Trolling Motor Battery Care
By Gary Carter

Fig. 1. Battery construction and materials composition in this simplified cross-section acknowledge a normal amount of expansion and contraction of plate materials in the course of routine cycling.

Trolling Motor Battery Care is not difficult, it just requires a little diligence. The buckling of trolling motor battery plates often leads to premature battery failure.  This can be prevented by the proper charging and care of the battery.  Lead-acid batteries of the type used to power trolling motors are designed to accommodate natural expansion and contraction that occur in normal charge/discharge operation. (See simplified cross-section view of a deep-cycle battery cell, Fig. 1.) Each cell comprises a group of alternating positive and negative plates. Negative plates always outnumber the positive plates by one, and the higher-capacity (amp-hour) batteries have a greater number of plates. Positive and negative plates are insulated from each other by separators, and are submerged in an electrolyte solution of sulfuric acid and water. At full-charge thesolution is mostly sulfuric acid and is mostly water in the discharged state.

Positive plates are made up of an active material (lead dioxide) held within a support grid by differing support (insulated) methods. These systems hold the active material to the grid as it expands and contracts during normal battery cycling. They also act as seperators to prevent the plates from contacting each other causing a "short".   Positive plates of a discharged battery have bulkier lead sulfate crystals, which take up more space than the lead dioxide of charged positive plates.

Negative plates have a thinner grid structure and no support system, because relatively little shedding of a negative plate's active material (sponge lead) occurs throughout the battery's service life. Crystals of the negative plates increase only microscopically in size as the battery is discharged.

Fig. 2. No two plates are exactly identical; this would be revealed by microscopic inspection of the "normal" positive plate assembly. Buckling, however, does not occur if the normal expansion incurred in battery cycling is uniform throughout the makeup of a given plate.

In a healthy battery, a simple visual inspection would not reveal any large thickness differences from one positive plate to another, nor from one negative plate to another. No two plates, in fact, are identical, but slight inconsistencies are normally of little importance. If all parts of the plates expand equally during battery cycling (charging and discharging), changes in plate size are of no consequence. Uneven heating, however, may result in buckling of the plates(Fig. 2).

The ultimate result of buckling is capacity reduction and diminished service life. Minimal buckling has no serious effect, but repeated abuses cause expansion and warping of plates. As a result, active material breaks away from the grid areas where electrochemical reactions occur. This can cause damage to the seperation material and result in the previously mentioned "short".

Effects of Uneven Heating

It is quite normal for heat to be generated during conversion of active materials to lead sulfate. It is also normal for this heat to cause expansion of the pastes and grids of the cell. Problems arise, however, when batteries are overheated. With too much heat, the large, irregularly shaped crystals of both positive and negative discharged plates continue to grow. As they are overheated, these sulfate crystals get bulkier, causing uneven expansion of portions of the plates. The parts covered with sulfate do not expand. The result is that those portions of the plate that have the least amount of sulfate expand, warping the plate.

Basic Causes of Uneven Heating

There are four basic causes of uneven plate heating -- and consequent buckling: Undercharging, overcharging, overdischarging, and charging at too high a current rate  will cause battery plates to buckle and possibly short out.

Undercharging.

Routine undercharging is the most common cause of buckling. In this scenario, weaker cells are simply exhausted in comparison to stronger cells. If charging is insufficient, weaker cells do not revert to a normal charged condition. Rather, they remain in an excessive lead sulfate condition of bulkier, irregular crystals, eventually straining the grid structure.

Overcharging.

When undergoing normal charging, cell electrolyte develops oxygen on the positive plates, and hydrogen on the negative plates. However, if a high charge rate is maintained after the battery is brought to a normal gassing condition, abnormally large amounts of oxygen and hydrogen are produced, and excessive bubbling occurs. Active material is loosened from the plate grids, and positive grids undergo corrosion. These physical changes are a direct result of overheating caused by overcharging. Routine overcharging and its accompanying overtemperature increases plate buckling.

Overdischarging.

Proper operation calls for a battery to be discharged to 80% of rated capacity before recharging. Discharging beyond this point brings operation into the zone where otherwise insignificant problems in plate composition and thickness become significant. Portions of the plate having lesser amounts of lead sulfate become overburdened, hotter, and expand more than higher sulfated areas.

Charging at too high a current rate.

This probably the most often occuring charging problem.  Proper charging requires the use of a precise charge rate. The finish or "normal" rate is the amount of current that can be applied at any time during the charge cycle, and which can be continued after completion of charge without excessive gassing or overtemperature. If the proper finish rate is exceeded -- and charging is consistently done at overly high rates -- battery temperature rises abnormally, resulting in excessive and irregular plate expansion.

The bottom line is to charge those expensive batteries with a quality charger.  There are many brands available in today's market place.  Use only battery chargers designed for charging your type of battery.  And, recharge the batteries as soon as possible to prevent sulfation of the battery plates.  Lastly, remember that the batteries also need periodic charging during storage -- charge your batteries the night before you leave for the lake.