Marine Battery Ratings

Most buyers of marine batteries compare products on the market by the number of plates, 20 hour capacity, reserve capacity, and the cranking performance. Before continuing, I would like to explain the meaning of the last three items.

The 20 hour rating is the capacity of the battery determined over 20 hours at 80° F (26.7°C). A battery rated at 100 A.H. for 20 hours means that if you divide 20 into 100 the battery can be discharged at 5 amperes continuously for 20 hours. Likewise, if the rating was at 8 hours then divide 8 into 100. This would mean that the battery could discharge 12.5 amperes for 8 hours.

Marine batteries are usually rated at the 20 hour or 8 hour rating. The 8 hour rating is usually approximately 82% of the 20 hour rating.

The reserve capacity is defined as the number of minutes a battery can be discharged at 25 amperes. The temperature again is 80°F or 26.7°C.

The cranking performance is a measure of the maximum load a battery can withstand for 30 seconds at 0° F or 17.8°C. The cut off voltage is 7.2 volts for a 12 volt battery.

The cranking performance of a battery is determined by the square inches of surface area of the positive plates.

The 20 hour, 8 hour and reserve capacity of a battery is determined by the amount of cubic inches of active material. To obtain the maximum in these ratings the battery would be designed with fewer, but much heavier plates.

In order to demonstrate the confusion that rating presents to the layman, I have made a comparison between Rolls and a leading R.V. type battery.

Both batteries are approximately the same dimension except the Rolls is higher. The R.V. is a little longer because of the rope handles. Both batteries will deliver approximately 90 A.H. when new. But this is where the similarities stop.

The Rolls is conservatively rated because of the thickness of the plate and density of the active material. Because of the thickness and density the electrolyte cannot penetrate the inner service of the plate. However, as the Rolls ages this unused material comes into use and the battery actually increases in capacity. When both batteries are subjected to hard service the R.V. type drops off in capacity very quickly. The Rolls increases in capacity and maintains this capacity over along life span.

The Rolls does not have as good a cranking performance because of the lower exposed plate area and also due to the thick woven glass mat. The thick woven glass mat retards the flow of electrolyte and thus reduces the cranking performance by approximately 10%. However, the glass mat adds to the life expectancy and reliability of the battery. The R.V. battery has a thin glass mat which is ineffective.

If the initial performance was the guide then the R.V. would be the better buy. However, when you think of the long term investment the Rolls is by far the better buy. Also, do not forget about the safety of a reliable product when at sea. Remember, "quality hurts only once".

When comparing batteries ask about the ratings as this will determine the physical size of a battery you will need. But do not stop, ask additional questions. It is most important that you know the type of insulation. Cellulose (impregnated paper) is the least desirable. Ask about the thickness of the glass mat. It should be nothing less than 0.020. Ask about plate thickness and height. The weight of the battery is a good guide as to the thickness and height of the plates. A thin plate battery will weigh less. If your dealer is knowledgeable and is not trying to confuse you, he will have the facts.