Which Battery Is Right for You?

Deep-Cycle Batteries Cost Over Life of the Battery

Battery Type . . . . . . . . . . . . . . . .Cost Formula Wet-cell . . . . . . .90 amp-hours ÷2 (50 percent discharge) = 45 amp-hours x 2,650 cycles = 0.0017¢ per amp-hour Gel-cell . . . . . . .86 amp-hours ÷2 (50 percent discharge) = 43 amp-hours x 1,400 cycles = 0.0027¢ per amp-hour AGM . . . . . . . . .92 amp-hours ÷2 (50 percent discharge) = 46 amp-hours x 3,000 cycles = 0.0014¢ per amp-hour

Please keep in mind that these costs are based on charge-cycle numbers that are quite high. In actual use, your cost per amp-hour will probably be much higher. However, on average the findings here are as good for comparison of battery types as any other method.

Although they will require more maintenance than other types, conventional wet-cell batteries are still the least expensive of the three types to buy and own over the long haul. AGM batteries offer all the advantages of the gel-cells plus a less-finicky charge cycle. Therefore, in almost all applications, AGMs are a better choice than gel-cells, but as of this printing you’ll still pay a substantial premium for AGMs over wet-cell batteries. AGMs are a particularly good choice for installations where acid spills are a consideration, as with personal watercraft (Jet Skis) and other sport boats, and they are excellent for use on boats that will be left unattended for months at a time.

Which Battery Is Right for You?

All but the smallest open boats should have at least two batteries. The starting battery is for starting the engine and needs a lot of cranking capacity to spin a heavy-duty starter motor. The house battery is used to run equipment such as cabin lights, stereos, refrigerators, and electronic equipment that isn’t connected to the engine. The starting battery should be a heavy-duty marine cranking battery, and the house battery should be a deep-cycle marine battery. One of my boats, a 15-foot dory I use for bay fishing, has a single deep-cycle battery that I use to operate my fish-finder and running lights at night. The engine is a pull-start outboard, so I don’t need a cranking battery. If I had an electric-start engine on this boat, I would consider an additional cranking battery, even for a boat this small. I hate paddling; it just takes too long against a 2-knot tide.

My other powerboat, a 25-foot V8-powered walk-around, is set up with a group 27 cranking battery and a group 27 deep-cycle marine battery.

Deep-Cycle versus Cranking Batteries

The difference between cranking and deep-cycle batteries is simple. Cranking batteries are designed to provide a burst of cranking power for a short period of time. Once the engine is running, the engine’s alternator will kick in and quickly recharge the battery, replacing the power used to start the engine. Cranking batteries are not designed to be discharged deeply over and over again. You would be lucky to get one season of boating out of a cranking battery used as a deep-cycle house battery.

Deep-cycle batteries, on the other hand, are built with heavy and comparatively thick plates and have much more lead in them than cranking batteries. You can actually tell the difference between the two by lifting them. They are designed to be discharged up to 50 percent of capacity and recharged over and over again without sustaining any permanent damage. Because of the heavier and thicker plates used in deep-cycle batteries, they take much longer to reach full charge than cranking batteries. Therefore, they aren’t a good choice for a starting battery, particularly where an engine will be started frequently and run for short periods of time.

Deep-cycle batteries are perfect for use as the house battery in cruisers that will be anchored away from shore power for overnight trips or for fishing boats that will be anchored for long periods with the fish-finder, radio, and beer cooler running.

Deep-cycle batteries are designed to take abuse, but even these can’t be completely discharged and recharged continually without failing. Thirty percent

of full capacity is generally considered a maximum level to which these batteries can be safely discharged. Fifty percent maximum discharge, however, provides a major increase in the life of the battery. Combination Batteries

Within the last few years I have seen what could be the answer to the battery prayers of many owners whose boats are not clear candidates for either pure cranking batteries or pure deep-cycle batteries. Previously owners of these borderline boats were forced to choose between two imperfect options. We now have batteries advertised as combination cranking/ deep-cycle units that are a perfect solution for the weekend powerboater who occasionally spends the night camping or fishing at anchor and spends the rest of the time at the dock plugged into shore power.

Combination batteries offer a compromise in performance. They won’t be as capable of sustaining repeated deep discharging and recharging as a true deep-cycle battery, but they will be fine for occasional deep-cycle use and more than adequate for starting your engine. As with any battery, a combination unit must be sized to fit your specific needs and the needs of your boat. Some Specific Recommendations

Let’s sum up all this various information and make a few specific recommendations for the right battery to put into your boat.

Small, open boats with manual-start engines, minimal electrical gear, and possibly an electric trolling motor need one or two deep-cycle batteries matched to amperage needs.

Personal watercraft need a single AGM or gel-cell battery large enough to meet engine-cranking requirements.

Outboard center-console boats need dual combination batteries, with one for starting and one to run the electronic systems.

Water-ski boats need a cranking battery or a combination unit of a size dependent upon the size of the engine and amperage requirements. Weekend cruisers need either one cranking battery for the engine and a separate deep-cycle battery for the systems, or two combination units, depending on amperage requirements and amphour loads.

Long-range cruisers need cranking batteries large enough to meet engine requirements and enough deep-cycle capacity to meet total daily amperage requirements.

When you’re choosing a battery for your boat and you find that you can’t decide between units of two power ratings, always go with the larger of the two. There is seldom any trouble with a battery that’s slightly too big, provided that it fits into your battery box, but one that’s too small can open up a world of problems. When you really need your battery, you won’t need a dead one, so always err on the side of too big rather than too small.

Batteries for Cruisers

If you’re a cruiser and spend days away from the dock, you need to get serious about batteries. Your needs are completely different from the day-tripping water-skier or fisherman. First, you should do an electrical-load survey on your boat and calculate the extent of your electricity needs while unplugged from shore power. This survey will help determine how big a battery you need and establish how many amphours you’ll need in it. You need a dedicated starting battery and a separate house battery to run the equipment you’ll be using while the engine is not running.

To do the survey, make a list of all the electrical equipment you have on your boat. List everything from the fish-finder to the light in the refrigerator. Now refer to the average power ratings found in figure 3-15 on page 37 and honestly estimate your average daily use of each item on your list. Multiply the amp-hours by the number of hours (or fraction of hours) for each item to get the amp-hours used each day by each piece of equipment. Next, add up the amp-hour-per-day figures, and the total will give you a pretty good idea of your total daily amphour consumption. Now double your daily amp-

hour figure, remembering that you don’t want to discharge the batteries to below a 50 percent charge, and you have an accurate amperage rating for your house battery. Purchase your batteries accordingly, and remember, when it comes to deep-cycle batteries, coldcranking amps is an irrelevant number. You need to be concerned only with the amp-hour capacity, sometimes known as the 20-hour rating.

The 20-Hour Rating

The 20-hour rating of a battery specifies the amount of amperage it can supply for 20 hours at 80°F (27.7°C). Figure 5-2shows the correlation between typical battery size by group number, amp-hour rating, and cold-cranking amps. Never assume amperage capacity based on size or group category of a battery. Always check the actual battery specifications with the seller. You might just find that a bargain battery might not be such a bargain after all.

10.5"

7"

24

21" 9"

9" 12"

7"

27

9"

21"

11"

Fig. 5 - 2 4 - D

10"

2 x 10.5" = 21"

6V Golf

7"

6V Golf 8 - D

10"

11"

Battery Amp-hours CCA

24 85 500–700 27 100 550–800 4-D 150–180 1000 8-D 200–225 1175 GOLF 200–225 1500–1600

Fig. 5-2. Common battery group sizes, amp-hour ratings, and dimensions.

Marine-Cranking Amps versus Cold-Cranking Amps

Most medium-sized boats need a cranking battery with enough cold-cranking amperage (CCA) to get the engine going, but it isn’t that simple any more. A new classification has been added to the mix, and some vendors are now rating the batteries they sell in marine-cranking amps (MCA) in place of coldcranking amps.

As defined by the ABYC, the two definitions are “Battery cold-cranking performance rating—The discharged load, in amperes, that a battery at 0°F ( 18°C) can deliver for 30 seconds and maintain a voltage of 1.2 volts per cell or higher. “Cranking performance (also referred to as marinecranking amps at 32°F or MCA at 32°F)—The discharge load, in amperes, that a new, fully charged battery at 32°F (0°C) can continuously deliver for 30 seconds and maintain a terminal voltage equal to or higher than 1.20 volts per cell.”

Notice the 32° variation in the two ratings. This means that if two batteries with the same amperage—one using the MCA rating and the other using the CCA rating—are being considered, the one using the CCA rating will be the more powerful battery. Battery potential decreases with temperature. So, a battery that can put out an equal amount of amperage cranking at temperatures 32° colder than the competition is theoretically a more powerful battery.

What does all this mean? Simple: Be careful! Do your homework and know what you’re buying before you pay your money. Manufacturers constantly play games with these numbers. If you’re getting your batteries at the local Sears or KMart, don’t expect the floor people to know any more than you do about battery ratings.

Typical CCA ratings for starting gasoline engines are shown in the following table.

Diesel engines, depending upon the type of starter motor and actual engine displacement, can