Article on Prius battery

Hybrid Batteries: None the Worse for Wear?
by Rick Cotta


According to a recent study by automotive experts Kelly Blue Book, the top three concerns among potential hybrid-vehicle buyers are:

1) Longevity and repair issues
2) Questionable fuel-economy claims
3) Battery life

The first certainly stands to reason. Until there are a lot of hybrids out there, it's unlikely you're going to be able to get one fixed at just any corner garage. To combat this, Ford, Honda, and Toyota all offer at least eight-year warranties on hybrid components, but that's of little help if you're stranded far from a dealership.

Fuel-economy claims that tend to be overly optimistic are being addressed by none other than Uncle Sam. The Environmental Protection Agency (EPA) has issued a proposal that a different procedure be used to establish EPA mileage figures for the 2008 model year. It is intended to bring the fuel economy estimates printed on a vehicle's window sticker more in line with what drivers can expect to average. This will likely drop the figures for all cars, but hybrids may show a bigger decrease than most others.

Now to the battery issue.

Toyota's Prius has been around for the last six years and is the U.S.' most-popular hybrid.
While Toyota's Prius wasn't the first hybrid to be sold in the U.S. (that honor goes to the low-volume two-seat Honda Insight), it was certainly the first to sell in reasonable numbers. Introduced in the U.S. for the 2001 model year, it was redesigned for 2004, offering more room, more power, and even better fuel-economy figures.


Helped by rising fuel costs, sales of the second-generation Prius took off. Toyota built about 52,000 of the first-generation cars, and so far has added more than 214,000 of the latest version. That means there have been well over a quarter-million Prius Hybrids sold in the U.S., making it by far the country's most popular hybrid.

And Toyota claims that not one has required a battery replacement due to malfunction or "wearing out." The only replacement batteries sold--at the retail price of $3000--have been for cars that were involved in accidents. Toyota further claims that the nickel-metal hydride (NiMH) battery packs used in all Prius models are expected to last the life of the car with very little to no degradation in power capability.

For those of us who have cell phones and other devices with NiMH batteries, that claim may sound unrealistic. Over time, the battery's charge longevity seems to wane, resulting in shorter and shorter usage between charges. Eventually, the battery becomes worthless and we buy a replacement.

But in the case of most electronic devices, the batteries tend to get fully charged, then nearly fully discharged before being charged again. For the power pack in the Prius, at least, Toyota says this would greatly shorten the life span of the battery.



A navigation screen converts to show the driver what type of power the Prius is using, gas or electric.
To get maximum life out of the Prius battery pack, the car's computer brain does not allow the battery to fully charge or discharge. Toyota says that for the best service life, the Prius battery likes to be kept at about a 60 percent charge. In normal operation, the system usually lets the charge level vary only 10-15 percentage points. Therefore, the battery is rarely more than 75 percent charged, or less than 45 percent charged.


If you're familiar with the Prius, you know there's a battery-charge indicator on the instrument panel. Toyota says this isn't the charge level per se, but rather a state-of-charge window. The top of the window represents about a 75 percent charge, the bottom about 45 percent charge.

According to Toyota, the life of the Prius battery pack is determined more by mileage than by time, and it has been tested to 180,000 miles. Supporting this are first- and second-generation Prius taxis in Canada that have reportedly traveled more than 200,000 miles without suffering any battery problems.

The Next Generation
Just as many electronic devices have recently switched from NiMH batteries to lithium-ion, so too might the Prius. Two reasons: First, Toyota says the cost of nickel has increased threefold in the past few years, largely because China is buying lots of it to make stainless steel. Also, lithium-ion batteries offer a higher power density that would allow a similarly powerful battery to be smaller and lighter.

But Toyota notes there are still some problems to be overcome before lithium-ion batteries are ready for prime time--at least in cars like the Prius. One is that the batteries have a dangerous tendency to catch fire. The second is that the life span of a lithium-ion battery in an application like the Prius can't yet match that of a NiMH battery. But neither of these problems seems insurmountable, as Toyota estimates lithium-ion batteries might be used in the Prius in as little as two or three years.

For now, however, Toyota stands by the durability of its NiMH battery packs. And it doesn't plan on selling many replacements any time soon