Types of Lead-Acid Batteries

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 5-1shows the construction of a typical 12-volt battery with cell dividers and internal plates.

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.

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

Fig. 5-1. A typical 12-volt battery.

battery is said to be sulfated and must be replaced. These chemical and construction variations in battery types also explain why some batteries have a tendency to produce more hydrogen (a process called gassing) than others.

Engineers have been able to reduce gassing to almost nothing by adding antimony into the plate material. Less gassing means less water loss, and hence the evolution to the sealed batteries which are becoming the norm today. We now enjoy modern gelcell or absorbed-glass-mat (AGM) technologies that keep the sulfuric acid in either a gelled state or absorbed in a mat material, much like conventional dry-cell batteries.

The first basic choice you’ll have to make as you try to pick out a battery for your boat is between the old-technology wet-cell batteries, the new gel-cell batteries, and the even newer absorbed-glass-mat (AGM) batteries, so let’s take a close look at each of these three.

Wet-Cell Batteries

Wet-cell battery technology has been around since the days of the first electric-start automobiles. This is the type of battery with the removable cell caps (generally, but not always) with which we are all familiar, because it’s the type still found under the hood of the family car. Wet-cells have the lead plates suspended in liquid electrolyte, and the durability of the individual batteries depends on the robust construction of the case and grids as well as the amount of material in the plates.

Even today, wet-cell batteries offer some important advantages over gel-cell and AGM batteries, the new kids on the block. They are usually the cheapest to buy initially and, as we will see below, they are by far the cheapest to use over the long haul—provided, of course, that you don’t neglect regular maintenance. They also stand up well to abuse such as overand undercharging.

The disadvantages of wet-cells are that they require more elaborate ventilated battery compartments. They will not hold a charge as long as the newer gel-cells and AGMs, which means that they can’t be left unattended for as long. They must be kept upright at all times, and they require regular topping-up with distilled water.

Gel-Cell Batteries

Gel-cell batteries work on the same principles as wetcells, and the materials are basically the same. The big differences are that the electrolyte is rendered into a paste about the consistency of grape jelly, and the plates (which have a slightly different composition, to reduce gassing) are suspended in this goo. Since there is no liquid to top up, there is no need for the familiar caps, and the cells are, for all practical purposes, sealed.

There are several important advantages to gel-cell batteries. You don’t have to worry about spilling the electrolyte by tipping the case over, for one thing. In fact, gel-cells work just fine on their sides or even upside down. They hold a charge much better than wet-cells and can be left unattended for longer periods of time—and, of course, you don’t have to worry about topping-up the electrolyte.

The disadvantages of gel-cells, besides the cost (which we will go into below), are that they cannot be overcharged without suffering permanent and often terminal damage. Also, because the electrolyte can’t circulate between the plates the way it can in a wetcell battery, gel-cell plates must be kept thin enough to accept a charge in a reasonable length of time.

Generally, over the last few years, gel-cells have developed quite a bad reputation for not living up to the claims made for them by manufacturers. Moreover, gel-cells have charging needs that differ from both the AGM (which we will discuss next) and wet-cell batteries. Traditional constant-rate ferro-resonant battery chargers, which are found on all older boats and on new high-production boats (Sea Ray and Bayliner, to name two), have destroyed many gel-cell batteries. Gel-cell batteries must be charged using a three-stage smart charger (covered in chapter 6) with the voltage set for gelled electrolyte.

Absorbed-Glass-Mat Batteries

Absorbed-glass-mat batteries are also of the nomaintenance type. They have a sealed case, just like their gel-cell cousins: you couldn’t add water to these

batteries if you wanted to. The primary difference between AGM batteries and gel-cells is in the way the electrolyte is supported. AGMs have a fiberglass mat between the cells that further supports the electrolyte. There are other important differences, though. Tests have shown that AGMs are less sensitive to charge rates than gel-cells, perhaps making them a better choice if you own an older boat with a constant-rate charger. (More on chargers in the next chapter.) No-Maintenance Batteries

Beware of batteries sold by some chain stores such as Sears and Wal-Mart that purport to be nomaintenance batteries and lack a filler cap just like gel-cells and AGMs. These are often wet-cell automobile batteries that have an internal reservoir of electrolyte that’s gradually used up as the battery is recharged. Once this reservoir is gone, the batteries are junk. These might be fine for your car, but they have no more business on your boat than any other automobile battery. A few may even be marketed as marine batteries, so be extra careful when buying from discount stores. The best policy is to buy your batteries from a reputable supplier who specializes in batteries for boats.

Battery Life

But what about charging cycles? How many times on average can the different types of batteries be discharged and recharged before they need to be replaced? This is a difficult question, because the number of charging cycles that you can expect out of a battery depends upon the rate and depth of discharge, the recharging method, and the quality of the battery construction. Also, batteries of the same nominal size will often have different amp-hour capacities, and the real concern is cost per amp-hour. The Typical Battery Charging Cycles table above represents the average number of times top-quality batteries of each type can be discharged and recharged. Cheap substitutes will provide a much lower average number of cycles.

Cost Comparisons

Conventional wet-cell batteries are the least expensive of the three types we have discussed, no matter which way the cost is measured. The question is, if I buy the new gel-cell or AGM batteries, what am I getting for my extra battery dollars? Are the AGM and gel-cell batteries worth the extra money? The answer is that there is no answer—at least not one that fits all circumstances. The type of battery that’s best for you depends on your needs and the use to which you put your boat. Here are some cost comparisons.

Comparing charging cycles to typical cost per battery gives you a feel for the true cost of these batteries over the long haul. Using a group 27 battery (a common size) for comparison gives a cost-per-cycle based on the average number of cycles in each type used above.

The table belowcompares typical group 27 deepcycle battery prices taken from the 2006 West Marine catalog, and the average true cost per cycle.

Using a cost-per-amp-hour calculation, we can use the advertised total amp-hour capacities for the three group 27 deep-cycle batteries West Marine lists in the same 2006 catalog to arrive at cost over life of the battery (see table on the next page).

Typical Battery Charging Cycles

Battery Type . . . . . . . . . . . . . . . . . . . . .Cycles Conventional wet-cell . . .800–4,500 cycles (2,650 avg.) Gel-cell . . . . . . . . . . . . .800–2,000 cycles (1,400 avg.) AGM . . . . . . . . . . . . .1,000–5,000 cycles (3,000 avg.)

Note: The above averages are based on manufacturer’s claims under ideal conditions. Actual averages will be considerably lower in the less-than-ideal conditions found on your boat.

Deep-Cycle Batteries Cost Per Cycle

Battery Type . . . . . . . . . . . . . . . . . . . . . . .Cost Wet-cell . . . . . . . . . . . . . . . . . . . . .$100, 3.8¢ per cycle Gel-cell . . . . . . . . . . . . . . . . . . . . . .$213, 15¢ per cycle AGM . . . . . . . . . . . . . . . . . . . . . . . . .$200, 7¢ per cycle