Just like all other equipment, your trolling motor battery must receive proper care to ensure long life. It is not difficult to do and only takes a little time and attention.
Overheating is the number one enemy of all batteries. It causes the plates to buckle and separate from internal connectors. If you make a habit of running your trolling motor battery all the way down, or if you charge it at too fast a pace, the battery will overheat and fail prematurely.
Lead-acid batteries are designed to accommodate the expansion and contraction that occur in normal operation. Internal cells contain a group of alternating positive and negative plates. Higher amp-hour batteries have a greater number of plates. These plates are insulated from each other by dielectric separators and are submerged in a solution of sulfuric acid and water. The solution is mostly sulfuric acid at full-charge, and mostly water when the battery is in the discharged state. This system of cells works fine for many years as long as the battery is not subjected to excessive discharge and charge rates.
It is normal for heat to be generated during battery use. You can feel this by touching the cables that lead to your trolling motor. However, problems arise when the heat becomes excessive. As stated earlier, excess heat will buckle the internal plates. This causes the battery to either fail outright or operate with diminished capacity. It is this second case that can catch you off-guard. The battery seems fine after charging but runs down faster than anticipated. This is a hassle with a trolling motor, but can leave you stranded if it happens to the battery that starts your boat motor.
Undercharging is a common cause of battery failure. Weaker cells become exhausted faster than stronger cells and this leads to damaged plates. This damage prevents the cell from ever reaching a state of normal charge. Repeated undercharging will eventually destroy the battery.
Charging your trolling motor battery at too fast a rate will also destroy the cells. A fast charging rate causes high heat and will most certainly damage the plates and separators. The sulfuric acid solution can boil to the point where large amounts of oxygen and hydrogen are produced. This causes corrosion of the plates.
Proper operation of a deep-cycle trolling motor battery calls for a 50% discharge of rated capacity before charging. Repeated discharging beyond this point causes excess expansion of the cells. This is the reason most people buy battery packs designed for twice the anticipated use.
A precise charge rate will ensure long battery life. High quality chargers have a slow cycle charge rate, and a finish rate to prevent overcharging. This finish rate is important to prevent damage. It has to be just enough to hold the charge without going above battery capacity.
Remember that batteries still need periodic charging while being stored. A slow charge once a month for a couple of hours should do the trick.