Basically, I go along with the bigger battery school of thought, but in the interests of pushing the boundary a bit - what were you thinking of using for a generator?  If it is a mains electricity type (eek!), then a battery charger for your type of battery will do the job.  If it has an auto-mobile type alternator, it might well have the regulator built in.  The normal rules for charging lead acid apply, voltage limiting to avoid over charging, and current limited to the maximum that the battery allows.  It assumes that if the motor wants more than the charger can give, the battery makes up the difference until the demand is removed, when the alternator recharges the battery.