Electronic – Vintage DC generator

A typical DC motor may be reasonably accurately modeled as an ideal motor in series with a resistor and inductor. At any moment in time, the voltage on an ideal motor will be proportional to the rotational speed, and the current proportional to the magnetic torque. The proportionality constants for speed and torque will be such that one watt of electrical power will yield one watt of mechanical energy.

When trying to use a motor as a generator, if shorting its leads doesn't remove enough mechanical energy from the source, you'll have to add a gearbox or use a different motor. Trying to extract maximum energy from a given source of mechanical energy may be tricky, but if even a dead short doesn't result in enough mechanical loading there's no need to work things further.

A battery is at least two cells, so what you want to do is charge a cell, not a battery. You'll need a voltage higher than 1.2 volts to charge the cell, and you'll need to limit the current into the cell being charged in order to charge it safely. You can find out the charge requirements by going to the manufacturer's website and reviewing their technical literature. You could also go to battery university, an excellent site for learning about batteries and cells.