The bigger problem is that you have a 6-volt generator. And a 6-volt generator will never be able to charge an 8-volt battery. So, your idea will work for about two hours.

How do you convert 6v to 12v? Here are some of 6V to 12V DC converter circuits that can be used to convert a small voltage of about 6 volts to a higher voltage of 12 volts but of course with a lower ...

The bigger problem is that you have a 6-volt generator. And, a 6-volt generator will never be able to charge an 8-volt battery. So, your idea will work for about two hours.