OK, here's how regenerative braking works.
First, a little background information about motors and generators. This is all directly derived from the basic electromagnetic physics of a conductor in a magnetic field.
Basically, a motor and a generator are the same thing, used differently. Every motor acts as a generator, and generates voltage as it turns, proportional to the rotational speed. Every generator acts as a motor, giving torque opposing the driving torque, proportional to the current.
The opposing voltage is sometimes referred to as "back EMF", but let's just simplify the picture, and consider them both to be the same thing. By thinking of just one device (as it actually is), we have a very simple and accurate model.
So we can describe a motor very simply (as a slightly idealized model ignoring resistance and similar effects), thus:
a) The voltage is proportional to the speed. Reverse direction, and you get reversed voltage. If you control the voltage, you control the speed. If you control the speed, you control the voltage.
b) The current is proportional to the torque. Reverse the current, and you reverse the torque. If you control the current, you control the torque. If you control the torque, you control the current.
You can't control both voltage and current at the same time. If you change the voltage, how much current is determined by how much torque is required to speed up or slow down the motor and whatever it is attached to.
Likewise, if you control the current, you apply whatever torque is indicated by that current, and the voltage will reflect what the load does, speeding up, slowing down, or staying the same.
So, I'm driving my Prius (I actually have two Gen 1 Prius's), and I step on my brake, but not hard enough to engage the friction brakes (it does have friction brakes).
Or, I brace for a stop on my Segway, and it begins to decelerate accordingly. (It does NOT have friction brakes, and relies entirely on regenerative braking. Friction brakes would be incompatible with its self-balancing function).
To do this, both devices draw power from the drive motor. That is, they take current in the same direction as the motor's voltage is pushing it. Power = voltage x current, so this is power out. Power integrated over time is energy -- and this is the energy that's stored in the battery.
Now, there's a problem. Batteries operate at an approximately fixed voltage, based on the electrochemical reactions that go on inside.
But voltage varies with speed. So a regenerative system, to work at a low speed, has to accept the current at a low voltage, and yet stuff it into the battery at a high voltage.
To do this, we can use a bit of switching-transistor magic, in a circuit called a voltage multiplier. Let's take, say, four capacitors. We accept charge quickly from the motor (high current) and use it to charge the capacitors in parallel, matching the motor's output voltage at this speed.
Then, we flip a few transistor switches, and stack the capacitors in series, giving 4x the voltage, but only 1/4 the current (so it's the same amount of power out). This then pushes the charge into the batteries.
To take the energy back out on starting up again, we just reverse the process. Charge up the capacitors in series, dump them into the motor at a low voltage but high current for a good high starting torque -- but carefully, so we don't spin the tires. Note that this gives us high torque right where gasoline engines are at their worst -- low speeds.
That's about it, really. The batteries to hold the charge aren't very big, if you're only holding the energy from braking. Both a Segway and a Prius have batteries that are much larger than that, and can store enough energy to drive for a considerable distances on battery alone. (24 miles, for a Segway). Electric motors are relatively light for a given output, compared to an internal combustion engine.
If you also use battery power to satisfy peak demand, you can save weight by using a lighter internal combustion engine AND add to its life by operating it in a lower-stress operating region. Extreme hybrids only need to size the engine to the average power demand.
Of course, in an electric vehicle, like the Segway, you have electric motors, motor controllers, and batteries anyway. You need ZERO additional parts, and ZERO additional weight to do regenerative braking. It's free energy. The same is true if you have a hybrid for other reasons (overall efficiency). There is never a reason to NOT do regenerative braking, if you have motors and batteries (or ultracapacitors) in place.
