Hybrid Battery from 8 to 1 bar instantly

What does "ICE went wild" mean? Do you mean that engine RPM dramatically increased?

If that is all you meant, then it is normal for the gasoline engine to have to work harder when the traction battery state of charge is depleted. You were wise to reduce vehicle speed from 70 mph to 50 mph. It sounds like your car is behaving normally.

 

It could be that your traction battery is starting to weaken.

 

Another thing to check is your battery cooling fan. It is possible that a passenger or items in the back were covering the intake located next to the passenger seat. This could have caused the battery to overheat... I removed the molding covering the fan and cleaned it on my Gen II at 60,000 miles. It is worth noting and making sure it is not covered in the future. Good luck!

 

i think, at 10 years old, you'll be replacing the battery in the near future. how near? difficult to say, everyone's experience is different, but it might be good to start researching. all the best.

 

As above,
It could be normal behavior for hill climbing...that's what hapens when your battery gets low, the ICE comes on and sounds like it is working hard. Sometimes you wonder why it got low, but that can have to do with A/C.

You did not really convince us it was instant change in the state of charge, but if so that is one sign of a weaker battery. Member UART down in Australia has posted a lot about how his battery does something like this.

 

I think you've more or less gotten the answer.
At 10 years old, it might be an early sign of the hybrid battery weakening.
But if for all other extensible purposes it is working OK now?
Then I'd just keep driving and keep an eye on it.

If it isn't anything...you'll continue to be OK.
If it IS something...keep your cell phone with you. And you'll continue to get more evidence a problem is manifesting.

Maybe not the most reassuring answer I could give you. But if it is nothing? It will be nothing. If it's the hybrid battery on the way out? More signs and eventually codes will happen.

 

Yep. It sounds like a classic undershoot of the SOC, which mostly seems to happen after the SOC has been "maxed out", and is a sign that that the battery capacity is degrading. Undershooting from 8 bars down to 1 seems like a particularly bad case (mine typically undershoots from about 6 down to about 3 - after it's coming back down from full), but it may have been exacerbated by the particular terrain on that trip and the use of cruise control. If I am expecting the undershoot I can sometimes drive so as to avoid or minimise it.

See my post about this issue here: Traction Battery SOC "Meta-States" | PriusChat