Marine Battery Basics

Your batteries are the heart of your boat’s electrical system. They supply power for engine cranking and all of your on-board electrical accessories. Treat them well and they will reward you with a long service life but abuse them and they will fail in very short order.

All marine batteries are a variation of he flooded cell lead acid battery originally invented in France in the late 1850’s. They’re big, heavy and can be the source of some very bad odors if not properly maintained. They produce electrical current by the immersion of two dissimilar metals (plates) in a liquid that conducts electricity (electrolyte). It is the reaction of the metals in the electrolyte that produces the electrical current. With one metal being more noble (the metals resistance to erosion) than the other it will erode at a different rate and produce an electrical current (potential difference) between the two. It is this continual erosion of the plates that determines the batteries life span and is the principal reason why no battery will last forever.

Marine batteries generally fall into two classifications, starting batteries and deep cycle batteries. Starting batteries are similar to what you would find under the hood of your car or truck. They have a large number of thin plates and produce a large amount of electrical current for short periods of time, just what you need for engine cranking applications. Starting batteries do have one disadvantage when used solely for marine applications (especially in sailboats), they will not tolerate repeated deep discharges. Using them in this way will reduce their lifespan greatly.

Deep cycle batteries on the other hand have fewer, thicker plates and while they may not produce as much peak current as a starting battery they will produce a smaller amount of current for a longer period of time and they can withstand repeated deep discharges. This is especially important in sailboat applications because engines are usually run for short durations (providing limited charging time) and batteries are called upon to power radios and instruments for long durations while under sail. It is for this reason that the best sailboat systems will usually feature a combination of both types of battery, a starting or reserve battery dedicated to engine cranking and deep cycle house batteries providing power for the on-board systems. In recent years it has become common practice to equip sailboats with only deep cycle batteries. This has proven to work quite well and is a practice that I employ on my own boat.

Fortunately maintaining your batteries is a relatively easy task. The electrolyte level should be kept above the plates and topped up with distilled water as necessary. The positive and negative terminals should be kept clean and as should the exterior of the battery case itself. Clean terminals ensure low resistance and proper current flow to the vessel’s electrical system and if the battery case is allowed to accumulate dirt this can provide a current flow between the terminal posts causing discharge.

Dirty Battery

The batteries pictured above are a good example of a poor maintenance, showing dirty cable clamps, residue forming on the case and sloppy wiring.

Monitor your vessel’s charging system with a voltmeter. If you don’t have one hard wired to your electrical system they are relatively inexpensive and easy to install. Charging voltages (with the engine running) will fluctuate according to load but should be within 13.25 to 14.25 volts, with all accessories turned off and providing batteries are in good condition. Voltages may intermittently fluctuate outside this range on a fully functional system but if they remain either higher or lower this may be an indication of trouble. Lower voltage levels may indicate a problem with the vessel’s charging system leading to dead batteries and higher voltage levels may indicate overcharging which will shorten battery life drastically. Overcharging may also be accompanied by a bad smell from the batteries themselves. If either of these conditions are encountered it is probably best to have your vessel’s charging system serviced by a professional.

A proper battery installation is also essential to ensure safe and reliable system operation. Batteries should be installed in approved acid resistant containers which will prevent spillage should a leak in the battery case occur. Also the batteries should be secured to prevent movement less than one inch in any direction and the area should be vented to allow for the discharge of gas produced by batteries as they charge.

Covered Batteries

The picture above shows batteries secure and in approved containers.

Recently two somewhat different types of batteries are gaining popularity with boaters and while they aren’t exactly new designs, both being used by the automotive and aircraft industries for some time now, they are referred to as Gel cel and AGM (absorbed glass mat) batteries. Both are a variation of the flooded cel design. The Gel cel uses an electrolyte which is thickened with a silica compound resulting in a cel with a thickness similar to petroleum jelly. The AGM battery uses an internal glass mat which reduces the internal movement of the electrolyte. Both types offer advantages such as reduced maintenance and quick re-charge rates but at this time they are considerably more expensive than their flooded cel counterparts. At this time the AGM appears to be the popularity winner an almost all applications.

With any type of battery proper maintenance of both the battery and the vessel’s charging system is the key to a reliable and long service life.