POWERBOATER’S GUIDE TO ELECTRICAL SYSTEMS The only other ingredients in your boat’s batteries are the grid, an inert (plastic) frame on which the solid active ingredients are suspended in the electrolyte, and the case. Lead dioxide, sponge lead, and lead sulfate are all very soft and fragile. The grid gives the plates the support they need to stand up to vibration and shock. The case, of course, contains the entire contraption and insulates one cell from the next. The voltage from a single cell in a fully charged lead-acid battery will always be approximately 2.1 volts (called the galvanic potential), regardless of the size of the battery. A lead-acid cell the size of your house is going to produce the same voltage as one the size of a peanut. Thus, when six of these cells are strapped together in series (positive to negative and negative to positive), you have a standard 12-volt battery. Recent engineering innovations have allowed manufacturers to produce plates that are slightly thinner than their predecessors but just as electrically capable. And due to advances in material technology, the new plates are much stronger than the old ones. Figure 5-1 shows the construction of a typical 12-volt battery with cell dividers and internal plates.
Types of Lead-Acid Batteries There have been enormous advances in battery technology in the past few years, and the result is a large and growing assortment of batteries that you can use on your boat. The days of the massive black case with gooey sealer and exposed lead cell-connecting bars are fading into history. Many of the heavy-duty commercial batteries are still constructed in the traditional manner, but even here things are changing fast. We now have low-maintenance, no-maintenance, cranking, deep-cycle, gel-cell, AGM (absorbed glass mat), standard automotive, and even special golf-cart batteries. Which is just the right choice for you and for your boat? Well, that depends on what you’re going to do with the battery once you buy it. Many boats today will have at least two types of batteries on board, and some will have more than that.
Fig. 5-1. A typical 12-volt battery.
To start, we can eliminate the standard automotive battery from all but incidental marine applications. These batteries might look just like their marine counterparts, but they are very different. Automotive batteries, even the so-called heavy-duty ones, are lightly constructed with thin plates hung on fragile grids; even the cases are thin plastic. This is because your automobile just doesn’t need a big, heavy battery. Your boat, however, does need a big, heavy battery, and car batteries wouldn’t last very long in the marine environment. Marine batteries must stand up to the vibration and deep states of discharge common on boats, and they must be able to withstand levels of neglect and abuse to which you would never subject your car battery. The difference between batteries is not only in the physical construction but in the ratios of lead peroxide and other materials such as antimony and a calcium alloy used in constructing the battery’s plates, and in the amount of material used in the plates. These variations affect the number of times a battery can be cycled (the number of times a battery can be discharged and then recharged) and still come back to useful life. The construction of a battery also affects how long it can remain discharged before the lead sulfate hardens to the extent that recharging can’t reverse the chemical reaction. When this happens, the
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