It's noted around this board that the Prius "full" reading is actually 80% charge. Why not 100%? Can it ever go above 80% technically speaking? Is 80% somehow better for an NiMH...? My understanding of NiMH is that it would not be. Does it have something to do with protecting from wide temperature variations?

 

I think that 82% state of charge has been reported at least rarely. The issue is indeed avoiding high temperatures. You might think about adding a watt to the battery: To a mid-charged battery, not much of that watt is turned into heat. For a high-charged battery, a larger proportion does become heat. At some point there is more heat than the battery management system can readily export.

I have become a bit cautious/neurotic about the battery. Use a device to determine what the maximum internal temperature is, and use the air conditioning to keep that below the high 40's oC, even if I do not feel cold in the car. I have no way of telling if that can extend battery life; it is just a bit more than what Toyota does with its multi-speed fan cooling.

DAS

 

It is for the cycle life.
The normal NiMH cycle life is only 500 to 1000 cycles using 0 to 100% SOC.
Toyota discovered we can get millions of cycles to limit the SOC range.
I would recommend everybody to view the following two Toyota presentations to understand Prius more.

http://john1701a.com/prius/prius-presentations04.htm
http://john1701a.com/prius/prius-presentations05.htm

Enjoy,
Ken@Japan

 

I figured it would be something brilliant like that. Man what a great car this is.

So, if you were to do nothing more than add a plug to the Prius, slow-charging it at night, you would actually reduce the life of the battery because of entering the "Overcharging area" regularly?

When people mod this for its batteries does the Prius automatically notice it's got more charge to work with, or does it stay in this absolute range? In other words is it a charge value or a percentage of capacity that governs those bounds?

 

I think you're correct, and Toyota would void the warranty.
Usually, modifiers bypass the battery ECU circuit when charging. There is no way for Prius to know the correct SOC value.

Ken@Japan