[Impact9]

Your best bet is to upgrade the alternator first then add a second battery if you still need it. I suggest a yellow top Optima or a Kinetic (strictly as a secondary battery), but at the same time once you drop $180-300 on a battery just a $100-200 more and you could have a powerful aftermarket alternator.

Shoot I see you already asked that over on CA forums after I did a search on the batcap since I've never heard of them.