Marine batteries come in three chemical types - flooded batteries, gel batteries and AGM batteries. The amount of attention that you should give your batteries to maintain them in top condition depends on which type you have.
Flooded batteries are the most common type and the one that people are usually most familiar with. They're 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 must also be adequately vented.
Gel batteries are sealed batteries and considered maintenance-free because their sulfuric acid is mixed with other ingredients to form a gel-like substance that does not evaporate and thus never needs replenishing. The hydrogen gas that's generated is reabsorbed and reused inside the battery, so these batteries need little or no ventilation. Also, because there is no liquid that can spill or leak (even if a crack occurs), 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 may vary from one type to another. Sticking to a single type in a bank will ensure consistent results.
As noted, flooded batteries require the most maintenance. The water levels in each cell should be checked at least once a month and distilled water added when levels fall below the top of the lead plates.
Although gel batteries and AGM batteries are sealed and "maintenance-free, you should still check their posts and keep them 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 it.
All batteries will perform better and last longer if kept fully charged and if the alternator on your engine is capable of delivering at least 25 to 35 percent of the amperes that the batteries are able to deliver.
When your boat is left untended for weeks at a time, it is a good idea to install a battery charger. At least attach a trickle charger to the battery. Make sure, however, that it's the type of charger that will shut off automatically when the battery is fully charged.
The most effective battery charging and voltage maintenance systems are the solar-powered units that use the new pulse technology. These units will ensure peak battery efficiency at all times and also help to greatly extend the life of the battery. One type is made by PulseTech Products Corp. (800-580-7554; www.pulsetech.net). These chargers prevent a loss of battery power even if batteries sit unused for long periods, because their unusual technology prevents sulfation buildup on the lead plates - a leading cause of battery failure.
One of the best and inexpensive ways to extend the life and performance of your battery is to use VDC Electronics' BatteryMinder On-Board Battery Charger/Maintainer/Desulfater to keep your battery at full performance and living longer. They provide several options, however, I have found the On-Board units quite good for my fishing boat's two batteries, one for the E-trolling motor, and the other for the starting motor.