I've written before about solutions to "range anxiety" -- the barrier to adoption of electric cars which derives from fear that the car will not have enough range and, once out of power, might take a very long time to recharge. It's hard to compete with gasoline's 3 minute fill-up and 300 mile ranges. Earlier I proposed an ability to quickly switch to a rental gasoline car if running out of range.

A company called EMAV has proposed a self-propelled battery trailer to solve this problem. While I am not sure how real the company is, the idea has value, particularly when it comes to robotics. As I have written, robocars can solve the "range anxiety" problem in several ways; mainly that robots don't care about how convenient charging is, and people don't worry about the range of a taxi beyond the current trip. But batteries are still an issue, even there.

The trailer proposal has the car hitch on the small trailer (which has room for cargo as well) and it provides the extra batteries you need when dong a long trip. The trailer is also motorized so it puts no load on the possibly small car that is "towing" it. EMAV imagines you might buy this, keep it charged, and only put it on when you need to do a long trip.

That could work, but presents a few problems. First of all, cars are much less nimble when they have a trailer on them. Backing up is much harder, and in fact novices will get completely stymied by it. You take an extra-long parking space if you can fit at all. There's also extra drag.

We might solve the maneuvering problem a bit with a mildly robotic trailer that has a link to the car controls, making backups and turns more natural. This can be done either with steerable wheels on the trailer or just independent motor wheels which can be turned at different speeds. Such a trailer might be able to couple much more closely with the car, possibly going right on the tail so that it acts like an extension of the vehicle. This might also solve the parking problem.

Things could also be aided by making the couple and decouple very simple and easy. That's a tall order because of safety issues, and the need for a high-current wire. The ideal would be an automatic decouple, so you could temporarily drop the trailer off somewhere if you needed to handle roads and parking where a trailer isn't workable. Even better but harder would be an automatic recouple, obviously requiring some more sophisticated robotics in the trailer, and a fully safe coupling system.

With standardization, trailers like this could be left on lots all over a city. Anybody with a compatible electric car could, if they needed it, stop off at a convenient lot to grab a trailer. (The trailer would also be in a charging station, making automatic coupling even harder.) With the trailer grabbed there would be no range anxiety. The trailer could simply provide power, or it could go further and charge the car at high speed, allowing the trailer to be dropped off at another charging station an hour or so later. (While this sounds nice, battery chemistries may doom this plan, since you now are putting two batteries through heavy use cycles to get one unit of charge into the car, doubling the battery lifetime cost of the energy.)

While eventually trailers would need to get back to their base after one-way trips, there are lots of ways to encourage various drivers to do that. As long as the dropped trailer is not entirely empty, you can offer drivers who take it back a ride without using their own battery, for example.

This approach might be better than the battery-swap stations planned by "A Better Place." The Better Place battery swap is cool, but requires all cars that use it be designed around its one particular battery configuration, and that people not own their own batteries. The swap stations are expensive and land intensive, while trailer depots would require nothing but a little land and a charging station for the trailer. A special trailer hitch is a much smaller modification of a car, too.

(One variation of the "PRU" trailer has the trailer contain a diesel generator rather than a battery pack. This of course has the range of liquid fuel, and doesn't even need a charging station where you drop it of. It's not being particularly green when used in this fashion of course, a bit worse than a serial hybrid car. If the trailer is heavy enough it could physically push the car and not need an electrical connection to it, though people might get highly confused by steering in such situations.)

As a cheaper and more flexible version of battery swap, this approach could be good for robocars too. Robots, unlike people, will not feel too burdened by the issues of driving a vehicle with a trailer, especially if they can control the trailer's motors or steering. Parking's easier too, especially if they can do robotic docking and undocking. While I have written how important it is that people don't care about the range of a taxi, the owner of a taxi cares about the duty cycle. If they robotic taxi has to spend too much of its time recharging, the return on investment is not nearly as quick. The trailer approach, like the battery swap approach, means downtime only for the batteries, not the vehicle. If the trailers are themselves simple robocars, they can move at low and safe speeds to come meet robocars that need them for a range boost. Even if not, they need not take up much space and they're easy to scatter everywhere for quick access. Indeed, the car itself might always use a trailer and thus have only enough battery power within it to get from one trailer to the next.