Marine Battery Design

There is considerable confusion in the marketplace concerning marine batteries. What is a marine battery? What is the difference between a marine, deep cycle, automotive and R.V. (Recreational Vehicle)? How to maintain, etc..

Many battery manufacturers sell their product to the marine trade by supplying their regular automotive and truck batteries with marine labels. This is difficult for the average layman to detect.

The Rolls definition of a marine battery is one that is designed for maximum reliability. Remember, there are no service stations at sea. Maximum reliability is achieved by utilizing heavy positive and negative plating, dense active material, reinforced grid design, premium insulation which usually includes a thick woven glass mat, cell protector, rubber container, heavy intercell connections and on larger batteries multi-cell construction.

The majority of failures in lead acid batteries are contributed to the positive plates. The grid which is part of the plate is referred to as the current carrying conductor. The plate consists of the grid plus the active material. The active material chemically produces electricity which is conducted by the grid via inter cell connectors to the terminals.

Every time the average battery is discharged a small portion of the active material becomes inactive or lost because of expansion. On recharging the current chemically converts the active material from a lead sulfate to lead dioxide. This same current also converts a small portion of the lead in the grid to lead dioxide. This is sometimes referred to as a grid corrosion.

By now, you have probably guessed why the heavy positive plates and the reinforced grid design. The heavy positive plates provides more active material. By using a denser active material, more material can be held within a given space.

The reinforced grid design is accomplished by providing more metal in the grid. More metal can be lost from grid corrosion without harmfully affecting the battery. Overcharging will accelerate positive grid corrosion. Starting batteries are subjected to overcharge conditions when the engine is running. The electrical system is designed to replace a little more current than is lost via self discharge.

We often use the expression "The chain is only as strong as the weakest link". This is true in battery engineering. That is why a high quality marine battery will utilize high quality insulation to compliment the heavy plate construction. In most cases a thick woven glass mat will be placed against the positive plate. This is used to maintain the active material on the grid which extends the useful life of the battery. The glass mat is also referred to as a retaining mat. The best insulation is PVC, rubber or polyethylene. Glass matting should be a minimum of 0.020 thick to be effective.

The cell protector, made of perforated vinyl, fits on top of the cell immediately under the vent opening. The protector mat prevents damage to the insulation when a hydrometer is inserted into the vent opening.

Heavy intercell connectors are important to provide that extra margin of safety during high discharge. Thin intercell connectors used in solid top construction may melt under severe loads.

A good quality marine battery is initially expensive although inexpensive when considering the life expectancy. Multi-cell construction can be repaired in the event of damage. Solid top construction cannot.