Swapping Batteries for Electric Cars

Most proposals for the use of electric vehicles suppose that the owner of the car would own the batteries and would recharge them at home, at work or in suitable battery charging stations. It takes time to recharge batteries. GM proposes that owners of its EV1 also own two chargers. The charger built into the car runs off 110 volts and takes 12 to 14 hours to recharge the batteries. A heavy duty charger runs off 220 volts and can charge the batteries in 3 1/2 hours. These figures come from an article in Road and Track 1997 April. The article also said that the range in their test was 48 miles using the radio and air conditioning and briefly drag racing a Mustang. GM claims 70 to 90 miles.

Someone said the EV1 is the best lemon on the tree.

The currently available batteries are lead-acid, and they give cars with reasonable performance only a very short range, e.g. 50 miles. Many proposals for electric cars seem to be left over from a time when battery performance was expected to be a lot better than it has so far turned out to be.

The Road Ahead for EV Batteries is an article in the EPRI Journal for March/April 1996 estimating progress in improving various kinds of batteries for electric vehicles. Many battery companies are working on the problems, but all of them are relying on the State of California requiring electric vehicles. It isn't clear to me that the electric cars permitted by even the long term goals of the United States Advanced Battery Consortium (USABC) will allow a car that the public will buy except in niche applications.

Maybe a different notion of how electric cars would be serviced would help.

Here's a proposal for battery swapping that would work even with lead-acid batteries.

There are battery swapping stations along roads used as follows.

1. The driver stops at a yellow line.

2. The car is identified electronically both as to type and as to the account to be charged. The identification should be made by radio using the cellular telephone system as soon as the car is identifiable to be the next car whose batteries are to be replaced. This way, the appropriate battery pack for this car can be made up automatically from standard batteries.

3. The swap machine (SM) has enough forward and back adjustment so that the parking doesn't have to be more accurate than is convenient. It opens the battery compartment door or signals the car to open it.

4. The SM is cantilevered from its stand. Whether it works from the side or from the back will probably be a matter of convention, although it may be better to settle on a versatile machine. This would give car manufacturers more freedom in design.

5. The SM is basically a fork lift. It pokes its fork into slots on the bottom of the battery pack and lifts it out after it has signalled a disconnect mechanism in the car.

6. It withdraws the battery pack and puts it on a belt to be moved into the recharging room.

7. Then it picks up the appropriate replacement pack from another belt and puts it in the car, and signals the car to reconnect. It closes the battery compartment door or signals the car to do it. Maybe the SM should use two forklifts so that the replacement pack can be installed while the removed pack is placed on the belt.

8. The batteries are the property of a battery company rather than of the owner of a car. The same batteries are unlikely to ever go into a particular car again.

9. The account charged for the swap is given credit for any charge left on the batteries. George Weinert of of Lawrence Livermore National Laboratory informed me by email that making a "state of charge" meter that will remain accurate as batteries age is a nontrivial problem that may still be unsolved.

   It occurs to me that since batteries deteriorate a cell at a time, it would be worthwhile to have an automatic mechanism in the car that could rearrange the batteries in the pack to isolate a cell that had suddenly gone bad. (They sometimes do that, you know). It might be worthwhile to keep enough fully charged cells to provide "limp capacity".

10. Since EVs have limited range, as much as possible of the equipment for the exchange should be in the SM, so that the car doesn't carry around unnecessary weight.

11. It should be possible for the SM to change batteries in 30 seconds. Doing it faster is not much of a gain, because the car has to enter the replacement station and leave again each time the batteries are changed.

12. Freeways would have drive through battery replacement lanes every 10 miles along the sides, although after a swap, the car might be able to go 40 to 50 miles before needing another swap. The ten mile interval is estimated from cars entering the freeway only partly charged and entering in several places. The time a driver would spend swapping batteries would be about the same as he spends paying tolls in toll highways. The slowdown of traffic would be similar and the bulge in the highway would be similar. The costs of the system would be similar to those of the current system of service stations.

13. The problem of finding a swap station when nearly out of energy will be more acute than that of finding a gas station because of the lower range. Moreover, if your batteries run down, you won't be able to do the equivalent of walking to the nearest gas station and walking back with a can of gas. You will need a roadside swap from an emergency vehicle. For these reasons it would be worthwhile to have the ability to have your car phone automatically locate the nearest swap station (using your location as given by your GPS receiver) and give you directions for getting there. [Second thought: maybe you could walk to the swap station and come back pulling a hand truck with enough of a battery to get your car to the station. A full battery for a car is about 1,500 pounds.]

14. A truly fancy system might monitor the state of your batteries, keep track of the most convenient open swap station on your route and warn you when you were cutting it too close.

15. Many other useful services would be devised by people out to make a buck - or merely liking to exercise their ingenuity - like me.

If the swaps can be done well enough and the replacement stations widespread enough, even lead-acid batteries may be usable in an EV system without great inconvenience.

It probably would not be economic to have swap stations every 30 miles on lightly travelled roads. If one had to drive on such a road, one might stop at a station and rent a battery trailer.

With all this, I oppose crash programs to replace internal combustion engines with electric cars while batteries are so poor. In particular, I oppose the California requirement that a certain number of "zero emission vehicles" be sold by a particular year. However, it does look like one of the disadvantages of such vehicles can be mitigated to a substantial extent.

1997 October update on batteries

Here are a few points of my own.

1. The main force are laws in California, Massachusetts and Europe. Very likely without these laws, battery powered cars would not be considered cost-effective.

2. All the main car manufacturers and battery manufacturers are involved. Claims that car manufacturers are dragging their feet are delusive. There's plenty of competition.

3. Lead-acid is here, and some of next year's cars are still lead-acid. NIMH will be available in some cars next year. Lithium-ion is still several years off. Zinc-air is being revived. There are no promises of reaching the cost-effectiveness of gasoline power, so a near term switch will have to be for air pollution and greenhouse reasons.

4. Some schemes involve battery swapping, but not in such a fancy way as proposed above.

5. Lead-acid batteries have a life of only 200 or so recharges, but of course the lead can be salvaged. One estimate is that this limited life would add 20 cents per mile to the cost of running a car. This would be unpleasant, but almost all present drivers would continue to drive. Lithium-ion batteries in computers seem to have good lifetimes, but computers don't go through deep discharge cycles as automobile power batteries would.
6. My guess is that lithium-ion or zinc-air will be sort of acceptable but will eventually be beaten by liquid hydrogen internal combustion engines for performance reasons. At the turn of the 20th century, there were more licensed electric cars than gasoline powered cars, but performance won out in the end. The coup de grace for electric cars was the electric starter for gasoline powered cars. This eliminated the need to crank engines by hand.

Send comments to mccarthy@stanford.edu. I sometimes make changes suggested in them. - John McCarthy