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I don't think hybrids have been out long enough for there to be good data on expected battery life

That's not true. The Prius has been for sale in Japan for over 6 years. There has been also Prius that was used as a taxi for over 200K miles. In that time Toyota has sold over 120,000 hybrids. The battery issue is something that scares a lot of people, so let me give you my thoughts on it. I own both a 2001 Prius and now a 2004 Prius.

The Toyota warranty on the original US Prius in 2001 is for 8 yrs or 100K miles. On the 2004 they changed the warranty for California to 10 years or 150K miles. (to meet the requirements for a better than SULEV "logo")

The battery in the Prius (and probably the Honda cars too) has its state of charge completely monitored and controlled by a computer in the car. This is unlike in an electric car where you charge it up and use it to go as far as you can or some short distance, then "you" decide when and where you want to charge it back up again. This causes lots of full charge-discharge cycles which causes a much shorter expected life. In the Prius the battery cycles between about 50% and 75% most of the time which allows a much longer lifetime. The design of the Prius battery is fully intended to last the entire life of the car (except for the extreme cases of a battery flaw or those you repair vehicles over and over to get lots of miles), but they do not want to make this claim...probably for legal reasons.

Like any component in a car, they calculate/estimate a bell curve for time to failure and have to set the warranty somewhere at the start of the curve to be economically in business long term. If they set it in the middle of the curve, they would have to replace ~50% of the components over the car model's lifetime. Using this basic knowledge, you can see that in the first cars Toyota must be expecting the typical battery to last at least 30%-50% longer than the warranty (or 130K - 150K). The warranty is not pro-rated -- it is for 100% replacement up to 100K miles (8 yrs). Of course, there will be some batteries that fail before 100K...and some that fail at 101K and some that last more than 200K.

There is also a myth about replacement of the battery. If your battery were to fail at 101K miles you would need to repair or replace it. The battery is nothing like the standard 12v lead-acid battery that all cars have now. The inside of the battery is made up of a few dozen modules of 6 batteries soldered together. In fact, in the first Japanese Prius model in 1997 the cells were D cells, just like in flashlights. Since then (for all US models) they got Panasonic to design a more space efficient shape since cylindrical cells don't pack together tightly. In 5 years I fully expect crashed and scrapped hybrid batteries to be recovered and allow replacement of bad modules for just a couple of hundred dollars. Currently there is no demand for this and no repair experience to do it.

The batteries are NiMH and are the same (chemically) as the AA ones you can go buy at the store and recharge yourself. The new 2004 Prius has a more advanced NiMH chemistry that is 30% more dense for the same power.

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