B under the D

One thing, there's really no need to use B on a short downhill. I'd only use it on long downhill runs, coming down a ski hill for example. There's no advantage to using it for short hills, it's actually detrimental to battery charging, and doesn't save the brakes in such situations. It's only extreme long downhill runs, where the brake might get fully charged, to the point it can't charge more, that there could be excess brake heat-up. Then you should use it.

 

I feel this is another one of those areas of Prius Chat that is seriously overblown regarding concern.

Unless you are descending a 3000ft+ grade in 100F+ heat I highly doubt you are going to overheat the brakes to the point of failure if you are loading the car within specifications.

And I would argue that if you are up that high the ambient air is nowhere near 100F.

I personally would rather repair/replace my brakes than my engine if I had to choose.

 

In the US, Toyota is required to provide a gear with more engine braking. You are not required to use it.

Some hints you should use it: separate speed limits for multiaxle trucks under 35 MPH, 1000 foot elevation drop, emergency braking ramps, brake inspecion area before descent.

 

You should try northbound I-5 from Frazier Park going down the Grapevine in the 100F+ summer time. It drops from just over 4000ft to just over 1000ft in 15 miles. It's not a lot but most of the it is during the last part of the descent. If you stay in D without braking, it'll get up to 90mph. If you put it in B, it'll slow down to 60 without braking even at full charge. I stay between 65 and 75mph to avoid getting a ticket. CHPs wait at the bottom of the Grapevine to catch speeders. If you have a Scangauge to monitor the engine temp, the engine actually cools down to abut 170f by the time you get down to the Grapevine.

 

The system will automatically use engine braking once the traction battery reaches capacity. So there's no reason at all to manually use B mode. That said, as it clearly states in the manual you can drive around in B mode if you like, you just forgo most of the regen charging you would normally get. Also good to note that light pressure on the hydraulic brakes will not cause over-heating, it takes repeated or continuous heavier braking to do that.

 

Unless you need engine braking, so you do not ride the brakes for 10 miles going downhill.

 

Err, either you use "b" and simply let the cylinders revolve, friction is slowing you, or you use the brakes, (on long steep descents) I use cruise control.