As a general rule, a battery doesn't need charging if left sitting for a few weeks, if you have to shag around charging your battery to get it to start your motor, it's buggered, batteries don't need additives, a good battery can sit for quite a long time if fully charged, my tinny battery can sit for a couple of months during winter and it never gets charged, I know when I get the boat ready for crabbing (last week) it will be OK, if it's not, I would replace it.