Why electric vehicles wont be showing up anytime soon…

And its not because of batteries, generation or transmission capacity. Its about recharge speed and how quickly you can get back on the road again. I’ll take you inside the numbers and show you what kind of electrical limits exist, and what you can do to get around it.

We’ll start with the only EV even close to mainstream, the Tesla Roadster. It comes with a 52kWh battery, and we’ll assume that it discharges to about 48kWh (240 miles at 200Wh/mi, leaving 4kWh spare) and that it has an AC to DC to battery efficiency of 85%. This would mean that from the wall, the grid is pushing 56.5kWh to recharge the battery. 

Most homes in the US run off of 120V (also referred to 110V) with a circuit breaker current limit of 20A. The Roadster’s primary charging unit requires a professional installation by a licensed electrician, who will probably run a dedicated circuit, most likely four circuits in parallel to provide a total of up to 80A. The high power charger is capable of up to 70A, and would deliver 16.8kW of power, and recharge the battery in three and a half hours. Lets assume this is the highest you’re going to get for in-home charging, as new electrical service panels in the US are rated for 100A – the other 30A at 240V will be for everything else in your house, like air conditioner or electric heater, etc. The only thing I can think of that might eat the rest of that really quick would be an electric clothes dryer on a 240V/30A circuit. 

The Tesla mobile charging units for the Roadster are rated at 120V/15A, which can deliver 1.8kW of power, which would mean it would take about 32 hours to recharge the battery fully, or roughly 7.5 miles of driving per hour of charging. Obviously this is not for regular charging. The 240V/30A connector is much better, delivering four times the power (twice the voltage, twice the current, the product is four). So that’s 7.2kW of power, which can recharge the battery in 8 hours, or about 30 miles of driving per hour of charging. Note that the miles of driving isn’t entirely accurate because of the way Li-Ion batteries recharge (CC/CV). 

OK, so we’ve got that as our baseline. Lets assume there were EV recharge stations as you’re driving on the highway from Los Angeles to Las Vegas (265 miles from the strip to downtown LA). 265 is just out of the range of the Roadster (225 miles or so on the highway, possibly less if you do 100MPH just like everyone on that stretch of highway). So if you stop in Barstow, you’ll want to get out, stretch your legs, and recharge the vehicle. This is where the fun begins. 

If they had the specially made Tesla high power charger, you could roughly get enough charge to make it to your destination (an additional 56 miles) in about 45 minutes. This assumes it delivers 16.8kW. If you had to use the mobile chargers, you might as well find a motel room. 

If we can assume that at some point, higher power battery charging infrastructure is made available (and the on-board AC/DC transformer in the Roadster can be made to handle more than 240V/70A), lets look at target charge times, the AC power needed to supply that, and what the conductor diameter need to be as well as the number of connectors.

As you can see, the very high current ratings of all three of these charging applications make it in no way safe for the average person to handle. At these numbers, you’re looking at some sort of solution where you would pull over a recharging station, and the vehicle would be recharged from underneath, with the conductors coming out of the ground. There are also a number of other issues like safety – how do you handle short circuit protection when dealing with numbers this large? Do you make the person get out of the car and stand away while its recharged? Then at the end, check to make sure that the car’s chassis isn’t electrically charged? 

This is by far the largest issue that faces pure electric vehicles. This does not apply for vehicles like the Chevy Volt, which only uses about 8kWh of energy from batteries before switching to the on-board gasoline generator (9.2kWh effective grid to battery). It would only take 5 hours to charge that amount from a 120V/15A line, and much less time (less than 3 hours) from a 240V/30A line. Using something similar to the Tesla high-power charger, the Volt would be completely charged in about 35 minutes, though I doubt the Volt’s on-board transformer will be able to handle 70A. Plus, with the Volt and unlike the Tesla Roadster, you can still run the Volt off of gasoline, and the engine is designed to be able to sustain highway speeds going up hills (we’ll assume they mean 75mph, the speed limit on the highway between LA and LV). 

So how do you get around the limits of how fast you can pump energy into the battery? Well, this means that instead of recharging the battery, you can swap out the battery for a fully charged unit. But there are lots of problems with this approach as well. First is that the batteries are heavy! Estimates put it at around 1,000 lbs of batteries and battery accessories like cooling. Second is that they are probably put inside the chassis of the car, so it would be difficult to get to – you wouldn’t want the battery pack falling out of the car while driving. Finally the issue is that you’re swapping a battery pack for one you don’t know the characteristics of. Does it have a high cycle count, which can lead to lower energy storage capacity and lower range? Who knows. And you’re going to be a part of a battery leasing program, since you aren’t going to want to buy the battery and then swap it out for batteries elsewhere in the exchange network. 

This is why my hope for the future is pinned on PHEV like the Volt. I would love for my next car to be a PHEV small/mid sized SUV (a plug-in Ford Escape, yes please!). Hopefully I don’t have to worry about buying another car until 2014 or so (after having bought a new car only 4 months ago). Maybe by then prices are a little more reasonable than what they’re expected to be when the Volt launches at the end of 2010.