Quick Tips #7 - Battery Charge
by Ted McGee
(Article Appeared in July 1999 TellTales Publication)

A good volt meter will tell you if your battery is fully charged. The following table shows the relationship between voltage and percent of charge. As you can see, the difference between a fully charged battery and a depleted battery less than a volt.

Voltage % Charge

12.6 or more 100%
12.4 75%
12.2 50%
12.0 25%
11.7 0%

A 12 volt battery consists of six cells. Each cell contains a number of positive and negative plates. The quantity and thickness of the plates depend on their use and capacity. More thin plates in a cell gives more cranking capacity for engine starting but battery discharges quickly. Thick plates reduces cranking capacity but provides more reserve capacity (deep cycle). Increased physical size will allow for cranking and reserve capacity Negative plates are made of a sponge lead and positive plates are made of a lead dioxide. These are dissimilar metals. When two dissimilar metals are immersed in electrolyte a voltage develops. When a load is connected current begins to flow. A complex chemical action begins. The negative plate tries to give away electrons, the positive plate tries to collect electrons. Typically a battery should be recharged once it is depleted to 50% (12.2 volts).

During charging the opposite occurs, the positive plate gives away electrons and the negative plate collects them. You will notice that during charging the voltage applied to the battery is higher than 12.6 volts. Voltage regulators are typically set around 14.2 to 14.5 volts.

A Word of Caution

Electricity requires extreme care. You are more likely to get an electrical shock if the boat is operating from shore power or from an onboard generator than from a boat that relies on a 12 volt battery for a source of power. However, a 12 volt battery is capable of delivering an electrical shock. In addition improper handling can result in harmful electrolytic acid spills. In overcharged conditions the battery can rupture. To ensure long battery life, follow the recommendations of the battery and charger manufacturers.