Battery fully charged (saturated) during a descent, but ICE stays on for long time after that

No - best to leave it in "B" - that's TOYOTA's advice too.

 

If it is steep enough that you have to hit the brakes over and over, like going down a mountain for several miles, B is better, that is if you don't want to wear your brake pads down, or over heat them. In one way it might not be a bad idea to give the brakes an exercise every now and then, seen some nasty looking rotors on older Prii, they don't get used and therefore they get corrosion and some other issues since they don't get used a lot compared to regular cars.

 

I haven't seen anyone mention yet that the catalytic converter also plays a role.
What i found going long downhills in Austria is that the ICE will keep running using a little amount of fuel as long as the catalytic converter is cold. And it quickly cools down because of the engine braking running it without fuel. Once it's doing that it can show that it's running in EV but still use some fuel to heat things up again.

 

Yes this is commonly referred to here as "stages". When it's warm and no longer running rich to make heat, it's in Stage 4. Before that the engine will run for various reasons and at a different mixture.

 

What i'm talking about is when it's already in Stage 4, and then because of a period of engine braking the catalytic convertor will cool down too much and the engine will start using fuel again to heat specifically the convertor up again. This typically happens on long decents

 

Hey guys,
I believe that the Gen 4 prius suffers the same problem as the new Leaf, ie: Rapid Gate.
After descending a longish grade, (not even that long) the batteries begin to reach 80% and then they hit around 45 degrees C, when it all goes pear shaped.
The computer detects the high temp and then puts the car into a kind of limp mode whereas the engine will not shut off at all, the batteries will not accept more charge even when they get low, and the car virtually runs like a conventional s*** box.
This condition continues for around 10 to 15 minutes until the batteries cool sufficiently.
Earlier Hybrids generally run on battery power alone after descending long grades for about a KM (with careful pedal inputs) but not the gen 4. There really needs to be a recall on this model to rectify this. Even if they have to get the cooling fans to work harder to keep it out of this situation.

 

I don't actually see a problem with it - apart from the sound of the ICE running. Somehow, it still gets remarkable economy when it's doing it.

I think that TOYOTA's THS II has sufficient safeguards built in - not just for the battery, but also the Inverter/Converter and the transmission - I'm quite happy for it to use the many inputs to make it's own decision. If it thinks it needs the ICE running, I'm happy to let it.

It seems more prevalent in summer - so you could be right in that it's heat related. I've tried turning ClimateControl ECO to OFF - thinking that it might cool the battery quicker - not sure if it's helped - as that will use battery power to run the A/C.

Have you looked at the EV% at the time? The trip % is generally in the same range as when it hasn't done it. My suspicion is that it's still using the battery + the ICE to drive the car - but I've no way to test that.