Actually, you need 13.8 to 14.0 volts to maintain a minimum charge in a 12 volt battery. While yes, some 6 volt generators may be able to give you 15 volts, they won't be able to do it with much current. As soon as the battery requires any kind of charge, the voltage will drop below the needed level. And the bearings and brushes will last about the same life, regardless of current output.
For just a few dollars more, you can just get a rebuilt 12 volt generator and do the job the right way.