Electric Vehicle Charging

There is little doubt that one of the key barriers to the adoption of electric vehicles is the issue of charging.

Most of the housing stock in this part of the world does not have a driveway or garage, and this creates issues with how and where to charge an electric vehicle. Some authorities have talked of a mass rollout of public charging points, but this seems to overlook the fact that charging takes a substantial time.

There has been media talk of fast charging being developed in order to make recharging an electric vehicle almost as convenient as liquid refuelling. I think we need to do some mythbusting on this one.

The Nissan Leaf, a short range vehicle, has a 24kWh battery. Longer range vehicles like the Tesla Roadster, 54kWh.

To put this in perspective, 24kWh is the energy required to run a small electric heater, or ten old-fashioned light bulbs, for 24 hours.

The  standard domestic electrical outlets in various countries do differ in their maximum power outputs, however the 13A, 240v UK style is typical and delivers a maximum of  3120 Watts, or 3.12 kW.

To deliver 24kWh from such an outlet will take 7.6 hours.

To deliver 54kWh would take 17.3 hours.

Since a full charge will take slightly longer than that, we could say 8 hours to change the Leaf, 18 hours to charge the Tesla. 

If we wanted to fully charge either car in an hour, we would need a current of 13x7.6 or nearly 100 Amps for the Leaf, 13x173  or 224 Amps for the Roadster. 

Those are quite substantial currents. The 100A for the Leaf's one-hour charge is just about within the capability of UK domestic supplies, but you'd have to avoid running any other high demand appliances at the same time.  The one-hour Roadster charge is well outside of UK domestic capacity, and somewhat outside of the heavier USA domestic supplies, which are usually 200A at 240v. European houses often have much smaller supplies than UK or USA houses, and achieving any kind of fast charging there would be a major problem.

Cables to deliver these kinds of currents to the car will be heavy and stiff, and require a fair amount of strength to manipulate. Long runs would not be advised owing to voltage drop and cabling cost issues.

Now let's take a look at the proposals put forward by some electric vehicle promoters, of super-fast charging in only a few minutes. If we take six minutes for the sake of argument, then that requires a further tenfold increase in current. The Leaf's cable alone, for a small runabout, must be able to handle a thousand Amps. For the electric 'muscle car' we'd be talking over two thousand Amps. 2240 to be precise.

The sort of cables required aren't even listed in the domestic catalogs. You'd need to go to a supplier of electricity distribution equipment, for the likes of a substation cable, to get a price. Apart from which, such cables are extremely heavy and stiff, and are not normally connected by plug and socket. The mere idea of having to connect and disconnect such a cable every day, is silly.

Of course, that is at 240v. If we were to up the voltage, then the amperage and hence the size of cable reduces. Higher voltages introduce greater safety hazards of course, and any connectors would need to be designed to allow such voltages to be connected and disconnected by an inexperienced person. Even with 10,000 Volts on it, the Roadster six-minute charge would still require a cable of the kind of size that feeds your house switchboard, though. 

That's assuming you can find a plug and socket that will allow an inexperienced person to safely connect and disconnect 10,000 Volts whilst standing in a puddle in the pouring rain. I think I can forsee opportunities for the forgetful or careless among us to pick up a few Darwin Awards there..

All of this assumes the battery can absorb the charge at that rate without overheating. Which itself is unlikely. Fifteen minute lithium pack charging is certainly possible, but that relates to single packs in well ventilated conditions. I would image heat buildup to be a serious problem if a large bank of tightly-packed cells were to be charged at that rate or higher. 

This should give you some idea what's involved in connecting a cable of the sort of gauge and voltage rating required for a six-minute Roadster charge:

By contrast, a hose can deliver an equivalent amount of liquid fuel in less than a minute.

Bottom line is that the idea of fully charging an electric car in a few minutes, is simply ridiculous. Like so many green energy ideas, the numbers simply do not add up. A one-hour charge might be achievable, but even that would stretch the capability of a domestic supply, and would involve the driver in manipulating a heavy and awkward cable.