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updated 9/27/2010 1:16:37 AM ET 2010-09-27T05:16:37

The biggest concern buyers have about electric vehicles is how far the battery will take them. For manufacturers, therefore, it's tempting to be optimistic in quoting the vehicle's range. Thus the claims almost always include the qualifier “up to,” as in “range up to 100 miles” (or 40 miles, or 200 miles, or whatever the manufacturer is claiming as maximum range).

And yes, it's probably true that at least one time, under optimum conditions, very carefully driven by a factory driver, with a topped-off, brand-new battery pack, that model of electric vehicle did reach that maximum range before the wheels absolutely stopped turning. And while some manufacturers state that their range numbers are computed by EPA standards, you still shouldn't count on reaching that range in everyday use, perhaps not even coming close. Here's why:

#1: EPA tests are not realistic. As former Tesla Motors spokesman Darryl Siry has revealed, the current EPA standard for electric vehicle range measurement is very optimistic and manufacturers like Tesla take advantage of that. That has allowed electric-car makers like Tesla to publish "EPA test" range numbers that few will ever get. EPA test rules allow procedures that a real-life driver should never do and the manufacturer may even forbid.

For example, the EPA standard allows the battery to be topped off to its absolute maximum charge (harmful to the battery in the long term, therefore a bad idea) and to run the battery completely dead (a very bad thing because that too reduces the life of the battery). In practice, to maximize the useful life of the battery pack, most manufacturers design their vehicle chargers so they don't top it off to absolute maximum, and the cars themselves are often designed not to allow the battery to be run completely flat in normal operation. Those factors alone will account for a significant difference between the “EPA tested” mileage claims and what a real-life driver would achieve.

#2: Battery packs lose capacity. Just like the battery in your cell phone, an electric car's battery pack will work best when brand new, but its capacity will decrease significantly over time. Factory range tests will, of course, be done with brand-new batteries, but over the years and miles the battery pack in your car will drop to 90% capacity, then 80%, 70%, and even lower until it's finally replaced.

#3:Weather and other conditions shorten range. Depending on where you live and where you drive you may encounter other conditions that reduce your range significantly. One such condition is hot weather: While on a gasoline vehicle heavy use of air conditioning may reduce range by 5% or so, on an electric car the A/C may reduce the range by 25% or more. In hot-weather urban driving the loss could approach 40%.

Ironically, very cold weather will also significantly reduce the range, in part because of the need to run the heater, but even more so because of the lower performance and capacity of batteries when they're very cold. (A heated garage or plug-in battery heater can help to some extent.)

#4: Use of headlights: At night you'll need to run the car's lights, which causes another significant drain on the batteries. In some locations and some weather conditions (such as fog or rain) you may need to run the car's lights in daytime, which exacerbates the problem.

The result is that when it's hot, cold, dark, foggy, rainy, or other less-than-optimcal conditions, an electric car's range will be significantly shorter than the manufacturer claims, at times drastically shorter.

#5: Radios and entertainment systems. Most people will want to be able to run the car's audio and entertainment systems while they drive, but like everything else they too drain the battery and reduce the range.

#6: Tire pressures. Most people don't pay much attention to tire pressures, and as a result it's common for cars to drive around with low tire pressures. That reduces the efficiency of any car, but due to an electric car's higher dependence on very low-friction operation, low tire pressure has a disproportionate effect on the range of an electric car vs. its effect on a gas-powered vehicle.

#7: How drivers think. Driver psychology shortens effective range. In theory an electric car with a "true" 100-mile range could be operated for 100 miles. But because the actual range varies with so many factors, and because no one wants to be stuck with a dead electric car far from the nearest charging station, prudent drivers always avoid testing the limits. Unlike a gas-powered car, there's no quick fill-up or “gallon of gas” for a dead or near-dead electric car. Some electric cars do hold back some of the battery capacity as an emergency reserve, but once that's gone, a long session at a charging station is the only thing that will bring the car back to life.

Example: The Nissan Leaf requires 20 hours for a full recharge using its built-in power cord and a standard 120-volt, 15-amp outlet. If a 220/240-volt, 30-amp outlet is available, the charge time can be reduced to eight hours. For commercial establishments or custom home installations Nissan makes a special 480-volt, 125-amp charging station that can provide a 30-minute quick-charge to 80% capacity, but it costs US$17 000 and Nissan warns that frequent use of the quick-charger will reduce battery life.

The result is that if the car is capable of an “honest” 100 miles most drivers will turn around less than 40 miles from home and plan to be home (or at a known charging station) at well under 100 miles total.

Showstopper?

Should those factors – and the reduced range they imply – keep you from buying an electric? No, not necessarily. If your planned uses for an electric car will never tax its range, then you should only be concerned about unplanned situations (could you ever have unexpected side trips from work that lengthen the distance home?) and evaluate how you would deal with those.

Over time the range of electrics is likely to increase and charging stations will likely become more available, though even then fill-ups will still take hours. But in thinking about an electric car purchase, consider what its real range might be for you, under the conditions you might have to face (heat, cold, night, fog, etc.) and evaluate the car accordingly.

 

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