First things first - Thank you Jason for creating such a worthwhile website!! I'm very impressed with its design and with the wealth of information it provides! The intelligent questions and equally intelligent answers from many of your participants sets this forum apart as one of the best I have found.

Now for my question. With long down hill coasts energizing the regenerative braking and charging the traction battery, is it possible to overcharge the battery? If the battery begins overcharging, heat will be created in the battery which could lead to its early demise. Knowing Toyota's penchant for perfecting every last design detail, I doubt this would happen, but I'd like to hear from someone who has had experience with long, continuous down hill coasting in the 2004 Prius.

Also, does the battery charge much on long, steep uphill drives?

It is theoretically possible to overcharge the battery, but the ECU will not let that happen. If you look around you'll see lots of posts from people talking about their ICE suddenly spinning up to high RPMs and scaring the bejesus out of them while they were coasting down a hill. That was the ICE reving to a state where it could utilize some of the battery charge and drain it to prevent overcharging.

If you're near full charge, at a stop, the ICE will cycle on and off several times in fairly quick succession to do the same thing--just without the high RPM (spinning up the ICE by the motor causes a pretty good HV batt drain).

I experienced some minor charging pulling a hill on a trip not too many weeks back. I can't quantify just exactly how much energy was generated, but the MFD was indicating that the output from the ICE was sufficient to pull a hill at 70 and do some charging on the side. My battery level was 3 bars from the top, but I did not get any new bars on this uphill pull. However the road that once went up went down about an equal distance and angle and all the way down I was coasting, ICE off, and the regenerative drag system kicked in and brought me up two full bars. I have not yet had the ability to see all bars full. There is a long highway hill in Idaho which I will probably have to traverse in the coming year which will will interesting to see the effects of.

If you force it, it will overcharge the battery. By force I mean use a charger. My friend did that to his 01 and the battery charge was over the top. It was very fast though, he clocked it at 8.7 seconds.

Can I ask a stupid question?

Why did your friend "overcharge" his battery. I suppose I'm a bit more on the cautious side, but the Prius isn't exactly an inexpensive vehicle and after what I paid for it, I'm not sure I would have the intestional fortitude to "plug" the car in someplace and do my bidding to the main traction battery.

But it is interesting to note the acceleration increase.

Um... hate to be the boring one, but I'm not sure I believe that. Making the Prius a plug-in hybrid takes a lot of advanced (and unique) technological work. There is likely only a handful of people who have even attempted this in the past.