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.

Isolators and Combiners

One way to charge two batteries (say, a house battery and a starting battery) is to connect the two together and then connect one or the other to the charger. Charge one battery and both will be charged. The problem with this approach is that the reverse is also true: If one battery is being used as a house battery, it will also be drawing the charge from the second battery — the starting battery. Run the lights, stereo, or something else a little too long and you may not have enough "oomph" left to start the engine. One solution is to always remember to use the battery switch to isolate whichever battery is needed. Another solution is an isolator, which is like a one-way valve that prevents one battery from stealing power from the other. One drawback: Isolators typically decrease the voltage that gets to the batteries by half a volt or more, so batteries must be topped off later by the alternator in order to be fully charged. An even better solution is a combiner, which connects the two batteries together whenever one of them is receiving a charge and then disconnects them when the charging stops. A combiner completely charges both batteries. Both of these devices eliminate the hassle of using a battery switch to alternate batteries for house or starting.


Unlike sophisticated battery chargers, most alternators charge batteries at a fairly constant rate based on the speed of the engine and the state of the battery. That's OK since it's usually only for a few hours at a time and there is usually a draw on the battery while the boat is underway. But boaters who motor extensively, like boats going up and down the ICW, risk the same problems caused by using cheap battery chargers — under- or overcharging. This problem can be solved by installing a "smart" regulator that turns your alternator into a precise three-stage charger. Most of these regulators have settings for AGM, gel cells, and conventional wet cells. Keep in mind that unlike set-it-and-forget-it battery chargers, slipping belts and excessive engine room temperatures can seriously degrade an alternator's performance.

Batteries are not tolerant of being given incorrect charging voltages. An alternator that overcharges or undercharges a battery by as little as a quarter-of-a-volt can hasten its demise. AGM and gel cells are even less tolerant, so if you don't have a smart regulator, it's important to make sure your alternator is putting out the right voltage. For boats that deeply discharge their batteries routinely and don't motor for long periods, up to 14.4 volts is OK for wet cells. For boat owners who only use the battery for starting and occasional house needs, or motoring for long hours, 13.8-14 volts is more appropriate. If your alternator is not putting out the correct voltages, it's likely the regulator (usually attached to the alternator) is defective and should be replaced.

Battery Maintenance

While many batteries are "maintenance-free," wet-cell batteries still lose some of their electrolyte during normal use and need to be checked occasionally. When topping up batteries, Trojan's Jim Lee recommends using only distilled water because it has no minerals that might contaminate the electrolyte. Fill up each cell until it is 1/4- to 1/8-inch from the top. Since they're sealed, AGM and gel cells can't be topped off or even checked.

Corrosion on terminals can cause resistance, which prevents the battery from fully charging. Terminal clamps should be removed from the terminal and cleaned with a solution of baking soda and water (be careful that none of the solution gets into the electrolyte). Remove the negative terminal first (make sure the battery is not under load, or it could spark) and coat both terminals with petroleum jelly or anti-corrosion spray after cleaning. Batteries can slowly leak a charge through conductive paths made by dirt or spilled water on their cases so keep the cases clean. Check for corrosion or frayed wires at the cable clamps — replace them if they're damaged. Make sure that wet-cell batteries have their vent caps in place.

Battery Checkup

In addition to pulling off the cap to check each cell's electrolyte level, a hydrometer can be used to test the concentration of acid (specific gravity). Available at auto parts stores for a few dollars, a hydrometer can tell you the charge state of a battery and, more importantly, if one cell is weak and dragging down the others. If the difference in specific gravity between any two cells is more than about 30 points, the battery will have to be replaced.


Improper storage is another major cause of battery failure, says Jim. If your boat is laid up ashore over the winter, he says it's best to remove the batteries and bring them inside, or at least store them in a place that won't freeze, since this can crack the case. A fully charged battery is safe to around 0 degrees Fahrenheit, but one that is partially discharged can freeze at higher temperatures.

Once a month or so, stored batteries should be fully charged, but Jim says to avoid trickle chargers — many don't turn off. And incidentally, the advice to avoid storing batteries on concrete floors is a myth. 

This article was published in the Spring 2012 issue of Trailering Magazine.


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