There's a simple approximate rule you can use to get started thinking about most electric motors.
- The voltage across the motor's terminals is proportional to the speed at which the motor is turning.
- The current through the motor is proportional to the torque exerted by the motor.
This ignores various non-idealities, most notably the resistance of the motor windings, but is a good place to start when you're trying to figure out which way around the voltage and current are.
I want to accelerate using the motor, in which case the batteries will simply be connected to the motor, which in turn will rotate and apply some extra torque on the wheel and thus the bike will accelerate.
This is true, but you are going to want some control better than on-off. Otherwise, either it will be a very abrupt acceleration (lots of torque applied) or it will not supply enough power to be useful. Thus, you need a motor controller, which can have a “throttle” input for you to control by hand.
Am i right in assuming that the generator (the motor driven by the wheel) will never produce more than the 24 V of the batteries, and I thus need to find a way to reduce the P.D. of the batteries, if I was to simply inverse the circuit?
This is almost correct, but you need to clear up your thinking about it a bit. Follow the rule I described above: if the motor is turning faster, then you can conclude there is more voltage.
Let's say you're out riding your 24-volt-direct-connection bike, currently on a flat road in Ideal Physics Problem Land — then the voltage in the system will be 24 V and no current will be flowing.
Now, say you start going down a slope. Gravity will be accelerating you, and so the voltage at the motor will be increasing beyond 24 V. This reversed voltage difference causes current to flow in the other direction, charging the battery. Thus, the voltage does stay around 24 V, but only because the battery is acting to regulate it.
So you have in fact got the right general idea about regenerative braking — if you want to slow down below the 24-volt-speed, you need a converter (a motor controller, in fact) which makes it look like you have a battery voltage somewhere below 24 volts. That, by itself, is enough to cause braking.
A motor controller can be thought of as a variable power supply specialized for motors. It works much like a switching power supply, using PWM control and big capacitors. To perform regenerative braking, the controller does in fact (or can; there are various designs) connect the battery “backwards” (using an H-bridge circuit) for very brief intervals to the motor. The inductances (in the motor) and capacitances (in the controller) translate this into a smooth application of reverse torque and a smooth reduction in voltage and speed.
(If you were to wire the battery to the motor backward with a switch, you would still get braking — or more precisely, the motor would try to reverse direction as fast as it could, throwing you off and possibly burning out from stall current.)
High-power motor control is not an easy problem — you need big MOSFETs, big heat sinks, big gate drivers, big capacitors, and control firmware that doesn't make mistakes which either blow up the motor controller or, as I have mentioned several times already, throw you off the bike.
If you want a practical, rideable device, you should buy an off-the-shelf motor controller. If you want to understand power electronics and control systems, you should build your own. (And either way, this blog is a fun read mostly on the subject of Stuff With Electric Motors In It.)