Got Juice?

Lessons Learned From Your Battery

By Chuck Fort

If the power you're expecting unexpectedly disappears, there's a reason why.

Photo of the cockpit of a 30 foot Searay

After a pleasant day-long stay aboard his 30-foot Sea Ray in Barnegat Bay, member Mike Lithgow went to start his engine, only to hear a "click" instead of a healthy whir. He thought it was strange that the boat had been plugged in all week and yet the batteries were dead after only a few hours. A jump-start from TowBoatUS got him back to the dock. He replaced the batteries.

The next time he went to use the boat, he noticed an unpleasant odor, but couldn't find the source. The engine started, but after another afternoon at the beach, Mike went to start the engine and "click, click, click " not again! Another call to TowBoatUS got him a jump-start back to the dock where he checked the battery's electrolyte level and was startled to see there wasn't any water! While the battery was plugged into the charger, he checked the voltage and found it was hovering at 16 volts — enough to boil off the electrolyte and ruin the battery in a matter of days. The strange smell had been the sulfuric acid electrolyte boiling off. The batteries ruined, he bought two more and a new battery charger. He's on his second season now, and says that the batteries are holding up fine.

Battery Killers

According to Jim Lee, technical support engineer at Trojan Battery, the most common cause of premature battery failure is improper charging. Undercharging causes lead sulfate to accumulate on the plates, which eventually destroys the battery. Overcharging causes accelerated corrosion of the plates and boils off the electrolyte, which exposes the plates to air and ruins them.

Most car battery chargers, Jim says, are not designed for charging boat batteries because, despite what the box says, they often don't shut down completely. And remembering to unplug a cheap charger when it "looks about right" is also risky and could overcharge and eventually destroy batteries, he says. Like babies, batteries are particular in how they are fed and how much they'll take. Jim recommends a three-stage charger, specifically one that will completely shut off when the battery is fully charged.

Batteries do best on a regulated diet of amps in the proper amounts, and in stages. First is a bulk stage, where they're brought up to 80 percent of their capacity. Discount store automotive chargers do this. But next, they need an absorption stage, where they are fed a constant voltage and tapering current that allows them to top off the last 20 percent or so. Finally, a float stage is used to keep their voltage high enough not to lose charge (batteries will self-discharge from three to 30 percent a month just sitting around), but not so high as to boil off the electrolyte. This is where an automotive charger can ruin a battery; it doesn't know when to quit. As complicated as this sounds, flooded cells, gel cells, and AGM batteries use a different voltage for each setting and it's critical to use a charger that can be programmed to charge them properly. Many chargers are designed for wet cells and some have switches for gel cells and/or AGM. Don't charge different types together, like AGM and wet cells, or one will be overcharged.

How big a charger do you need? A good rule of thumb is to select one that is rated at 10-15 percent of a battery's (or battery bank's) amp-hour rating. For example, a 100-amp-hour battery would require a 10- to 15-amp charger. An undersized charger won't last long and may not be able to fully charge the batteries. Undercharging can also lead to another problem - sulfation.


When wet-cell batteries are lightly used — discharged by only 10-30 percent of their capacity — the acid becomes concentrated at the bottom of the battery and sulfate crystals begin to form on the plates. This severely reduces the capacity of the battery, though its voltage will show that it's OK. A "sulfated" battery works fine, but dies much sooner than it should. To counteract these effects, some chargers have an equalization setting that temporarily boosts the charge voltage to mix the solution and dissolve the sulfate crystals. How often batteries should be equalized varies from manufacturer to manufacturer; a good rule of thumb is once a year, or whenever a battery checks low using a hydrometer, despite having the proper voltage. Equalization can only be carried out on wet-cell batteries; other batteries can be ruined by the process. Also, it's important to disconnect batteries from the electrical system during equalization because the higher voltage used can damage equipment.

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