dual battery on atv

 I want to add another battery on my atv. I got a winch that needs 500CCA, my battery only has 200CCA. I am not to worried about being able to keep them charged up, because I'm not running a lot of extras. I only use the winch once in a blue moon, its just on that blue moon I really need it. I would down size the winch but I would lose half of my money, so if a dual battery  can be done with out costing an arm and leg. How can I saftly add an extra battery.  Atv output 330 watts @5000rpms and I don't turn 5000rpms a lot. But it keeps the battery charged and runs one extra set of lights fine. Are there any Isolators that would work. My main thing is to keep from frying the electrical systems.

add a battery isolator between the two batteries so that the extra battery keeps a charge. With an isolator the extra battery only gets charged when the primary one is fully charged.crazy nate