The electric gas station
By John Donovan, Low-Power Design, for Drive for Innovation
When you run low on gas you look for a handy gas station. What happens when you’re driving an electric vehicle (EV) and you run short of electrons? Unless you’re doing an out-and-back run from your home charging station, you could have a problem.
Right now the market for EVs is primarily early adopters, such as engineers who love the cool technology and who, coincidentally, may also want to save some money—having first parted with a goodly chunk of it—along with the environment. EVs will have to go mass market in a big way before the recharging infrastructure begins to rival that for internal combustion engines. It’s Catch-22: lack of infrastructure slows adoption of a technology, which in turn needs the infrastructure to really take off. The solutions lie at both the personal and system level.
Charge It!
Let’s say you drive a Chevy Volt and you’ve just come back from a long drive that required an assist from the gas engine to get you home. The Volt’s 16 kWh battery will be at 30% state of charge, which is where the motor/generator maintains it after it kicks in. The battery is now good for a theoretical 4.8 kWh, so it needs 11.2 kWh to recharge to 100%. The most you can pull from a dedicated 120V/20A house circuit without popping a circuit breaker is 2.2 kW, so you’d need a minimum of five hours to fully recharge while running that circuit at maximum capacity. This situation is not one I really recommend.
In practice, the Volt requires about 10 hours to recharge at 120V and 4 hours at 240V. You can recharge it overnight from a wall plug or much more quickly if you buy the 240V/40A charger that GM recommends—for about $2,000, not counting the cost of the electrician to install it. If you live in a house with a garage, you don’t have a problem; if you live in an apartment complex, you do.
If you’re on the road, you need to look for charging stations, of which there are three types:
- Level 1 chargers plug into standard 120V/20A outlets and can recharge your Volt battery in 10 hours, which is fine unless you work evenings and leave early, as a lot of people do. Or you live in an apartment and need to charge the car using an extension cord, with the drop in voltage that implies.
- Level 2 chargers provide a dedicated 240V/40A circuit and can recharge the Volt battery in 4 hours. This is the solution that GM and other manufacturers recommend. They use an outlet/plug combo specified by the Society for Automotive Engineers (SAE) J-1772 standard.
- Level 3 chargers provide 480V at an unspecified current rating. The SAE is yet to agree on a standard; right now Level 3 chargers are anything rated about 14.4 kW. These are the ‘fast chargers’ that can recharge your EV in 30-60 minutes—they’re the electric gas stations of the future. Unfortunately the operative word is ‘future,’ since the lack of an SAE standard makes charge station manufacturers reticent to invest in them, which means the few that are on the market are anything but cheap. That will change as the market matures.
Even when fast charging stations become plentiful, you may want to think twice about using them frequently. Repeated rapid charging can lead to degradation of the cathode, resulting in shortened battery life. The Volt’s electronics monitor the state of the battery and regulate the charge rate to provide the fastest rate that is compatible with long-term battery life. That’s true for the Level 2 charger you can buy today; Level 3 chargers move that tradeoff way over to the speed side, so look before you leap when these chargers start popping up in your town. However, I’d expect auto makers to stay ahead of this problem with control electronics that can handle it.
Grid-Level Challenges
You’ve just bought an EV, and now you’re the cool guy in the neighborhood. So what happens when several of your jealous neighbors follow your lead and go out and buy one, too? Blackout!
Most pole-mounted power transformers in the U.S. are rated at 50 kVA and serve 4-8 homes. Let’s say each of six homes is pulling 8 kW at any given time for a total of 48 kW. Now if everyone turns on the air conditioning; someone runs a load of wash; and three people start to charge their EVs, each of which pulls down 5 kW, the result is a blown transformer. Right now EVs are few and far between, so this is a hypothetical problem. But do the math: when EVs become popular, this will become a real problem and an expensive one to solve.
An obvious solution is larger pole-mounted transformers, but that’s not going to happen; there aren’t enough EV owners to pay for the expensive upgrade, which would be an unfair burden on non-EV owners.
The ‘smart grid’ could provide a partial solution, detecting when too many 240V sources are running and selectively turning off one or the other to maintain a minimal service level. This could mean your car might not in fact be charged in the morning when you expected it to be—an unhappy surprise. But hey, no blackout!
In the end, neighborhood fast charging stations with their own large step-down transformers may be the answer. Here’s a great marketing opportunity: put a bank of fast chargers in front of a McDonalds or Starbucks and you’ll have EV drivers circling the block waiting to get in!
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Sounds like eventually money is going to have to be spent to put in larger transformers, and the question will then be whether EV owners should carry the brunt of the cost. I expect an answer will be a surcharge based on usage. But perhaps this is one of those expenses that should be considered a necessary improvement and so paid for equally by everyone…think education…I hope we all can agree that it is for everyone’s best interests to contribute to a good education system whether or not a particular individual has children in the school system at a given time. Thinking the same way, should we all pay for improvements to the electric grid in order to increase the viability of EVs? I’m not sure. I don’t have an EV, and I certainly don’t want to see my electric bill go up.