The ABCs of Marine Batteries
YOU NEED TO KNOW THE DIFFERENCES BETWEEN BATTERY TYPES BEFORE YOU CAN PICK THE RIGHT SETUP FOR YOUR BOAT.
Irecently received an e-mail from a reader who said that while he liked this column and looked forward to it every month, he also viewed it with trepidation because every time he finished one task he found he had more things to add to his to-do list. For his benefit and that of other harried readers I offer this column as a Christmas present. I promise it will be informational only and contain no maintenance tasks whatsoever.
The subject is marine batteries. This may not seem terribly exciting at first glance. Most of us give little thought to our batteries until they don’t work. Fortunately that’s rare. Marine batteries are remarkably durable and reliable, but like old soldiers, they eventually fade away. When they do, knowing some battery basics can help you pick the best replacement.
Let’s start with the various application types. There are two: starting (or cranking) and deep-cycle. Starting batteries are designed to provide a big jolt of electrical power (or amperes) for a short period of time, then be quickly recharged. Deep-cycle batteries are built to supply a limited number of amps over a longer time and then be recharged more slowly. You can use a deep-cycle battery to start an engine and a cranking battery to supply electrical gear, but the cranking battery won’t provide power for as long, and the deep-cycle battery won’t crank your engine as fast or as long. And if repeatedly misapplied like this neither battery will live as long.
A battery’s ability to crank an engine is indicated by its cold cranking amps (CCA), the amount of juice delivered at a specific temperature, usually 32 degrees Fahrenheit. Since few pleasure boats operate near freezing, CCA is valuable mainly for comparison, and most people just use the amphour rating. Diesel engines require batteries with more amp-hours because they have much higher compression ratios, which make them harder to crank.
Generally a battery should be recharged like it is discharged. Hence a cranking battery is quickly recharged while a deep-cycle battery can take hours. Fortunately, most boats now have “smart” battery chargers, which automatically apply the correct charge rate.
In terms of construction, virtually all marine cranking and deep-cycle batteries use a combination of lead plates and sulfuric acid (electrolyte) to store electrical energy in liquid form. There are three common types of lead-acid batteries: flooded (or wet cell), gel cell, and absorbed glass mat (AGM). Flooded batteries have a plastic case with a series of cells inside each containing a grid of lead plates surrounded by electrolyte. At the top of the battery are a series of holes through which you can add liquid (in the form of distilled water) as needed.
Wet-cell batteries are cheaper and more tolerant of inaccurate charging, but if their case is breached caustic electrolyte will leak