Electric Car Battery Basics: Capacity, Charging and Range

Fuel Tank Analogies Don't Always Apply


  • 2011 Nissan Leaf Battery Picture

    2011 Nissan Leaf Battery Picture

    Rather than focusing on the rated capacity of an EV battery, such as this one for the 2011 Nissan Leaf, consumers should use it to compare the relative size of EV batteries. | January 24, 2011

4 Photos

You could think of the electric vehicle battery as the new gas tank. And while this comparison works on many levels, the analogy doesn't completely hold true. Americans who are considering the switch to electric cars and are struggling to understand the new technology should understand some fundamental facts about batteries before they buy an electric vehicle (EV). The three aspects of electric car batteries that consumers should understand are capacity, charging and range. Here's a brief overview of each one.

Capacity
An electric vehicle's battery capacity is measured in kilowatt-hours (kWh), the same unit your home electric meter records to determine your monthly electric bill. In the EV world, kilowatt-hours are to batteries as gallons are to gas tanks. But a full battery can't be completely equated with a full fuel tank. "It's important to understand that the rated capacity of the battery is something you will never be able to use," says Dan Edmunds, director of vehicle testing for Edmunds.com. In order to preserve battery efficiency and battery life, a "state-of-charge" management system never lets the battery become 100 percent full or 100 percent empty. A more relevant measure might be a battery's usable capacity, but that's swathed in mystery, too.

"Usable capacity is not often reported by the manufacturer," Edmunds explains. "That is unfortunate, because the difference is significant. State-of-charge battery management, a very necessary feature of modern electric cars, nevertheless leaves you with about 60-70 percent of the rated capacity to work with."

In other words, don't rely too much on the fact that the Nissan Leaf has a lithium-ion battery rated at 24 kWh, or that the Tesla Roadster's battery is rated at 54 kWh. You'll never be able to tap all of that energy, anyway. Rather than focus on the specific figure describing a given battery capacity, then, consumers should use the rated capacity figure to compare the relative size of batteries. "It's kind of like cargo capacity or any other interior dimension," Edmunds says. "Use the number to approximate which car has 'more,' and by how much."

Charging
Charging comes down to two familiar resources: time and money. How long an EV takes to charge depends on its battery size and the voltage of charger that the consumer uses. How much the charging costs depends on when and where the vehicle is charged.

Until recharging stations are reliably available in shopping areas and workplaces in sufficient numbers (a development that might take years), most owners will charge the vehicles at home. And most of them will use the 120-volt "trickle-charging" cord that comes with most EVs. A trickle-charging cord is intended to top off batteries from any standard electrical outlet that can be reached without an extension cord (an extension cord is a big no-no in this process). Unfortunately it can take more than 20 hours to complete a full charge on 120 volts.

Instead, an EV buyer will likely want to install a 240-volt home charger, which will enable much faster fills. The Nissan Leaf's battery, for instance, takes seven hours to recharge with a 240-volt charger, according to the Environmental Protection Agency. The cost of a 240-volt charger plus installation varies widely due to the differences in home wiring systems, local utility policies and a variety of regional subsidies and tax credits. A reasonable estimate is $2,000.

The cost of charging, meanwhile, depends on where the EV is driven and whether the car is refueling during peak or non-peak hours. There are more than 3,000 electric utility providers in the United States, and rates vary widely. In the Pacific Northwest, hydroelectrically generated power is cheap, and so are average electric rates. In Hawaii, electricity costs are so high that recharging an electric car could rival or exceed the cost of filling up a conventional car. (Edmunds has prepared an article that describes the cost of charging in more detail.)

Basically, cost-conscious EV owners should recharge their vehicles when the cost is the lowest, and allot sufficient time for a full charge. This will usually be at night, when the car is least likely to be needed and when, happily, electric rates are lower. Automakers recognize this is so crucial to the cost of ownership that the latest vehicles (the Leaf and plug-in hybrids such as the 2011 Chevrolet Volt for example) have programmable charge timers built in. These help drivers take full advantage of off-peak rates that might exist in their areas.

At the same time, it's important to remember that the very act of charging carries a hidden cost. Battery charging is inherently inefficient because of the heat that's generated. Fans and cooling systems in the car operate during the process, and they use some more of the electricity that's coming down the charge cord.

"Let's say your charger had a meter — most don't — and it read '12 kWh' after you finished charging," Edmunds says. "Only about 10 kWh of that charging actually made it to the battery. But, of course, you're paying for all of it." Charging losses of 15-20 percent are pretty typical of most electric cars, he says.

Early performance claims from EV manufacturers typically ignored such losses, but the EPA does not. Its official electricity consumption figures are measured at the wall, where your home meter will measure it and add it into your bill. The EPA has accounted for the hidden electricity usage on the vehicle window sticker.

Range
Range is the stickiest question facing new EV drivers. That's because it varies much more for an EV than it does for a conventional car, Edmunds says. Nissan says the Leaf's range is 100 miles. EPA testing puts the car's range at 73 miles. Tesla, meanwhile, says its Roadster can go 245 miles. But for all EVs, range will vary.

High and low temperatures affect battery performance and reduce range, which is why some EVs are first being introduced in areas that aren't very hot or very cold. Also, quick acceleration and fast driving discharge the battery faster. Even aggressive braking hurts significantly, because it cheats the EV's regenerative braking system of the chance to recapture some energy and recharge the battery.

Because of all these variables, "if the manufacturer says 100 miles of range, it could be 60 miles or it could be 130 miles," Edmunds says. And if the manufacturer says 100 miles, you're going to want to allow a buffer, he adds.

Further, Edmunds notes, "You can't tempt fate with the low fuel light like you can in a gasoline car, because the only place to fill up may be your own garage. So if you're wrong, you're walking." And that's why there's so much talk about range anxiety.

While capacity, charging and range might be foreign concepts now, it's because EVs are still in their early days. Hybrids also were mysterious to most car shoppers when Toyota introduced the Prius in the U.S. just over 10 years ago. Now hybrids are a staple in many manufacturers' lineups. In the very near future, EV owners — and maybe even some EV tire-kickers — will quickly get the hang of these new rules of the road.

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