Hybrid cars 101: How long should batteries last?

Japanese automaker Honda found itself in a second US court case early this year over a batch of problematic hybrid-car batteries that customers say sabotaged their fuel efficiency by as much as 40 percent.

Like other hybrids, the Honda Civic Hybrid relies on a battery system that gives the gasoline engine an extra boost in efficiency. These energy cells recharge while driving, but lose capacity over time. Now a class action lawsuit alleges that Civic Hybrids from the years 2006 to 2008 had shoddy batteries that reduced their fuel economy from the promised 50 miles per gallon to 30 m.p.g. within just a few years.

How long should such a battery last? Hybrid owners already pay a premium for their fuel efficiency. Replacing the battery midway through the life of the vehicle would tack on an additional $2,000 to $4,000.

Fortunately for hybrid owners, the Civic's battery problems seem to be an anomaly.

"What appears to be some sort of failure in Honda Civics has not been reported in the other models," says Bradley Berman, founder of the news and reviews site HybridCars.com. "I'm not saying that these batteries will last forever, but the general sense is that they will last for, quote, the lifetime of the vehicle."

Cinda Gobeille had to garage her Toyota Prius when the battery gave out after a healthy life of 10 years and 144,000 miles. The car looks great, she says. But without a new battery, which dealers say will cost up to $4,200, her hybrid simply won't turn on.

"It could get 50 miles per gallon on a good day," says Ms. Gobeille, a nurse case manager from Woonsocket, R.I. "When the battery was dying, it dropped down to probably 35 [m.p.g.]. It's a great little car. I really enjoyed my Prius, but this battery system is just too expensive to replace on such an old car."

Gobeille has held on to the dormant Prius while she investigates third-party batteries, which she says could cost $1,300.

In some cases, old hybrids show no signs of aging. Last year, Consumer Reports compared a 2002 Prius with 206,000 miles with the results that it had collected 10 years earlier on a nearly identical 2001 Prius that had only 2,000 miles. There was barely any difference in fuel efficiency. The near-mint 2001 model had logged 40.6 m.p.g., while the aging 2002 Prius averaged 40.4 m.p.g.

So why are hybrid batteries sprightly after a decade, while laptop batteries start wheezing within just a few years? The chemistry is different – but in some ways, humans are to blame.

"Generally, when you're charging your device battery, you're charging it to 100 percent," says Mr. Berman, "and you're depleting it to zero percent. Right? That is the worst thing you can do for the longevity of the battery."

Hybrids come with software that purposely stops the battery from charging once it hits about 80 percent of full capacity. Similarly, the battery will say it's empty when it falls to about 20 percent. The exact numbers are company secrets, but the point remains: By undertaxing their batteries, hybrids ensure longer life spans. (Some gadget gurus suggest a stricter "40/80" rule for laptops and phones. Of course, that limits the usefulness of a single charge in exchange for long-term gains.)

If hybrids do conk out early, most batteries come with an eight-year or 100,000-mile warranty.

Even as her Prius's efficiency slipped, "it always had good power," says Gobeille. She now drives a Hyundai Sonata Hybrid and says she plans to stick with hybrids – or maybe electrics – from now on.

For more on how technology intersects daily life, follow Chris on Twitter @venturenaut.

[Editor's note: This is an updated version of an article that appeared in the February 13 issue of The Christian Science Monitor magazine.]