Most generators (specially Chineese) are manufactured for AC (110V) generation. They can supply 12 V for charging purposes but the output is generally low. Make sure to look for a good unit that has a high current output for 12 V.
It is a better idea to usesolar panels + wind power for charging. They are more silent, does not need gasoline and work 24 hours.
Some makes of wind generators are too noisy. Used a three blade one with blades narrowing on the tips. In fact I stopped it due to extra noise.
I alsoused a five blade one with blades not narrowing. It was very silent. You cannot even know if it is working.