Engine revs high when stopping on a downhill slope

Hi all,

Does anyone know why a Gen2 Prius would rev the engine really high when stopped with a fully charged hybrid battery, but only on a downhill slope?

There is a particular road near me that has a long downhill stretch (well maybe 10 seconds at suburban driving speeds), and there are traffic lights at the bottom just before it flattens out, so I always end up at a complete stop while the car is nose down at a maybe 10-20 degree angle. Usually by the time I reach a stop the battery is fully charged or close to it (usually one bar off full). At this point the Prius will often start the engine again and idle it, although the display does not show the engine running or any of the power going to the wheels or battery.

This evening it gave me a bit of a fright because it didn't just start, but it revved really high, again only while the car was stationary. The engine typically does not run on the downhill stretch (it charges the hybrid battery in silence) it's only maybe 3-4 seconds after coming to a complete stop that the engine starts back up again.

I know the engine tends to rev when climbing a hill depending on how much current the batteries can deliver, so an aging battery with less oomph tends to make the engine rev higher, but that's when climbing a hill - this is when stopped, and only when stopped with the car still on the downhill slope. If the car is stopped on a flat or uphill slope it does not start the engine while stationary (unless of course the hybrid battery is almost flat but that's to be expected).

Does anyone know what could be causing this odd behaviour? It does it regardless of the level of fuel, however last time I replaced some unhealthy cells in the hybrid battery it stopped doing it for close to a year, but now it has started up again. So it would seem it's related to a hybrid battery that's nearing repair time again, but my question is why?

I get that a weak battery would cause the engine to rev high to make up the shortfall when driving normally and especially when climbing a hill, but why would it happen when stopped and there's no work for the engine to do, and why only when stopped on a downhill slope and not flat or uphill?

I found a post on here that said the Prius will turn the engine with the electric motor if the battery charge gets too high, but I couldn't find any confirmation that the Gen2 does this. I can also hear the electric motor turning briefly when it tries to start the ICE and it sounds like a very quiet whine (barely audible really), but when the ICE starts it's very clearly an ICE engine running. When this happens at the bottom of the hill it sounds exactly the same as the ICE engine running and nothing at all like when the electric motor spins to start the engine, so I don't think it is "bleeding off charge". It also happens when the battery is still one bar away from full - you'd think the battery would have to reach full charge before it would try to reduce the charge level, so I'm not convinced that's what's happening here. However it is a valid point that it happens after a long downhill stretch where the battery has been charged via regen the whole way. I suppose I should try to find a long downhill stretch where I can stop on a flat or uphill bit at the end and see if the behaviour is the same.

Any other ideas what could cause this type of behaviour?

 

Very interesting, thanks for the feedback! What metrics would you be looking at in the OBD data? I can connect something and see if I can see anything. I seem to be able to reproduce it close to 100% of the time on this particular hill.

It's winter time here so while our winters are quite mild (18C / 64F) the journey I made last night is only six minutes long so the car wouldn't really have had much time to warm up, but I guess it's possible that a failing cell in the battery might be causing excess heat.

If you've ever driven a Prius with a dying battery up a hill, foot to the floor, you'll know the sound of the engine revving at what sounds like its max RPM. That's the sound it gave me last night while stopped - it's much louder and somewhat higher pitched than normal ICE idling. It would be like in a manual car putting your foot most of the way to the floor while it's in neutral and holding it there with the engine revs way above idle. In my experience it's rare for the Prius engine to run at such high RPM, and I usually only hear it that high when trying to accelerate quickly and the battery is only at one or two bars.