Reva Electric Car Co. Lets You Overdraw Your Battery

Good to the last volt

Bank cards let you overdraw your account when you must on the understanding that you’ll pay it back when you can. If you could do the same for your electric car by overdrawing the battery, it’d sure alleviate range anxiety—the fear that you might get stranded far from an electric plug.

Overdrawing your battery simply means taking advantage of a power reserve that today’s control systems deliberately build in to preserve the electrodes and thus extend battery life. The reserve can amount to 30 percent of the battery’s capacity. In the Chevrolet Volt, a plug-in hybrid that will use a gasoline engine as a range extender—and thus can afford to protect the battery’s life very carefully—the pure-electric reserve will reportedly come to 40 percent.

Reva Electric Car Co., based in Bangalore, India, and the world’s leader in electric vehicle (EV) sales, is parlaying data from the 135 million kilometers (84 million miles) logged by its EVs into an expert system that can give drivers an extra 10 km. The system, called REVive, uses remote communications to enable the system to determine how much extra charge can be accessed without doing harm.

”This is really a very complex problem,” says Chetan Maini, Reva’s chief technology officer. ”This is not something that you could fix in an algorithm that meets all situations.”

Photo: Reva Electric Car Co.
Doubly Green The Reva NXR runs on volts alone.

When REVive receives a text or voice message from the driver, it remotely accesses the vehicle’s three-year store of data on such crucial parameters as the battery’s age, the number of charge cycles, whether it's been scorching in Mumbai or freeze-thawing in Oslo, and how aggressively the car has been driven. REVive feeds that data to algorithms, resets the range gauge, and puts the car in a ”limp mode” akin to that of a laptop on a power-saving regime. 

REVive will be a standard feature in future Reva EVs, starting with a lithium-ion-battery version of its NXR subcompact to be released later this year in India and Europe. The car comes with an already substantial 160-km range, double that of the carmaker’s EVs powered by traditional lead-acid batteries.

Reva’s solution has merit, according to Andrew Burke, a research engineer and battery expert at the Institute of Transportation Systems, at the University of California, Davis. He says that although deep discharges are in general bad for battery life, they can be allowed once in a while without causing real damage, given that the extra range is profoundly reassuring to customers.

Photo: Daimler
Enough to Limp Home: The Daimler Smart ForTwo electric car lets you get below the red line.

Burke was himself reassured a decade ago, when he drove an electric car that Honda marketed briefly under California’s zero-emissions vehicle program. On three or four occasions he miscalculated how far he’d be driving but nevertheless made it home, thanks to the car’s limp mode. Inching along was no fun, recalls Burke, but ”I was happy to get home any way.”

Limp mode is a feature on the updated electric-drive version of the Smart ForTwo, which Daimler began test-leasing in December, according to Pitt Moos, the vehicle’s product manager. But Moos sees no need to proceed further to the ”hassle” of a system such as REVive, betting instead that time and consumer experience will dispel range anxiety. ”People who are very scared of getting stopped by zero percent [state of charge] won’t buy or lease battery EVs,” says Moos. ”Those who [do] will learn the cars and feel perfectly safe after a while. They will pass their experience along, and the market will grow.”

Still, if REVive can assuage range anxiety, it will boost the sales of the Reva, push other manufacturers into similar battery-management schemes, and expand the EV market that much faster.

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