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After fully charging the battery have it load tested by your local marina or automotive store.

Batteries lose capacity over time. As the battery ages an increasing fraction of lead becomes tied up in the lead sulphate form rendering less plate material (lead/lead oxide) available to react with the sulphuric acid. An old battery can "take a full charge" and the battery voltage will seem fine...until placed under load. Load testing tells the true story of the condition of the battery.

Also, if you have power available at your boat slip use a good battery maintainer: BatteryMINDer and Battery Tender are the best: http://www.batteryweb.com/batterymind...arison.cfm. BatteryMINDer also makes a solar version.