2011 Toyota Prius II. Most days on my way home from work, I have to come down a large hill and then turn into my driveway, which is level. So I would be rolling down the hill with the ICE off, and usually by the time I've reached my driveway, the battery has fully charged since its a long downhill. I've noticed that often times the ICE will turn on after I park the car. This can happen with A/C, heat, lights, radio (etc.) all turned off. I don't know why the engine would need to turn on here. The battery is fully charged, there's no real electrical power needed, and the engine is warm (commute is ~ 1 hour). So why does it need to turn on? It just makes for a scary situation because the engine wants to turn on right when I press the power button to shut the car off. I don't like interrupting it like that.
My guess that cold weather and the engine is not as warm as you think. A Prius will never get cold enough to pollute the air.
Before driving down the hill I was driving for an hour. Shouldn't the engine be warm enough? The downhill is long, but by the time I reach the driveway, the ICE has only been off for about 30-60 seconds. I wouldn't think it could cool down that much in that short time. Even if it was cold, why would the engine need to turn on if no power is drawn and the battery is full?
i think the cold weather is triggering a software parameter, which reduces the allowable amount of battery charge. this is triggering a burn off of electrons. prius does this often in large downhill situations using the engine as brake to burn excess capacity
The HV battery does not like being fully charged, my guess is that it is burning off excess charge by using the engine as an air pump.
The car is protecting the battery from overcharge by using it to spin the ICE. You're not burning any gas during this phase, so it's probably okay to shut off while the engine is still spinning. But, you might want to try B mode when coming down this hill. You'll be using the spinning engine to help slow the car and maybe not fill the battery up with regen braking. Not sure you've noticed this, but if you are keeping your foot on the brakes during this descent, when the battery fills up, the regen braking ceases and the braking switches to friction entirely. You'll actually feel the transition if you have your foot lightly on the brake pedal when it happens.
