How to Rapidly Kill a Boat Battery
There are three ways to shorten the lifespan of your boat battery.
Undercharging: Consistently failing to recharge batteries fully leaves them with lead sulfate that hardens on their plates—they become sulfated—and gradually lose their ability to perform. Increased resistance when charging causes falsely elevated voltage readings, essentially fooling the battery charger, leading to further undercharging, in a downward spiral. Beyond a certain point, a sulfated battery cannot be returned to a healthy state, and you need a replacement. Keep your batteries charged, and equalize your wet cell batteries every six to eight weeks in temperate climates, and more frequently in the tropics.
Overcharging: Especially fatal to Gel and AGM batteries, consistent overcharging (NOT equalization) boils the electrolyte out of the cells, and can even lead to thermal runaway, with the battery becoming hotter and hotter.
Excessive deep discharge: Don’t completely discharge a deep cycle battery if it can be avoided. The deeper the discharge the less life you will get from the battery. The ideal method is to charge and discharge the batteries through the middle range (50 percent to 85 percent) of their capacity and, if they are flooded batteries, to equalize them periodically. Leaving the battery in a fully-discharged state, for example during winter storage, causes it to become sulfated.
Tips for battery longevity:
Shallow discharges lead to longer battery life.
80 percent discharge is the maximum safe discharge
Don’t leave batteries deeply discharged for any length of time
Charge batteries after each period of use
Don’t mix old batteries with new ones