Dual Purpose batteries are deep-cycles that are capable of delivering cranking amperage for smaller motors. I wouldn't use one on anything with more than three cylinders, and not on any EFI of Direct Injected models as a primary battery. They still charge slow. Use the Deep Cycle setting and I wouldn't go over 10amps.
Hey, Have you seen? They make Really Cool life preservers nowadays.
So "Looking Dorky" is no longer an excuse for not wearing one.
The most effective way to charge and/or maintain a battery with a charger is slow and steady. A 2 amp trickle charge is fine. If you are having a problem and need some quick juice to get the battery going, then you can hammer it with a 10 amp charge but only for a short period of time. Do a search on batteries here and you'll find a lot of info on how to maintain your battery. There is no such thing as a "maintenance free" battery. All batteries should be checked periodically, especially the fluid levels, at least once a season, usually spring time. Pop the caps and see WITH A SMALL PEN LIGHT, NOT A MATCH OR LIGHTER, if the fluid inside is at the tops of the plates or slightly above. If not then boil some water, let it cool down to room temperature and add it slowly until the levels are up (boiling it distills it.) Also if you are going to charge a dead battery you should pop the caps off as it will create some heat and gases that you'll want to allow to escape. Do this in a ventilated place like your garage with the door or a window open. Every now and then you can look inside carefully and see if bubbles are coming up in the electrolyte. This means you're getting a good charge.