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Lightning in a Box

A little care keeps your boat batteries safe.

By Fritz Grell

Originally published in the February 2008 print edition.

Maintenance-free does not mean care-free. This sealed battery blew its stack after running low on water.

I turned the key to start the diesel engine of the 47-foot motor yacht and the bridge deck shook with an explosion in the engine room. What gave? We had to find out, and fast!

That the engines were diesel helped eliminate some of the possible sources; it's unlikely to have explosive fumes with diesels. Still, the engine room was full of smoke, but no active fire source could be detected. We activated the blowers to clear the engine room. When it was safe to go below, I found the port engine starting battery had exploded. The top of the battery was blown off, taking the top of the box with it.

The batteries on this particular boat are difficult to access, so when the batteries were last replaced, maintenance-free batteries were installed. The sealed batteries were great as they saved a lot of time—we no longer had to check the level of the battery water. But while we were ignoring them, they were gradually running low on water. It took a little over two years, but eventually one of them failed—catastrophically!

Fortunately, we were at the dock and not in The Bahamas. Treasure Coast Battery of Stuart was not surprised that the flooded, sealed batteries did not last long, but we were told to expect twice the life from newer maintainable batteries, assuming they are checked periodically and distilled water added as needed. A 2004 boat with original batteries we recently load tested had only one of five batteries in need of replacement. These batteries are checked monthly. Usually, a little distilled water is added to them and the levels are never low enough to expose the plates.

Once the plates are exposed, the batteries are ordinarily ruined. They will overheat and may explode since the battery is then full of explosive gas instead of fluid. A number of things can trigger the explosion, including jumping the battery, excess power demand from the battery or hooking up or cycling on the battery charger.

On our boat, we went back to maintenance-type batteries so we would better be able to avoid a catastrophic failure. Once you have experienced an explosion, you know it is not worth trying to stretch a little more time out of a battery. Some boat builders have placed batteries in areas that are almost impossible to service and, in those cases, maintenance-free batteries should be installed, keeping in mind that flooded type cells must be replaced every few years. Charging flooded batteries produces hydrogen and oxygen gasses which vent from the batteries and deplete the battery fluid over time, but distilled water can only be added to the standard batteries with battery caps.

Alternatives to the flooded batteries are gel cells and absorbed glass mat. Battery gassing is nearly eliminated with these batteries since fluid is in gel form or absorbed by glass mat, and so they are sealed and maintenance free. Gel cells and glass should last longer than their wet cell counterparts, which helps offset their higher price. Don't mix different types of batteries together as they will not charge properly.

Bottom line is, if you choose maintenance-free batteries, you still need to check them for heat and to make sure the terminals are clean. If the batteries get hot while charging, turn off the charger and determine the problem. Old batteries which have been performing properly but then start heating up excessively probably need to be replaced. If the batteries are new and heating up, turn the charger off and contact a marine electrician, as you have an incompatibility or other charging problem.

Flooded batteries with removable caps should be checked every other week at first, then at least monthly if they are doing well. Even if you don't have to add water when the batteries are new, keep checking because you will need to add water eventually. If the batteries require weekly additional water, my guess would be that you have a charging problem that needs to be corrected. Use only distilled water when servicing batteries—it's available at most grocery stores.

Numerous marine and RV battery manufacturers offer information on their Web sites and West Marine Advisor,, offers a concise battery review to get you started. Nothing is completely maintenance-free and you get what you pay for, so a little research is time well spent.

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