Divide and Conquer
Got a few giant 8D or 4D batteries onboard your boat? These big kahunas can be so inconveniently located and so freakin’ heavy that swapping out old for new typically requires the assistance of a musclebound gorilla, the agility of a ballet dancer, the physical dimensions of a dachshund, and hell’s own appetite for draconian maritime punishment.
Take heart. Thanks to the unhandy location of the marine batteries onboard a brand-new motoryacht we recently seatrialed, and her manufacturer’s justifiable concern over the difficulties an owner might face when eventually having to remove and replace giant 8Ds or 4Ds, a different approach to marine batteries and their storage has come to our attention. And there’s a very good chance you may find it helpful.
Here’s the gist. Consider your average 8D marine battery: It has a capacity of approximately 245 amp-hours, and, as it weighs in at about 153 pounds, is both wickedly heavy and often just about impos- sibly unwieldly. Now relax for just a moment and consider the average Group 31 marine battery: It has a capacity of approximately 105 amp-hours, weighs in at about 69 pounds, and is certainly much lighter and much easier to deal with by comparison.
So, the question quite naturally arises—why not, for convenience sake, hook up three Group 31 batteries in parallel to duplicate the performance of the single 8D? After all, even if the three 31s weigh about 54 pounds more in total, they don’t take up appreciably more space when tightly aligned. And hefting a Group 31 versus an 8D under cramped or otherwise difficult circumstances? No contest!
Of course, we’re conveying one manufacturer’s general approach to a fairly common issue here. And if you decide to give said approach a go at some point in the future, there are certain things you should bear in mind when substituting three smaller marine batteries for a more powerful but heavier single. First of all, you need to realize that three Group 31s are going to cost you a good bit more than one 8D, by as much as 30 percent in some cases. Secondly, you need to consider other measures of battery oomph besides amp-hours when choosing your smaller substitutes. More to the point, you need to make sure that, when combined, your three smaller batteries approximate or even surpass the cold-cranking amps, cycle life, and reserve capacity of the larger, single battery. And finally, you need to remember that when one of the three smaller batteries begins to fade, it’s best to remove and replace all three at the same time. — Capt. Bill Pike