[Will M]

If you want to "control" the car and stop it from depleting the battery, thereby forcing it to charge whenever it can, just downshift. You might have to pull that whole mountain in 3rd gear. This control works fine and it doesn't void any waranties. That still only revs the engine to around 3,000-3,500rpm. Many cars cruise at that RPM on flat highway.

Nobody gets good gas mileage while climbing a mountain. Deal with it. It takes more energy to climb. Energy is fuel. Some cars won't seem to burn that much MORE gas going up a mountain, simply because they are already wasting so much gas the rest of the time with unneeded capacity. You are getting a good deal with this car and the way it manages fuel.

The battery charge is just another form of fuel. You can burn it recharging the battery, or you can burn it in 3rd gear. Your choice.

 

[Will M]

The thing is the system tries to preserve the battery chemistry by not letting the batteries sit at the bottom of the charge. If allowed to sit there too long, other factors will dip the charge even lower; below the 20% limit the system tries to enforce. That low a charge can make a permanent change in battery chemistry, shortening or ending the battery life.

The battery is meant to act like a flywheel. It absorbs energy some of the time and gives it back when you need it. Unfortunately, a mountain of several thousand feet requires too much sustained power for the battery to really be much use. You pretty much have to use low gears and run that little gas engine in the higher rpm range where it has enough unassisted power to push the car up the mountain. Ignore the light suggesting that you shift to a higher gear.

If the battery had enough capacity to take you up that mountain, it would weigh much more and the rest of us who don't need that capacity woud pay for it in less gas mileage and less storage space. It would be nice if engineers didn't have to compromise, but...