your destination.

In the era of all cars being short range, I wrote about towable trailer generators and packs which had been proposed. Towing a trailer just for that is a burden on road trips, as it makes backing up and parking a pain, but they are much easier to rent.

One could design a car so that a temporary generator/tank unit could be inserted in it, again probably at a service station. The simplest approach might be to make the generator live on a trailer hitch style mounting, making it easy to deal with heat and exhaust. A 20kw generator could support sustained 55mph. The BMW's, at 34hp/25kw, is slightly larger. It could also be possible to go much smaller. If you consider a 10kw generator, which might sustain only 30mph, you could turn it on at the start of a trip with a 260 mile battery, and 4 hours later it would have added 120 miles. That's a nice boost for non-stop driving, but real driving usually contains many stops where the generator could run. The generator could also fully recharge the battery at night. This might not strike you as a very green option, but it's actually lower in fuel consumption than almost any gasoline car. Combined with the ability to use DC fast superchargers, and occasional overnight charges at hotels, it could produce a very low emissions trip from a vehicle with the "go anywhere" flexibility of gasoline powered cars. A 10kw generator could be small, cheap and light -- its main issue as that smaller generators tend to be less efficient.

As a simple calculation, if you specify a maximum daily range of 500 miles, and your battery has 250 miles, and your travel day is 12 hours, then you need about 70 kwh. If you did it perfectly, 6kw could do the job, though when you are stopped or otherwise putting power into the battery rather than the wheel motor, you suffer battery losses, and so might want 7kw, or around 12hp mechanical energy.

In theory, a Stirling engine generator could be a highly efficient and quiet choice, but in practice these are not made in mass volume anywhere. Stirlings are no good for driving cars because they take time to fire up and can't accelerate, but they're great for providing constant power like this. They can use almost any fuel that burns, too.

Regulations

It should be noted that regulations might stand in the way of a generator pack, because they would convert the vehicle from a pure electric to a plug-in hybrid. In some cases that might invalidate your ability to use things like carpool lanes, though in practice, plug-in hybrids are still treated like BEVs but that won't last forever. As long as the generator were truly a temporary add-on, one would hope the regulations would be kind as this is still much greener than a full-time gasoline car or hybrid.

If necessary, software in the car could prohibit the number of days per year the generator could be attached.