Wednesday, September 1, 2010

How Fast Can You Really Charge Your Plug-in Car?

plug-in cars

We are quickly approaching the launch dates of the Nissan LEAF and Chevrolet Volt-the first two globally-distributed and mass-market plug-in cars the world has ever seen.

Beyond those two groundbreaking vehicles, every major automaker has now committed to delivering some sort of plug-in vehicle within the next five years. As the public's attention shifts to the battery-powered drive-train and its perceived shortcomings, the question of how long it will take to charge the battery has rightly taken center stage.

To this point, much of the conversation regarding plug-in car charging times has revolved around what kind of charging station you use. In the US, as many of us know, there are essentially three types of charging:

A standard 3-prong household outlet, also known as "Level 1 charging"
A specialized home charging station, also known as "Level 2 charging"
A commercial quick charging station, known alternately as both "DC fast charging" and "Level 3 charging."

Listening to radio and TV shows, and reading through internet threads devoted to the topic of "How long will it take me to charge my electric car," it is apparent that there is a very big information gap out there when it come to charging times and what you might reasonably expect for your Nissan LEAF or Chevy Volt or Coda Sedan or whatever other electric car come down the pipe.

A battery is just a storage device for energy. Any given battery's potential energy storage is rated in terms of kilowatt-hours (kWh). For instance, the Nissan LEAF effectively has a 22 kWh battery, and the Chevy Volt effectively has an 8 kWh battery. In the US, your standard household outlet (Level 1 charging) can deliver about 1.6 kW (after accounting for losses and other items). To figure out how long it will take you to fully charge a given battery, simply divide the battery's size by the outlet's output. For instance, a 16 kWh battery will take 10 hours to fully charge from a standard outlet (16 kWh/1.6 kW).

So, if you install a Level 2 home charging station in your garage, how much shorter will the recharge times be? In the US, Level 2 stations are rated up to 14.4 kW (240 Volt / 60 Amp) outputs, but most of them will probably be installed on standard dryer circuits (240Volt / 30 Amp) and be able to output about 6.5 kW (after accounting for losses and other items). Of course, you can go higher if you buy a station that is rated higher and you pay for the upgraded wiring and circuitry to get you to 60 Amps, but for the sake of discussion, let's assume an output of 6.5 kW for a Level 2 station.

Using the same logic as for your standard household outlet above, you'd think a Level 2 station could charge that 16 kWh battery in about 2.5 hours, but this is where things get a little tricky. As it turns out, the station is just the energy supplier in the charging world-the actual device that regulates charging speed is on-board the car. And, as it also turns out, this on-board charger is the absolute critical piece to understanding how fast you can charge your brand spanking new electric car.

If you wanted to take maximum advantage of your typical Level 2 station, you'd want an on-board charger that could handle at least 6.5 kW. In fact, looking back at previous electric cars that were released back in the California mandate days, the original Toyota RAV4 EV was equipped with a 6.6 kW charger. Today, things are quite a bit different though. The first gen LEAF is shipping with a 3.3 kW charger, same as the first gen Volt. The Coda Sedan, however, is shipping with a 6.6 kW charger.

So even if your Level 2 station is rated at 6.5 or 6.6 kW, if you have a LEAF or Volt, you'll never be able to push more than 3.3 kW to the battery at any given time-resulting in charging speeds that are half that of what you might expect based on the charging station's stats. However, if you have a Coda, you'll be able to take full advantage of it. Even more confusing, the on-board charger doesn't affect how fast you can charge your plug-in at a DC fast charging station. In that case, the DC station is just dumping energy very quickly into the battery and kind of bypasses the on-board charger. You could get a Nissan LEAF battery from 0-80% full in about 25 minutes at a DC fast charging station. The Volt and Coda don't have DC fast charging capability.

In their defense, Nissan has said that the inclusion of the 3.3 kW charger was a choice they wouldn't make again in retrospect, and they plan on upgrading to a 6.6 kW charger for the next generation LEAF. At that point they also plan on making the 6.6 kW charger available for installation in the first gen LEAF, likely for some additional cost. My guess is that the Japanese Nissan LEAF engineers, working in a secretive Japanese world when first designing the LEAF, based their assumptions on Japanese outlets. A standard Japanese outlet is rated at 200 V and 15 Amps, or about 3 kW. In Japan there won't be any Level 1, Level 2, or Level 3 charging-just standard household outlets and DC fast charging while on the road-so there's no need for a charger rated higher than 3.3 kW. Whoops...

Yet, in the end, all of this talk of charging speeds and times from empty to full really doesn't make much sense because we're rarely going to be filling our plug-in cars from empty to full. More likely you'll drive the thing 40 miles in a day and then come home at night and plug it in. When you wake up in the AM it will be fully charged no matter how long it took. But if you only have to drive 40 miles a day, even a charge on a 3-prong outlet is a reasonable 6 hours, so focusing the charging speed becomes less important.

source: reuters