http://www.automoblog.net/2008/12/31...ius-batteries/

This also includes a Toyota statement about Prius battery performance that puts things in perspective.

Bob Wilson

 

It appears that Honda has more pack failures than does Toyota. I wonder why this is? H

 

I agree, Harold. There's a feeling of MORE Honda failures than Prius failures. And I think it's more than a feeling...

If I had to determine why, I think it's because of a less-robust pack temperature management algorithm.
I think this is due to the lack of temperature-controlled AIR to the Honda IMA battery packs (only passive air-flow). It's hotter than it should be often in the summer (especially in the hottest states), and cooler than it should be often in the winters.
I think this contributes greatly to the quicker weakening of the individual cells, eventually totalling an IMA pack.

 

I am not farmiliar with how Prius deals with the temp. of the bat. pack, but they must do something that helps thier packs last longer! I have never heard of a prius pack failure, but I'm sure the have some. They are very popular for taxi use in Van, and Victoria as Bob mentioned. I have never seen a Civic used as a Taxi, but that could be because of the truck size? I like my HCH however, H

 

Hi,

The Prius has a temperature controlled fan and pulls air from the cabin, and exhausts out of a small vent on the side. When it get hot, you can feel the heated air being dumped overboard. It works on the principle that the temperatures people like work well for batteries.

Two years ago, I did some hill climb tests in 95F weather and while taking a break, the fan came on. This was before I realized the importance of controlling battery temperature.

As for cold weather, charging NiMH is exothermic. It doesn't take long to charge-pump the battery temperature.

Bob Wilson

 

Bob is correct, of course, about managing battery heat. The other factor is that the HCH batteries are the same generation as the older Prius (2001-2003) batteries that have also shown some failures (a small number) as they age. The Panasonic "Prismatic" cells in the 2004+ Prius are somewhat improved batteries. Hopefully Honda and Ford will have access to improved batteries soon.

JeffD

 

I could be mistaken, but I don't believe NiMH becomes more than very slightly exothermic until near full charge, or when under excessive charge rate. NiCads heat under charge and discharge, but not the NiMH cells I've worked with. The NiMHs only heat up during discharge, and under excessive charging currents.

For example, I can do a 2C max safe rate charge (20 minute full charge) on an NiMH pack and not sense any temperture rise until just before full charge. I'll push those cells to 180F on discharge, but get worried if they even reach 100F on charge, or even a couple degrees above ambient during the first 95% of charge.

-- Alan

 

The heating at maximum charge is the generated hydrogen gas that recombines. We're talking about heating due to a combination of ohmic and ordinary chemical reactions during the charge.

I have a paper at home, "Thermal Behavior of Small Nickel/Metal Hydride Battery during Rapid Charge and Discharge Cycles", Takuto Araki, Masato Nakayama, Kenichi Fukcuda and Kauo Onda about charge exothermic action. The put the small NiMH cells (AA or AAA) in an insulated cell with a quality temperature probe to measure the change:


I have observed a similar temperature range during charging in my Prius traction battery during a forced charge:

A 7-9C increase in battery temperature is not too bad when it is cool. But starting out on a hot, summer afternoon after solar baking . . .

What is sad is if you go through a lot of charge-discharge cycles, it adds up. You can 'pump' your batteries to some significant temperatures.

Bob Wilson