How to make a wheel recharge the battery as it decelerates?
OK you need some form of energy harvester.

That is, when you turn off the motor the back-emf will still be present in the motor and it will act as a generator driven by the inertia of your mechanics, which, for a ball-thrower I am assuming is pretty high and it spins on for some time after you cut the power.

As such you need an energy harvester circuit. This is basically a switch mode boost convertor that will "suck" current out of the motor and boost the voltage out up to greater than your battery voltage by a volt or two.

Since the back emf will fall as the motor slows down you need a booster that is capable of running down to small voltages so you can harvest the maximum amount of energy out it. Also, the booster needs to be current limited as you do not want to draw current out of the motor too fast or it may overheat.

There are numerous ready made boost convertors or harvesters out there though 24V ones may be fewer.

Here is a typical one, though it would be nicer if it went down below 5V.


Source

If you want to design your own, Linear Technology etc. sell various DC-DC convertor ICs.

Note the harvester should only be turned on when the motor is off, also, although some automatically shut down when the voltage drops from the motor you may want to add your own supervision.

You may also want to consider augmenting your system by adding some large capacitors in parallel with the battery rather than charging the battery itself. Have those charge up above battery voltage, and then have your controller draw from those. That will be more efficient and increase the life of the battery. 

Or if you are feeling adventurous something like this should work..