Battery First Aid

Posted: September 1, 2012

Your batteries demand just a little attention, so don’t deprive them.

By: Deane Hislop

Marine batteries come in three chemical types: flooded, gel and AGM. The amount of attention you should give your batteries to maintain them in top condition depends on which type you have.

Flooded batteries are the least expensive of the three and require the most maintenance. Flooded batteries contain a liquid electrolyte (sulfuric acid) that evaporates and must be replenished by topping off each cell with distilled water at regular intervals. Flooded batteries produce hydrogen gas, so they must be properly vented to allow the gases to escape into the atmosphere. The compartment or space in which they are located also must be adequately vented.

Gel batteries are sealed and are considered maintenance-free because their sulfuric acid is mixed with other ingredients to form a gel-like substance that does not evaporate and never needs replenishing. The hydrogen gas that’s generated is reabsorbed and reused inside the battery, so it needs little or no ventilation. Also, because there is no liquid that can spill or leak, gel batteries can be tilted or installed on their sides if necessary.

Like gel batteries, AGM (absorbed glass-mat) batteries are sealed and maintenance-free. They can be installed at any angle, and they do not discharge hydrogen gas. They’re also the heaviest and most expensive of the three types. AGM batteries are generally more tolerant of being deeply discharged without causing damage.

All the batteries in a battery bank should be of the same type, because charging voltages vary from one type to another. Sticking to a single type in a bank will ensure consistent and optimum results.

Since flooded batteries require the most maintenance, we’ll start with them. The water levels in each cell should be checked at least once a month and distilled water added before fluid levels fall below the top of the lead plates.

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, and then check the voltage by using a portable multimeter across the terminals. A fully charged wet-cell battery reads about 12.6 volts across the terminals. Anything below 12.4 volts indicates it’s time for a new battery.

Another test for fully charged flooded batteries is checking each cell with a hydrometer, a turkey baster with a float inside, to measure the specific gravity of a fluid. The electrolyte 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 entire 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.

Although gel batteries and AGM batteries are sealed and “maintenance-free,” their posts need to be kept clean and lubricated. Loosen and remove the cables, then clean the posts and the cable clamps or ends with a wire brush. Be sure to securely retighten each cable when you replace them.

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