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Twelve-Volt TLC

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This cutaway of a conventional wet battery shows individual cells.

Most boats alternate between the genset and the yellow cord, so it's easy to overlook the batteries—but lead and acid need love, too. Taking care of your batteries usually demands little more than visual inspection and a quick wipe down with a rag. Ignore your batteries, though, and one day you'll turn the key and hear the slow groan of an underpowered starter motor. Here's how to avoid that.

Most important? Maintain secure connections; a loose or corroded clamp can make even a fully charged battery seem anemic. If a yard stores your batteries every winter, don't assume it cleaned the posts and clamps in the spring. It takes only a wrench to remove the clamps and some emery cloth to clean them. Scrub both the post and the inside of the clamp with the cloth until they shine like silver. Some people cover their terminals with grease, but I prefer fighting corrosion with regular inspections. Keep the battery tops clean and dry, too.

If your batteries have caps (most deep-cycle ones do), open them and check the fluid level. Add clean water—distilled is best—until you see a bull's-eye when you look down into the opening. (It'll be obvious.) Don't overfill, or the excess fluid—now acidic electrolyte—will vent out through the caps when the battery heats up during charging. Check the fluid level monthly. If you always have to add water, maybe you're overcharging the batteries, a common problem if your charger is an old-fashioned model that never completely shuts off.

Charge the batteries, and let them sit overnight. In the morning apply a 15- or 20-amp load to each for a minute or two to remove any remaining surface charge, then check the voltage. Don't rely on your instrument panel's voltmeter; instead, use a portable multimeter across the terminals. A fully charged wet-cell battery reads about 12.6 volts across the terminals; Absorbed Glass Mat (AGM) batteries should read 12.8 volts. Anything below 12.4 volts indicates it's time for a new battery. If in doubt, call an expert: There are more sophisticated test methods for analyzing battery condition (i.e., a load test), but they require special equipment and skill.

One test that doesn't require much skill, other than being able to remove the battery caps, is checking each cell with a hydrometer—basically a large eyedropper with a float inside it that measures the specific gravity of a fluid. The electrolyte in a fully charged battery has a higher concentration of acid, and therefore a higher specific gravity (at least 1.265), than it does in a partially or fully discharged battery. All cells should read within a few hundredths of each other. One dead cell kills the whole battery, but often the problem is sulfation of the plates, which can sometimes be reversed with an equalizing charge applied by a three-stage charger. Otherwise, open your checkbook.

If you do, buy only those made for marine use: They're built to withstand shock and vibration and have thick plates that are more forgiving of deep discharge, even if they're technically "starting" batteries. I prefer AGM batteries vs. "flooded" models because the electrolyte is trapped in glass-fiber mats, so it can't leak out even if the battery splits open. (I haven't forgotten gel-cell batteries; we'll address them in a future issue.) AGMs also recharge more efficiently, produce less hydrogen fumes, and have a lower rate of self-discharge (voltage loss while idle) than flooded batteries. Yes, they cost more, but what doesn't?

Ask the Experts: Peter Kennedy

The question: My battery charger is so old, I think it was built by Edison. I want a new "smart" one but don't know what size or type. Can a PMY expert help me?—M.S., Teaneck, NJ

The answer: This is a job for Peter Kennedy, an ABYC-certified marine electrician and corrosion expert. Born in Ireland, Kennedy is a Royal Yachting Association-certified Offshore Yachtmaster with thousands of sea miles under his deck shoes. Since moving to Annapolis in 1990, Kennedy has specialized in designing, installing, and repairing marine electrical systems.


Peter Kennedy

While old-fashioned ferroresonant chargers are simple and reliable, they also lack sophisticated control circuits, Kennedy says. Although they start at full output, they start cutting back immediately as the battery charges, eventually falling to nothing. The result is, on average, you get about half the rated output. "Three-stage chargers give full output for most of the charge cycle, compensate for battery temperature—when the battery is warm, the charge rate decreases to prevent boiling off the electrolyte—and can be set for different battery types," he says. (AGM and gel batteries require different charging voltages than flooded batteries.) The new chargers are also smaller and lighter, he adds.

What size charger should you buy? "There's no rule of thumb," Kennedy says. "Do you have a generator? If so, you want to load the generator while it's running, so choose a high-capacity charger to charge the batteries quickly." If you plug in every night, you can use a smaller model, since it has plenty of time to bring the batteries back up. How many battery banks do you have? Most smart chargers have two or three outputs, but a knowledgable installer can engineer the system to handle more batteries than the charger has outputs. "And if you have a system with extra big loads or big batteries, it has to be engineered," he says.

Maybe you'd be better off with an inverter/charger, with a transformer that runs in both directions: When connected to an A.C. source, it's a powerful battery charger; other times it produces A.C. from the D.C. provided by the batteries. "It makes more sense to pop popcorn with output from the inverter," Kennedy says, "not start the generator for ten minutes."

Whatever charger you choose, add proper circuit protection, an inline fuse in the output wire near the battery terminal. If you have several wires connected to the battery, shift them to a terminal block, and run a single wire from the block to the battery. "Then when you have to disconnect the battery, you won't have so many wires you forget which is which," Kennedy relates. You might want to add a monitoring system, too. "A voltmeter can be fooled by a surface charge, so it doesn't always give you an accurate representation," he says. "A monitoring system gives you better information; it tells you not only the voltage but also the percentage of charge left, amp-hours used, and so forth."

Finally, consider where you'll mount the new charger. Don't mount it directly over the batteries because of the corrosive fumes vented during charging. The engine room is often too hot, while a cockpit locker is too damp. And there must be good fresh-air flow for cooling.

This article originally appeared in the July 2008 issue of Power & Motoryacht magazine.