The most common causes of boat battery failure are sulfation, overcharging, and undercharging. 2. Sulfation is a build-up of sulfur on the lead plates in the battery that reduces the battery's capacity and lifespan. 3. Overcharging can cause the battery to overheat, leading to grid corrosion and shortened life. 4. Undercharging can also reduce the lifespan of a battery, as it doesn't allow it to charge fully or discharge. 5. It's essential to use an automatic deep cycle battery charger to avoid these problems. This will help keep your batteries charged all year round when not in use.