How to avoid overcharging Prius...?

The car takes care of everything and will not overcharge the battery.

 

If you want to exert some manual control, you can shift into B at the top of the hill and back to D at the bottom. (as a reminder to shift back, you can't use Cruise Control in B) As haole man correctly points out this will not prevent over charging, that is automatic. It will keep the friction brakes somewhat cooler.

B under the D in the Drive gears? | PriusChat

 

How steep of a grade and how fast will you be going, and are there multiple lanes?

Keep in mind that if you do a lot of engine braking, your engine RPM will shoot up to over 4,000 at high speeds, so don't be too frightened if the car starts to roar when you switch to B. (Personally I'm always afraid that it'll put a lot of wear on the engine or it'll break in too quick if I'm going 65 mph and I hit B.)

 

Cruise Control will manage everything for you, except on very steep downgrades. When the battery is "full", engine braking will kick in but it'll have less ability to apply drag so be prepared to use the brakes (which cancels CC). It all depends on the grade of the hill.

FWIW, engine braking puts very little strain on the engine even at 5000RPM. The system adjusts valve timing to reduce stress.

 

Highway 17 up from Santa Cruz? That's steep and windy, but not very long by mountain standards. Last time I drove that, it was very busy and the drivers seem to completely ignore whatever the posted limit is.

Don't worry about the car; it will take care of itself. Just drive.

 

I suppose you could turn up A/C a notch to help use up some of the excess energy. As mentioned above once the batt is full then either (1) your brakes shoes are being used more, or (2) you shift into B mode for engine braking. Depending on length of downhill, using A/C might avoid the need for (1) and (2).

 

Be aware that the engine will appear to rev very noisily when going down hill. It probably won't be using any fuel but is being spun by the HV battery to 'burn' excess power or charge. It worried me a little the first time I heard it, so be prepared. It's quite normal - so don't worry.

 

I've used B very occasionally in cases like this. Still, I'd be inclined to just do nothing, at least the first time, and observe if the car continues to charge all the way down. I've heard the car will do a little "B mode" behind the scenes, even if you haven't selected it.

One reason I don't like B: it raises RPM alarmingly on steep down grades, a little freaky. For the most hills I don't think it's warranted; I've yet to see charging cease on hills, though to be fair we've got no really long downhill runs in our area.

 

I live in mountainous terrain in the Black Hills. My battery is fully charged a lot of the time. I rarely use B because I do not like the noise it makes. I may use my brakes a little bit more than I need to, but it causes me less anxiety than listening to the Noise in B mode. I have never had an overcharging issue in 120,000 miles on my 2006 or 3500 miles on my 2013.