Where does extra electricity go?

They all turn green at 80% charge.

B doesn't convert excess energy to electricity; it applies engine braking.

I have noticed that initially when you put the selector into B that the added braking force seems to charge the battery. When the battery indicator shows full charge, you will notice that the engine speeds up significantly. I believe the control computer manages the flow of energy, and the energy flow changes depending on the conditions.

"B" mode just tells the computers to engage the generators. If the batteries can use the charge, then the extra load the generators provide will absorb energy from the wheels and give a similar effect to "downshifting" a "regular" car. When the batteries are fully charged, the generators will no longer attempt to charge them and the load will be greatly reduced. I found this out the hard way when I took my '02 to Pikes Peak a couple of years ago. Thinking my regenerative braking was saving my disk pads, I came down with that superior feeling until I had to go to car "time-out" when my brakes overheated! But my batteries were really charged.....

'B' is for engine braking. It's like trailer trucks' jake brakes - the engine spins to bleed off your speed. Under most conditions, you actually get LESS battery recharging when in B mode (so it is NOT B for battery, but actually B for braking).

When the hybrid battery reaches its upper charge limit (shows as 100% on the screen, but in reality it's only around 80% of full charge on the battery), the excess generated electricity is used to spin the gasoline engine faster. You'll probably hear the engine still spinning once you get down to flat land and cruise along on surface streets in electric-only, while the battery tries to get rid of that excess charge and get back to its happy medium.

The owner's manual says "B" applies engine braking. It still charges the batteries if they will take a charge. Driving down Pike's Peak is an extreme case.

Which probably answers one of the other questions I had on this same trip ...

When going down a long hill, as long as the battery is still charging, is it "better" to leave it in 'D' and use the brakes (rather than shift into 'B')?

Every time I've tried to use B going downhill (including the Grapevine on I-5 in either direction), it slowed the car too much, so I wound up keeping it in D and using the brakes. Keep in mind that except for panic stops and when the car is going very slowly, the brakes are the regenerative brakes and not the mechanical, friction, brakes.

Wouldn't it work to put it in B and accelerate to the desired speed? I mean, wouldn't light pressure on the gas pedal overcome engine braking as with a conventional stick shift?

On long downhill treks, the batteries will fill up, and braking will revert to the old-fashioned mechanical brakes. So I'm thinking that Michelle has the right idea, leave it in B and give it a little gas as needed .. that way, the deceleration will be performed either by regenerative braking or enging slowing?

Yes, that works also. In the Prius, overcoming engine braking merely means that the computers will stop using engine braking whenever you are commanding acceleration via the gas pedal; there will not be one mechanical system fighting another and wasting energy. Just remember to shift back into D at the end of the downgrade.

yeah, but that uses gasoline.

Um, that's not my idea; my idea is to leave it in D and use the brakes.

Right.

Not really. It just tells the computer to reduce the braking effect. Remeber, the "gas pedal" is not connected to the engine. The gas pedal is simply a suggestion to the computer that you want to change speed.

I use B on the grapevine in conjunction with cruise control (2002 model) and it hold speed well even if there is an uphill grade followed by a downhill.

In the 04 and later models, putting the transmission into B disables cruise control.