In spring, a boater's fancy quickly turns to getting the family craft ready for the upcoming season. Your first thoughts should go to the condition of the batteries. Batteries that refuse to hold a charge or discharge unusually quickly are due for replacement. So too, are batteries with dead cells, loose or badly corroded terminals, leaks, or cracked or visibly distorted cases. Even if they seem healthy, but are three or four years old, you're about to find yourself shopping for new ones

Your local marine service center should be equipped with a high-end tester that will tell you exactly how a battery is performing.The test can be done with the battery in or out of the boat. The technician hooks up the tester leads, dials in the ratings printed on the battery label and pushes a START button. Then the tester simulates a series of loads and reports (on a digital screen) whether the battery can be charged or should be trashed.

Before plunking down your money for new batteries, evaluate your needs and keep in mind that the boatbuilder probably installed the cheapest batteries he could find that would meet the as-delivered electrical requirements of your boat. So, take another look at your boat's DC electrical requirements. Have you added a bunch of high-draw navigational equipment, such as radar, or a battery-powered inverter to run a mini-microwave? Maybe you've re-powered your boat with higher compression engines. Boaters who operate on big water and have only a single battery may want to add a second battery and a battery selector switch for emergency use.

Wet-cell lead-acid batteries designed for starting marine engines are called cranking batteries and are rated in terms of marine cranking amps (MCA) or cold cranking amps (CCA). These indicate the amount of current (amps) the batteries can deliver in a sudden draw to start the engine. (To be specific,MCA is the number of amps a 12-volt battery at 32 degrees F can deliver for 30 seconds and maintain at least 7.2 volts. Typical MCA is about 650. Some high-compression outboards, such as the Mercury Optis, require 1,000 MCA batteries.)

Large, high-compression engines require batteries with relatively high MCA ratings and may even require several batteries hooked up in parallel to handle the load. All batteries also are rated in amp-hours, a measurement of the battery's electrical storage capacity. It is an approximate measurement of the amount of current (amps) available over an amount of time (hours). So, a battery capable of delivering 10 amps for 20 hours would be rated at 200 amp-hours (10 amps x 20 hours = 200 amp-hours).