This has to be one of the most repeated questions and should be a sticky somewhere so the curious can maintain their batteries properly. Most answers are opinions and we all know about them, they're like and some can stink pretty bad. It has been said many times that most batteries are destroyed, mostly by neglect and users being unaware of what happens to them. Without cutting and pasting the whole article(there are many, simply google battery sulfation), here is just a small excerpt: http://www.boatelectric.com/sulfation.htm
The worst sulfated battery can be created by just leaving a battery unattended for a long period of time. A battery is like a bucket with a tiny hole. Electricity leaks little by little through what is called "natural discharge". It may take three to six months to drain a battery completely, and sulfation created in this process is quite possibly the worst kind.
For deep cycle batteries used in Renewable Energy, Marine and RV applications, sulfation buildup is a real problem because these batteries are not regularly charged to their full capacity.
Do yourself a favour, get a battery maintainer such as the CTEK from CDN Tire or Battery Minder from VDC Electronics and let them keep the battery in its best condition. I know, they cost money, how much does throwing a battery out every couple years cost. They take pennies worth of electricity during the off season, maybe 1% of the cost of a new battery. My boat battery is over 9 years, my last RV battery lasted me 11, I must be doing something right. Don't be the doofus that pulls up the boat ramp with a dead battery and says well I charged it last fall.
I'm not pointing fingers at anyone who made earlier replies, just trying to help you avoid blowing money on batteries when you could be buying more fishing gear.