Lead acid batteries will self discharge when left sitting unused. Discharged batteries rapidly undergo a process called sulfation. You see, when batteries are discharged the active lead material bonds with the sulfate and electrolyte forming sulfate crystals. These sulfate crystals harden and block the effective area of the battery and reduce its capacity. A battery with no capacity is useless; it can't provide the power required for its designated function. A smart battery charger, such as the Noco Genius brand of chargers, will help prevent sulfation from occurring by keeping your battery in a constant state of charge. They will never over charge your batteries. These chargers are designed to be left connected to your battery until you need to use the vehicle or device again. Our friends at GeniusChargers.com have compiled a short video addressing this very issue (see below).
If you find that your battery is already sluggish and suffering from the effects of age and sulfation, then you may want to consider the Xtreme Charger XC100-P from Pulse Tech. These incredible desulfating chargers will renew your battery to an "as new condition" by using patented Pulse Technology that increases the duty and life cycles of any size or type of 12 volt lead-acid battery by minimizing and removing lead sulfate crystals.