[Ed: due to technical problems, Part 2 of this article will now be published tomorrow, 31st Dec 2019]

 

Being the owner of an electric car (a Mitsubishi I-MIEV) I thought I’d set out a few observations.

Costs

Although there is a government subsidy at the moment, electric cars are expensive to buy and there is massive depreciation. However this does mean that, second hand. they are more affordable.
The new cost of my own car was around £30,000. A government subsidy at that time was £6,000. I bought it as a one year old (ex-demo) from the dealer (Hereford) with 375 miles on the clock. It had depreciated by £7,000. Unsurprisingly, they had not succeeded in selling a single car at full price.  At that time, it was the cheapest electric car on the market.

This, by the way, is a small city runabout which was modified from an earlier petrol form. The reason for such massive depreciation is the perceived risk of battery failure. A new battery for my small car cost £7,000 at that time. Ergo, battery failure makes the car instantly worthless in cars less than nearly new. This happened to me at 16,000 miles, fortunately the battery was still under guarantee.

The Technology

There is an electric motor, a control system, a battery and a battery charging system. The inherent problem is that batteries supply Direct Current (DC) and the motor runs on Alternating Current (AC) as do all electric motors. 

In days of yore, conversion of DC to AC (and vice versa) was done mechanically by a device known as a “commutator”. These are still in use though they are inefficient, expensive to make and subject to wear. The problem of converting DC to AC electronically has finally been solved. The problem is that the waveform of the generated AC must be a sine wave. If it isn’t, there are significant losses. Also three separate waves must be created (three phase AC) in order that the motor is self-starting, easily reversible and compact. The speed of the motor depends on the frequency of the generated AC. 

This means that the electronic device (known as an inverter) must be able to generate a three phase, variable frequency, sinusoidal AC and also be able to control the voltage. This is a big ask and today’s electric car technology revolves around this.

The traction battery

We have had low tech electric vehicles for over a hundred years. The problem has always been that of battery capacity and weight. The recent development of lithium batteries mean that batteries of half the weight can hold three times the energy.  Lithium is quite a common element but only in a few places have natural events concentrated it in commercially viable deposits. There’s not enough to go round for everyone to have personal transport. Also needed is cobalt with a similar problem. There is a lot of cobalt at the bottom of certain oceans. Recovering it is certain to be controversial.

My own car has a 16Kwh battery weighing nearly a quarter of a ton. Compare this with a petrol engine car which carries around 100-200Kwh in the form of petrol. It’s immediately obvious that with so limited an amount of energy we are going to have to be pretty frugal and efficient in its use.

The traction battery is generally reckoned to lose around 1% of its capacity per year. This could be a lie, the public has no way of verifying this. Also the charging regime can affect battery life.
If buying, be sure to enquire about the battery guarantee. Some manufacturers lease the battery out for a fixed monthly payment. Some sell you the battery. It’s optional with others. You need to sharpen your pencil and do the calculations.

There is also a small 12V battery to run lights, windscreen wipers etc.  This is charged off the main traction battery by yet another electronic gizmo. If flat, the car can’t be started and neither indeed can the main traction battery be charged.

The motor

Big steps too have been made in the efficiency of electric motors. It should be understood that all electric motors can also be generators, there is no difference between the two.

Traction motors work by the interaction between an alternating or revolving magnetic field (or flux) and a fixed one. The stronger the magnetic fields are, the more compact and powerful the motor will be. There have also been big advances lately in motor design. In days of yore the fixed magnetic field had to be generated by a coil of wire which consumed electricity. Nowadays we can produce the same by means of  neodymium-iron-boron rare earth magnets, so saving the electricity previously used. As we have seen, the commutator has also been dispensed with. Electric motors used for traction must have independent means of cooling, a simple fan on the motor shaft won’t do. The motor could be running at low speed yet still using full power thus needing maximum cooling. This usually takes the form of a water jacket round the motor and a conventional radiator.

Regenerative braking

As previously mentioned the motor can also be a generator.  This means that when descending hills the “potential energy” can be recovered by converting it to electricity and putting it back into the battery. Also, when the car is slowing down, the “kinetic energy” can be recovered. So if you drive up a hill and then descend, about 25% of the energy used going up can be put back into the battery on the descent.

In practice,  the driver takes no action to enable all this. Speed control is by a “single pedal” If the foot is lifted the car goes seamlessly into regenerative braking mode. This means that 99% of driving is by single pedal. The mechanical brakes need only to be used in an emergency and to bring the car to a halt from 5mph or so. Light pressure on the brake pedal also brings regenerative braking into play.
My own car has a lever to switch braking and power modes for different terrains. There is a “city mode”, a “hilly/winding lane mode” and an “open road” mode. The result is an intuitive and relaxing drive.

 

[Ed: due to technical problems, Part 2 of this article was not published as planned on Sat 29th Dec, but will now be published tomorrow, 31st Dec 2019]