Engine turns on and off at stop light

OK, 3,300 ft elevation down to sea level. Once you see regen current drop to zero, how much more of a descent is left on your drive? What would happen if your descent started from 6,000 ft?

When I say "overcharged" I mean that the hybrid vehicle ECU spins the engine via MG1, to use up electricity. I looked in the 2004 model year New Car Features manual but did not find mention of this particular behavior. Nevertheless I have noticed the behavior over the 11 years that I've owned Prius vehicles - and am not the only one to comment on the topic.

Is it possible that the behavior is at least partially prompted by low ambient air temperatures which translates to the engine needing to run to maintain its operating temperature? I suppose that is possible.

I assume the ambient air temp at sea level in Maui is around 80 degrees F. When I visit Lake Tahoe the ambient air temp is typically around freezing. That is a pretty big difference.

 

Mine, a 2004 Pkg#9, exhibits the same behavior. 153k miles and it has its own mind. You are right, the engine coolant temp "needs" to be 158 to 165 F in order to keep the computer happy. If I pull up to a stop light, the ICE shuts off then restarts and runs. The PCM also " looks" at the mixture fuel ratio sensor (a sophisticated Oxygen sensor). Hotter is better, but when its cold out the engine is commanded to run until everything is making miles per gallon unless another variable is introduced.
Sorry for the long report, but many, many things make the ICE in the Prius run when it does. I still love mine eleven years later.

 

In my normal trip down the mountain, the battery SOC goes from 6 blue bars to full in the first 10 minutes, and stays there for the next twenty minutes, with regen current going to near zero. At the bottom, the energy display bar chart show 6 lines of 99.9.

One of the reasons why it is possible to misinterpret the finer points of the energy flow is that the MFD will still show the charging arrow going to the battery, but instrumentation shows that the actual amperage is quite small, just enough to maintain the base load of electronics and A/C. In this condition, that small current does not change when hitting the brakes, instead the engine will rev higher to augment the friction braking.

A longer descent (once in a while I go down from 1500m ASL) does not change the battery condition after it becomes full. But as mentioned, the longer the descent, the more this allows the ICE to cool, especially in a cold climate, allowing it to fall out of S4. At the bottom, that can cause the engine to burn fuel at low speed to reheat, even though the MFD shows all green bars. No need for electricity to be "used up". The car's first priority is as a ULEV, and that means keeping the combustion portion at optimum temperature.