It is widely accepted that a barrier to the uptake of electric cars is the lack of a readily accessible network of chargers. If the only place to charge up is at home, it will severely limit how far a private motorist is confident of driving.
Another limitation is the speed of charging. A full charge of an exhausted battery takes six to eight hours, something quite difficult for drivers used to filing up with petrol in five minutes to get used to.
The main factors constraining speed are the electricity supply infrastructure and battery characteristics. Fast charging of electric vehicles – such as luggage trucks at airports – is routine in industrial settings.
However, the scale of the problem of achieving filling station rates of recharge is illustrated if you consider that the effective rate of energy transfer in filling a tank with diesel in three minutes is 10MW. The biggest current wind turbines are capable of generating 9MW. Even a power station the size of Drax, which generates 4,000MW, would only produce enough electricity to refuel 400 cars simultaneously at this rate.
Charging points installed in homes and the 280 public points already installed around the UK (of which 220 are in London) use the standard 240V, 13A supply to provide a 3kW charge rate and provide a full charge of a typical electric vehicle battery in six to eight hours. According to Dave Greenwood, project director for advanced technology at Ricardo, the domestic supply infrastructure could cope with charging at 32A without significant investment, and this would reduce charging times by more than half. To go beyond this, however, would require considerable upgrading work and wouldn’t be cost effective.
Charge rates of 63A can be achieved with a three-phase supply, which is available on industrial and commercial premises. This type of faster charger would thus be more suitable for installation at off-street sites such as car parks. A three-phase, 63A supply would allow charging at a rate of 45kWh, cutting the time of a full charge to around half an hour.
Even faster rates can be achieved using a 500-600V DC supply. This is done in industry, but would need dedicated, supervised sites such as filling stations, which would likely charge for their service.
At faster rates of charge practical problems start to assert themselves. One is the size of the cables and connectors needed. A 45kWh charger would need cables with 10mm2 copper cores, said Greenwood, and these would be very difficult to handle.
The battery also imposes limits. Charging generates heat and if the battery gets too hot its life is shortened. Charging is typically 95 per cent efficient, so five per cent of the energy used in charging appears as heat and must be dissipated. ’At 13A that’s not too bad, but at 45kW, five per cent is quite a bit of heat to get rid of. The temperature of the battery must be limited to 60-70˚C, on a stationary vehicle,’ said Greenwood. The charging process also becomes less efficient at faster rates.
The heating effect is proportional to the square of the current, so reducing the current and increasing the voltage at which charging takes place helps reduce the heat generated. But as each cell needs a fixed voltage across its terminals using a higher voltage requires more cells to be used, making the battery more expensive and creating greater potential for failure.
George Paterson, a spokesman for battery pack manufacturer Axeon, said: ’We could design a battery that could charge from 20-90 per cent within six minutes and not heat up more than 15˚C, but the price would be prohibitive.’
Energy boost induction charge
More convenient charging systems are currently under development
A big leap in the user friendliness of charging electric vehicles is expected within the next five years or so with the advent of induction charging. In this system a large electric coil buried in the road surface at a parking bay or in a garage floor charges the battery of a vehicle parked above it by inducing a current in a coil in the vehicle. Its big advantage lies in eliminating the need to connect to a power supply.
Development work is looking at the question of efficiency and what level of ground clearance is needed while still achieving a good rate of charge. Early applications could include loading equipment and material handling vehicles that frequently return to a loading bay, for example, and would receive a small ’opportunistic’ charge every time they were there.
Elektromotive’s Greg Simmonds sees it as being suitable for recharging taxis in cab ranks.
There is also the question of standardisation – ensuring that the car and the charging infrastructure must be compatible with each other. Today’s on-street charging points are in fact just electric power points, with the battery management electronics situated on the car. Car manufacturers are unlikely to want to provide the electronics to deal with multiple charge rates as standard.
For the very high rates, the charger power electronics become too large and heavy to install on the vehicle, which is another reason why high-voltage DC charging will have to take place at dedicated sites.
Paterson said the electronics in a charger supplied for Modec vehicles, which provides 12kW from a three-phase 32A supply, weigh more than 140kg and are ’the size of an industrial electric distribution cabinet’. This is too big and heavy to carry around and is installed at the charging point.
Already different vehicle manufacturers are going in different directions.
The Mini E, currently undergoing trials with private motorists, is provided with a standard adapter for use with a 13A supply but also with a garage-installed wall box, providing a 240V/32A supply, giving a full charge in 4.5 hours rather than 10.
The Mitsubishi i-MiEV city car, also undergoing trials, can be charged at 13A or via a 500V DC charger supplying 50kW. This allows the battery to be charged from flat to 80 per cent capacity in 30 minutes. While charging, air-conditioning fans are used to cool the battery.
Last December, Mitsubishi installed the country’s first fast-charging unit for the car at its UK headquarters.
The London mayor’s strategy envisages the capital’s public charging points being a mix of standard 3kW points, ’fast’ 7-43kW using a three-phase supply, and ’rapid’ 50-250kW DC chargers that will top up a near-empty battery in 10-20 minutes.
Greg Simmonds, technical director of Elektromotive, which has supplied most of the on-street charging points installed so far in the UK, said: ’In less than two years, cars on the market will be able to charge at 7kW. In the next five years we’ll see cars with AC and DC charging capability.’ Semi-public chargers of up to 21kW AC using the three-phase supply will appear off street in car parks, he added. Within the next five years it will also become apparent, he believes, whether there is a viable business model for DC fast-charging stations.
The psychology of how fast-charging capability affects driver behaviour is complex. In Kanagawa, Japan, a trial involving i-MiEV drivers showed that people started using the cars much more after rapid chargers were installed. While a similar study in Tokyo noted that use of fast charging dropped off after a few weeks as participants became used to the car’s range and capabilities.
Ricardo’s Dave Greenwood questions exactly how quick fast charging needs to be. The average daily distance covered by UK motorists is 38km, which would require just 2.5 hours of daily charging at 13A. ’My gut feeling, supported by the Tokyo evidence, is that the fastest rates of charging aren’t necessary.’
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