photo-20070831-162018-000125 data 

This graph shows a Behaviour of the Prius that I didn't know about before. It was on a long slow downhill road in the carpatians. Most of the time going under 69km/h (ICE at 0 RPM)
At the 2nd 5-Minute interval from the left you can see that the battery is reaching the 80% mark (full). In order to keep the cars behaviour of slightly braking when you lift your foot off the accelerator the Prius seems to start the ICE and make it brake the car (Green line below 0)