I use B-mode not only for descending but also: -For taking an exit from a highway. -Slowing down more rapid heading for a stoplight when i see that gliding won't do the job within the stopping distance i have. I do both with the intention to spare my brakepads/discs. Is this bad for ?: -Some parts of the car -The mileage/fuelconsumption ? -The environment ? And last but not least... I also have the feeling when i'm in B-mode and simultaneously put my foot on the brakepedal that my brakeforce is 'much more efficient' then in D-mode.
The simple answer is you are "wasting" some energy. In "B" the engine converts some energy into heat. In "D" it doesn't convert as much of the "extra" energy into heat. If you are careful with the brakes you don't wear the brake pads. It's amazing how much braking you can get with regen. Especially with the GIII. You will recover about 50% of that energy eventually (the rest is lost in conversion). In "B" the energy lost to engine braking is not recoverable, unless the coolant is below the control temp. and the energy is then used to keep the engine and cabin warm. As far as being bad, it's just fine either way. It won't cause significant wear on any engine parts either way. You will probably drop your fuel mileage just a bit, though you may not be able to see it. Regeneration accounts for about 30% of the fuel economy -improvement- of the Prius, so you may see up to 1 l/100 km improvement using it fully. Assume using "B" wastes about half, so you may see a 0.5 l/100 km drop in fuel economy. This is easily swamped (lost in noise) by other factors. Slowing down on freeways and coming to lights is not a big part of driving, time wise, so that would drop that 0.5 even further. As far as the environment, you will probably end up burning a bit more fuel, so that is slightly worse for the environment (a bit more CO2 emitted).
Use the brakes, the system is really clever and uses regenerative braking as much as it is able so there is little use of the actual service brakes in the wheels under normal and light braking. Brake as long and soft as you can for maximum energy recovery, better still try not to brake at all if you can get away with it. Otherwise David has already answered the questions asked.
While driving and taking that exit or driving towards the stoplight... B-mode = charge bar > fully filled Braking = charge bar > 1/4 filled That made me thinking that B-mode recovered more battery...guess not ?...
To let you see how well regen braking is take a look at your disks. I've always driven pretty easy on the brakes for all my cars including my new Prius. I've had my car since the end of July and a few weeks ago I noticed rust starting to form on the rotors meaning the way I'm braking isn't using the rotors at all. Since then every once in a while I'll do some hard stops to clean the rotors off. Going like this I don't know if I'll ever have to change pads and I keep my cars for a LLLLOOONNGGG time. My last car that I just traded in (Cash4Clunkers) was a '95 4Runner with 230k miles and I think I did the brakes on that three times.
