Aug 272010
 
NiMH Charge curve
Image via Wikipedia

THE whole point of paying an extra $5,000 or so for a hybrid car was supposed to be that it would deliver more miles to the gallon, possibly a bit of extra pep, and a warm feeling of superiority over the majority of carbon-emitting motorists. For these luxuries, customers were assured that their vehicles’ rechargeable battery pack—two-thirds of a hybrid’s extra cost—would last at least as long as the rest of the car. Try telling that to those who bought Honda Civic hybrids between 2006 and 2008.

Since spring, irate owners of hybrid Civics have been venting their frustration on the web. Some describe how their cars’ battery can suddenly die while trying to overtake or labour up a hill. Others talk of leaving their car with the battery fully charged, only to return an hour or two later to find it flat. Being barely three or four years old, the Civics in question are unlikely to have done more than 75,000 miles (120,000km) at the very most—about half their expected life.

Honda’s answer has been to issue a couple of software patches that make the Civic’s battery work less energetically, and its petrol engine harder still. However, after having the fix downloaded, many owners claim not only that their cars have lost power, but also that their fuel economy has fallen from about 45mpg (5.2L/100km) on the highway to around 33mpg—little better than a non-hybrid version of the same model, which costs $5,000 less. Some have asked, unsuccessfully, to have the old software re-installed. Others have threatened to sue, and had their pricey batteries quietly replaced.

The surprise in all this is that the batteries conking out are the reliable old nickel-metal hydride (NiMH) variety—not the more powerful but finicky lithium-ion versions being adopted in the latest generation of electric cars and plug-in hybrids. The rechargeable nickel-metal hydride cell has been one of the great success stories of battery development, at least technically, if not commercially. It is a direct descendant, with all the benefits and more, yet few of the drawbacks, of the nickel-cadmium (NiCad) battery used for decades for heavy-duty tasks like starting aircraft engines and powering forklift trucks.

As in a NiCad battery, the NiMH’s anode is made of nickel oxyhydroxide immersed in an electrolyte of potassium hydroxide. The cathode, however, is composed of a harmless hydrogen-absorbing alloy instead of toxic cadmium. Weight for weight, an NiMH cell can store two to three times more energy than its cadmium-based predecessor. This alone has made it a favourite among hybrid carmakers such as Honda and Toyota.

The best guess about what is happening is that it has something to do with the way the batteries are charged and discharged during use. Unlike pure electric cars, hybrids normally have a rather shallow charge-discharge cycle. A hybrid’s battery is replenished constantly by the engine as well as by the vehicle’s regenerative braking system. As a result, it is hardly ever fully drained. That ought to make it last longer than the battery of a plug-in electric vehicle.

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