Charging in the Electric Robocar World

I've suggested that robocars enable the electric car by solving the battery problem. People today demand long range from their cars, and that requires a large, expensive and often heavy battery pack. They also are bothered by the idea that once an electric car runs out its range, it must sit for hours charging before you can use it. They are scared of losing the effectively "infinite" range of a gasoline car that just refuels at a gas station in 5 minutes.

The first step to enabling the electric car through robocars is to realize that when taking most robocar trips, you will know in advance how far you're going. If you are taking a series of trips you will specify that. You can then be sent a robocar that has enough range to make that trip. You don't care that after it's done it will have to limp off to a charging station for 4 hours. That's not your problem. Even if it's your car.

If you do need a car again soon, and your car is low on juice, you might hire a robotaxi, possibly arranging for your personal stuff locker to be exchanged first. That taxi will handle your needs.

And if you do need to go cross-country, or have not settled on your destination, then you'll get a liquid fuel (perhaps gasoline) car that works the old fashioned way, and can just refuel quickly and go.

The charging downtime of an electric car does remain a problem. There are a few ways it can be dealt with:

1. For vehicles that get a lot of use, it may make sense to have them come with a swappable battery cartridge. They can go to a depot (anywhere not too far away) that offers a battery swap. In this case the batteries would be owned and maintained by the depot company, not the robocar's owner. This makes a lot of sense for a robotaxi that you want to have working all day long. The "refuel" will take similar time to a gasoline fill-up. The company "A Better Place" is working to install battery swap stations in many locations, notably Israel.
2. Alternately, people have proposed and even built simple trailers that hitch on the back of an electric vehicle. The trailer can either be full of batteries or have a gasoline engine. The trailer is motorized so that the car does not have to be powerful enough to pull it, and in fact the trailer can charge the car by pushing it in the car's regenerative braking mode. The main downside is that trailers make it harder to backup and park -- but robotics can help with this. A small trailer might even clamp right on the back rather than being on a ball hitch.
3. For vehicles that don't get a lot of use, it may be perfectly fine to have them charge slowly. In fact, if you know they won't be used more than their range that day, they can wait to charge at night, when power is cheaper. Or, if needed, they'll charge with more expensive day power -- whatever makes the most economic sense.
4. In many cases, if a vehicle is down charging, and it is needed, the owner can just summon a robotaxi that is charged enough for the trip they need. You don't get your own car, but in most cases this may not matter.
5. For cars with moderate duty cycle, "Rapid charge" centers can offer charging at very high current, so that a battery pack can be recharged in 10-15 minutes. This technology has already been proposed for the general electric car market, but it can serve the electric robocar well.

When you charge depends on your own economic valuation of how much cheaper night electricity is, how much extra battery packs cost or indeed how much extra robocars cost. Some robocars may be so cheap that it's simpler just to leave discharged ones idle while fresh ones go into service.

While the ideal is to use a vehicle that has no more battery in it than a given trip needs, in reality this will not always make sense. So vehicles will come with capacity for 2 or 3 trips if that's what people need. Adding battery capacity adds cost and weight, but delays charging and makes the vehicle more flexible.

With swappable cartridges, it's also possible to swap in cartridges of different capacity. So a car being sent for a 10 mile trip might swap in just enough juice for that, to reduce the weight.

Charging Stations

In an electric car world, a charging station is nothing fancy. In fact, it will just be a pole in some parking spot. The robocar will drive up to the pole (very accurately) and then extend a robot arm to plug into it. It will send some data over the line to authorize payment for the power, and start getting juice. Such poles could be found all over the city.

(Actually, it need not even be a pole. It could be recessed in the ground, so that the car's arm reaches down while over it to remove the plate and plug in. So these could sit at the side of streets, or be embedded in street parking spaces.)

Robocars could also plug into ordinary power plus for very slow charging. This might create a bit of power theft, but the charging is so slow I think this will mostly be done in garages.

Rapid high current charging will require special stations. So will battery depots. As gas stations close, I suspect we'll find a place for them.

Power Sources

As noted, nuclear and coal plants don't alter their output very quickly. They prefer to run constantly, but power demand is much lower at night. Power at night can be much cheaper than power in the daytime. People charging batteries, be they car owners or battery swap depots, will decide what makes the most sense -- own more batteries/cars and charge them all at night, or own fewer, and do some daytime charging. The market will figure this out.