Originally Posted by David Beale

Quick version, the difference between "B" and "D" is in "B" the engine is used in compression braking when you release the throttle. In "D" it is not.
I think in "B" the "base rate of regen" is a little higher as well, but I could be wrong on this.

You use "B" for descending long hills to keep the car under control.

Actually NOT...!!

Your statement "under control" implies, implicitly, adverse roadbed conditions, snow, ice, etc. One should never, NEVER, bias braking toward the front of the vehicle in any way when traveling downhill on a slippery surface.

Absent any other method (say frictional braking w/ABS) of slowing the vehicle a light or slight application of the E-brake (almost always rear implemented) would be appropreate.

The Prius is FWD, ALWAYS.

Engine compression braking and/or regenerative braking, in those conditions can turn DEADLY instantly. To that end Ford now has a US patent that applies to techniques they developed for use in the hybrid Escape and Mariner.

If the OAT is near freezing or below the level of regenerative braking that can be used is significantly reduced. The second technique involves the INSTANT disabling of regenerative braking should ABS activate when you have pressure on the brake pedal.


Generally the Prius will coast better than most other vehicles, so when you descend a long hill you will release a lot of energy into the brakes or regeneration system. If you just use "D" and the brake to control the vehicle speed, you will quickly fully charge (to 80%) the "traction battery" and will then be using the friction brakes only to keep the speed under control. In "B" the car will use the engine to dump the excess energy (and it does so aggressively - I've seen the ICE RPM go up to 5000 on really long steep hills such as a mountain pass!). So you reduce the load on the brakes in "B". On short hills you will be "wasting" the energy by dumping it into the engine compression rather than using it for recharging the traction battery (regeneration).