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Marine Battery Maintenance

2010 February 1
Keep the power going over the winter months with a few simple steps

With winter approaching, as it does early in Maine, it was time to haul Sonata, the 36-foot cutter I owned with my wife Liz. We'd lined up winter storage at a small boatyard perched on the edge of Rockland Harbor.

"You want the batteries off the boat, right?" I asked the boatyard owner. "So you can trickle charge them over the winter?"

"Oh, no," he said. "Just leave 'em. We'll charge them in place."

I wondered how he'd do that. There was no obvious electrical hookup in sight. "You must have a very long extension cord," I said.

The boatyard owner laughed and went about his business.

For two winters Sonata remained at the yard and my brand-new deep-cycle batteries suffered. Sitting idle for months in severe cold while slowly running flat between the one or two charges they received did not do them any good. If batteries are left in a discharged state, like mine were, deposits of lead sulfate will collect on the plates and hinder or eliminate the chemical reaction that produces electrical current. Extremely hot temperatures and overcharging will also cause batteries to sulfate.

Clean energy
Lead-acid batteries are like living things. They need tender love and care to achieve maximum performance and long life regardless of battery type: flooded (also called wet), absorbed glass mat and gel. Fortunately, effective battery maintenance is easy both in winter and during the sailing season.

A clean battery is a happy battery. Flooded lead-acid batteries are subject to acid spatter or minor spillage that can be conductive and encourage a faster rate of self-discharge. Cleaning the case with a mixture of fresh water and baking soda will neutralize the acid. Rinse off all residue and ensure that the batteries are kept dry in winter storage and in the boat during the sailing season.

Dirty or chronically damp batteries also invite corrosion at the battery posts and cable terminals, which will inhibit battery performance. At least once every year, it is a good idea to check for the telltale greenish hue and pitting that reveals the presence of corrosion. If you see corrosion taking hold, disconnect the terminals and clean them with a wire brush before applying a thin layer of petroleum jelly prior to reconnecting the terminals.

Terminals should be tightly crimped to cable ends and sealed with heat-shrink tubing. Electrical tape works fine, but it unwinds over time. Whenever the battery is in service, terminal connections with the battery posts must be tight, and if the battery is the flooded type, check the level of the electrolyte in each cell and top off using only distilled or deionized water. Checking electrolyte levels in flooded batteries should be done regularly, particularly before and after extensive charging of batteries that have been run down. Obviously, you don't have to do this with sealed AGM or gel batteries.

When the boat is on the hard, it is best to remove the batteries and take them home or to store them in the battery shack at your boatyard. Ideally, the temperature in the shack will be kept cool, but not below freezing. Flooded lead-acid batteries in a discharged state can freeze, causing damage. The advantage of a cool but not freezing storage temperature is a slower rate of self-discharge as opposed to a hot storage temperature that encourages a faster rate of self-discharge. If you do leave batteries on board, make sure to disconnect the terminals and adhere to a regular charging schedule.

If you take your batteries home for winter storage, don't worry about storing them on your garage or basement concrete floor. The myth that the concrete will increase self-discharge rates is just that, a myth based in historical fact. The hard-rubber cases of older batteries were porous and could allow current to flow to moist concrete if an electrical ground was present, but this isn't true with today's plastic cases. Be aware that spilled battery acid will stain concrete.

In addition to the temperature, the age of the batteries will also have an impact on self-discharge rates, which can reach as much as 60 percent of capacity in a single month in some cases. Older batteries lose their charge faster than newer batteries, and charge times take longer. Charging older batteries once per month is ideal, though newer batteries with full charges can be left for two months before you need to charge them up again.

A battery charger that delivers 10 to 20 amps should be sufficient in most cases. Using an automotive charger isn't ideal and can actually damage your batteries. Smart marine chargers are highly recommended because they deliver just the right level of charge and will automatically "equalize" the batteries. Equalization occurs after the normal charge cycle is completed and basically means the charger continues delivering a low-level charge until the batteries reach a full charge and each cell is in balance with the others.

Take care to match the charger to your batteries. A charger that's too small won't ever fully charge the batteries and a charger that's too big will cause excessive gassing in flooded lead-acid batteries, releasing potentially explosive hydrogen gases into the air. Using an oversized charger may also boil away the electrolyte, exposing the plates and causing sulfating.

Periodic testing of flooded batteries is good preventive maintenance and can save you money. If you have a bad battery in the bank, current from good batteries will flow to the bad battery, bringing the state of charge down and reducing the life of the good batteries. Think of the electricity as water flowing downhill from the good batteries to the bad battery. You want to know if this is happening, so you can replace the bad battery before it does significant damage. Always buy exactly the same size and type of battery to match the others in the bank; mixing size and type may cause damage.

A voltmeter will provide you with a general idea of the state of charge in each battery, but you'll need a hydrometer to test each cell and to see if it is fully charged. Don't try to take a hydrometer reading just after you top off the cells with distilled water. Let the battery charge to mix the pure water into the electrolyte, or you'll get an artificially low reading. Also avoid testing in low temperatures, which tend to make the electrolyte denser and will give an inaccurate reading. The best temperature for testing is around 70 degrees. You can also buy temperature-compensating hydrometers.

Battery capacity is listed in amp hours and it is standardized so you can easily make comparisons between batteries, whether they are deep cycle, starter, or dual-use batteries that can be used in both starting and house bank roles. The formula for figuring capacity is based on a battery's ability to deliver electricity at a constant rate of discharge for 20 hours prior to dropping to a charge of 10.5 volts. So, if a battery has a capacity of 200 amp hours, it will run a 10-amp appliance for 20 hours before the charge drops to 10.5 volts. Capacity is also given in reserve minutes, which is the total number of minutes a fully charged battery will run a 25-amp appliance before the charge drops to 10.5 volts.

In an ideal world, you should deplete only about 20 percent of the capacity of your batteries prior to recharging. Such practices will greatly prolong the life of the batteries and shorten charge times. You'll need to size the batteries for your boat's daily hunger for amp hours and add extra amp hours to the bank to avoid discharges that exceed 20 percent of total capacity, if practical. Add up the draw in amps for each appliance aboard based on the time it is actually in use in a given day, and that will provide you with a general idea of the total amp hours you use in a 24-hour period. To achieve the best results, select batteries in a size that together in the bank can deliver at least three or four times the total amp hours you require in a given day.

Flooded, absorbed glass mat and gel batteries are better now than ever. Maintaining them is simple and easy, and will go far in extending their years of service aboard your boat.

Quick Tips

  • A clean battery is a happy battery. Keeping cases and terminals clean will improve performance and extend the battery's life.
  • Check the electrolyte levels of flooded batteries regularly. Top off with distilled or deionized water as needed.
  • To slow discharge rates, store batteries in a cool place. But don't allow them to freeze, as it can damage the batteries.
  • Occasionally charge stored batteries to prevent them from fully discharging-about once a month for older batteries or once every few months for newer ones.
  • When sizing the battery bank, determine the boat's electrical needs and add extra amp hours to avoid discharges that exceed 20 percent of total capacity.