Originally Posted by bubb, the real one
I second the idea of using a regular 3 stage battery charger, the 12v charging port on your generator will do a poor job of fully charging the battery and it may overcharge it, unless of course the generators manual says its 12v outlets output is controlled by a 3 state battery charger,
bubb is right; don't charge with the "12v battery charge" output directly from the generator. They're low power, completely unregulated, and will happily over-charge a battery. I think it's a testament to how unusable this output is that the larger portable/contractor generators with battery start don't even wire their own starting battery to that output! They know the battery would be boiled dry before the machine had even 10 hours on its clock.. (to qualify that statement: the higher-cost type, usually permanent-install and over maybe $3000, often have an alternator with circuits to properly charge a battery. That seems to be universally omitted on the portables, though.)
I give a second vote for the Black & Decker charger Nat recommended. I have the same one and like it quite a lot. Another I like, though in a much lower power class, is the Battery Doctor #20085 2/4/8 amp smart charger. It's the only one I've ever seen that automatically powers on to the same charge rate you picked last time, so if you hard wire it to a battery, you simply plug in the mains cord and walk away. It doesn't remain off, or default to the lowest rate, if you forget to click a button the way so many others do.