Use the B position of the Mode Selector Lever in exactly the same circumstances as you would change to a lower gear to slow down a conventional car. If you are going down a long and steep hill, controlling the car's speed using only the brakes can cause them excessive wear, get them hot and may also tire your foot. The solution in a conventional car is to select a lower gear. The engine then spins faster and the energy it takes to do this holds back the car. Less brake pressure is needed to maintain a slow rate of descent. The Prius, however, has no gearbox in the usual sense of the term. Toyota have made special provision for engine braking and to activate this you put the Mode Selector Lever in the B position. By a special configuration of the motor/generators, the engine spins and helps hold back the car just as in a car with a gearbox (see How B Mode works, below).
There is no benefit to using B mode in a situation normally calling for the use of the brakes (except one, see Advanced uses of B Mode below). If anyone, even someone who should know what they're talking about, tells you that in B mode you recover more charge to the battery, they are wrong. You recover less for the simple reason that the purpose of B mode is to throw energy away by spinning the engine. The B does not stand for Battery, it stands for Brake. However, as far as I know you will not hurt the car by using B mode inappropriately. If you press the accelerator, there is no difference between B and D modes. So, if you don't care about recovering braking energy to the battery and you get a kick out of flipping the Mode Selector Level to slow down, by all means do so. Just don't be surprised if other Prius drivers roll their eyes at you.
There are two situations in which B mode has been found useful other than descending long steep hills.
The first is in slow descents of bumpy roads when you might not normally think of using engine braking. What happens in such a descent is that the car's traction control system disengages the regenerative braking system. This means that you are not recovering very much of the energy of the descent as charge in the battery. You won't hurt the car and there is no need to do anything about this except to brake a little harder, which you will do automatically. But, if you want to recover as much energy as possible, you can switch to B mode. A side-effect of the way in which B mode works causes an increase in battery charging at lower speeds when regenerative braking is disengaged. Note that if regenerative braking were to operate, for example if you reached a smooth section of road, you would get the most regenerated charge by switching back to D mode and increasing brake pressure. It may be necessary to release the brake for an instant to let regenerative braking reset itself.
The second is in stop-and-go traffic queues. Normally, when you move the car forward you will use battery only mode and the engine will not start. After a while, however, the battery charge level drops and you will find the engine starting for each short movement and then shutting down. There is nothing wrong with this, but if you find it annoying you can switch to B mode during a forward lurch. When you come to a stop, the engine will continue to run and charge the battery for a while. Why this happens is not understood.
All material Copyright © 2003 Graham Davies. No liability accepted.