Why battery-electric cars don’t work

I’ll get to the point quickly: they are not cars.

A car is a device which transports you, your friends/family and your stuff over distances between about 1 and n miles a day. For me, 98% of the time n is less than 50, and 99.4% of the time it is less than 250.

So, for a huge majority of my journeys an electric car which can only do 250 miles per day would be fine. There are two problematic aspects however: one is that if I am not at home overnight I will find it more difficult to recharge the car; the other is the 0.6% of my days in which I want to travel more than 250 miles.

The problem is recharge time. With my current car, I can do about 320 miles between charges and takes less than 10 minutes to recharge. The best electric car I can find can do 211 miles between charges and takes 4 hours to recharge if you fit a custom 70A charger to a 240V supply in your house. If you don’t have that available it takes 48 hours (2 complete days) to recharge with a 15A/120V charger you plug into a normal socket. Presumably this would improve to 24 hours with the 240V supply we have over here.

Recharging the electric car requires 70A × 240V × 4h = 67.2 kWh ≈ 242 MJ of energy. To transfer that amount of energy within 10 minutes you would need a 403 kW charging station: that’s a whopping 1680A on 240V. To transfer that over a copper wire would require that a wire around 25mm thick. Even if your house could supply 1680A (it can’t) you would be unable to bend the charging cable and might even have difficulty lifting it.

The reason the battery-electric cars are not competitive with gasoline/diesel from a recharging perspective is that you can get far more energy per second from a fuel pump than you can from an electrical outlet.

What about an electric car which was recharged using a fuel pump – would that be worthwhile?

Recharging my gasoline car requires 55l × 34.8 MJ/l = 1948MJ of energy. My car weighs about twice as much as the electric car I used for comparison and goes 50% further per charge, so let’s scale up the previous electric energy figure by those amounts and see how much electrical energy it might need if it used an electric motor instead: 242MJ × 2 × 1.5 = 726MJ. This is a very approximate calculation, but it looks like an electric motor would be 2.6 times more fuel-efficient than my petrol engine. If instead of a petrol engine I had a 100% efficient electricity generator and an electric motor I could do 832 miles between recharges, which would be brilliant.

Unfortunately the best fuel->electricity generators suitable for this kind of application are not 100% efficient, but 36% efficient. This means that I would only be able to do 300 miles between recharges, so right now it’s more efficient to combust the fuel in a conventional petrol engine than to convert it to electricity and use an electric motor.

I personally expect the efficiency of electricity-generation to improve very rapidly so that it will soon be more efficient to have a gasoline-fueled electric car than a conventional gasoline-powered car. Although such a car would still use fossil fuels it should be relatively simple to replace the gasoline tank and fuel-cell with a hydrogen tank and fuel-cell once a hydrogen infrastructure exists, without the huge environmental cost of scrapping or building an entire vehicle. I feel that this is the most plausible path to renewable-energy motoring, and I will be near the front of the queue when such vehicles become available.

2 Responses to “Why battery-electric cars don’t work”

  1. James Anderson Merritt

    Cars exist (and near-future cars are being designed) that allow not only charging from an outlet, but also quick swapping of the battery (within the time intervals that are customarily for filling our liquid-fuel vehicles). One example: the forthcoming (now in beta) Tesla Model-S, which can be fitted with a 300-mile battery pack that may also be quickly swapped when necessary. Project Better Place (http://www.betterplace.com/) is working with other manufacturers to establish standards for battery form-factors and replacement procedures, which will allow quick-swapping on an industry-wide basis.

    Most of the time, we can expect the batteries of true electric vehicles to hold enough electricity to carry us around for several days. Plugging them in at night, or whenever we expect to leave them parked for more than an hour or two (at work, e.g.) will be enough to top off the batteries and obviate the need for a full battery swap. But on long-haul trips, battery swapping will provide the “quick fill” that you think is necessary for EVs to be considered as “real” cars. If I can get a Model-S with a 300-mile battery pack, that’s plenty “real” enough for me, even without a quick-swap option.

    Reply
  2. Bob Tay

    What happens to that 200 mile range when you are driving at night in lashing rain and the kids are frozen and need the heater on….then you encounter a tail back..
    You are now powering
    A heater
    window wipers
    headlights
    radio
    and your car.

    Reply

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