Simplest and cheapest way: use one or two 12 V 20W halogen bulb , connect them in series with the battery, connect to the DC motor. Monitor the charging current, vary the number of halogen bulbs in series to get the right current.
You get charging and light at same time.
Sharing knowledge is one thing that defies basic arithmetic logic --- the more you share, the more you get!
Normally a DC motor used as a generator will not have a regulator in it.
The Voltage is determent by the speed of the shaft. So, about one fourth the shaft speed will give you about the output voltage you need. Other facts that you need to know: DO NOT go over the Max. rated current of the motor/generator; -- a friend of mine, used the Max. Watts and end up copper flowing out of his generator!!! Look up the manufactures SPIC sheet on the motor and if you are run near Max. current keep and eye on the temperature, for there are usually several different Max. temperature SPIC!
If the batteries are Lead Acid your max charging current is about one tenth the Max Amps/Per/Hour rating but it is always best to get the manufactures SPIC sheet. Even when you get the batteries to stop taking current, the manufacture may have you keep the batteries on a low charging current for several hours to get the max charge into the batteries.
Lot of thing look like they simple straight forward projects but when get down to the "NUTS & BOLTS" there are a lot of little that can "BIT YOUR ASS".