With a little of the right knowledge, boat owners can choose and install batteries that should enjoy a long, full life.
Of the many onboard items that make boating as we know it possible, batteries are arguably the most important. Unfortunately it’s a sad fact that batteries rarely die a natural death. Most are victims of unwitting owners. Knowing how to properly select and install boat batteries can ensure they live to a ripe old age.
Batteries convert chemical energy into electricity via a galvanic cell, which is formed by suspending two dissimilar metal plates within an electrolyte. Once the plates are surrounded by electrolyte, an electrical voltage develops between them, the amount of which varies depending on the types of metal used and the electrolyte itself. Lead-acid batteries (the focus of this story) consist of a sulfuric acid electrolyte and plates of lead dioxide and sponge lead, thus the name: lead-acid.
Before discussing the various types of batteries available, here’s a review of a few terms that will aid in selecting the right one.
Cold Cranking Amps (CCA) represent the maximum number of amps a new, fully charged 12v battery can deliver at 0 degrees Fahrenheit for 30 seconds while maintaining a voltage of at least 7.2v (1.2v per cell).
Marine Cranking Amps (MCA) is a term used to rate how much cranking power a new, fully charged 12v battery provides at 32 degrees Fahrenheit for 30 seconds while maintaining at least 7.2v (1.2v per cell).
Amp-Hours (AH) express a battery’s storage capacity, and the measurement is determined by multiplying current draw (in amps) by the length of time (in hours) that it takes a battery to discharge. A battery capable of providing 5 amperes for 20 hours (5 amperes times 20 hours) would have 100 amp-hours of capacity, as would one that delivers 4 amperes for 25 hours. The rate and discharge time can vary, but the battery’s electrical storage capacity remains the same.
Reserve Capacity (RC) of a battery is the number of minutes it can supply a constant, specified voltage (normally 25 amps) at 80 degrees Fahrenheit while maintaining at least 10.5v. It’s designed to give the buyer an idea of how long the battery can continue to supply power to essential accessories after a charging system failure.
The 20-Hour Rating states the amperage a new, fully charged battery can supply for 20 hours at a temperature of 80 degrees Fahrenheit. This is also known as the Amp-Hour Capacity of a battery.
Cycle Life is the number of discharge and charge cycles a battery can deliver over its service life. One battery cycle is defined as discharge from a full charge to a complete discharge and then back to a usable full charge again, usually from 100 percent to 20 percent and then back to 100 percent, although the percentage of discharge varies between ratings.
What’s it all mean, you ask? For starters, the burden is on the consumer to ensure the playing field is level when he compares and contrasts the various ratings manufacturers use in their advertising.
Battery A, which costs $100, offers 900 CCA. Battery B, which costs $75, offers 900 MCA. Both provide the same amperage, so B appearsto be the better value initially, but the informed buyer (that’d be you) knows that MCA is a rating performed at 32 degrees Fahrenheit while CCA uses 0 degrees as a reference point. Any battery will have higher MCA than CCA. Since battery performance degrades with lower temperatures, if battery A provides the same amps at 0 degrees that battery B does at 32 degrees, we can assume it’s likely a stronger, more robust product.
Manufacturers may also use different discharge periods to generate higher AH numbers for their product. For example, a battery can have a 20-hour rate of 344 AH, as well as a 100-hour rate of 429 AH. It’s easy to see how buyers comparing two batteries with similar capacities but different hour rates would logically assume the one with the higher AHs is more powerful, unless they’re aware of the difference in hour rates.
KNOW YOUR TYPE
One of the first decisions to be made when battery shopping is whether to go with a wet-cell battery or one of the newer technologies, such as gel-cell or AGM. All wet-cell, gel-cell and AGM styles are lead-acid batteries in one form or another — meaning they all use the same chemistry despite variations in construction — and each can be designed for starting or storage applications. Familiarization with the pros and cons of each will help match the right style with the job at hand.
Although the oldest design wet-cell, aka flooded, batteries are still the workhorse of the industry and considered by many boat owners to offer the best value. Their design consists of flat, lead plates immersed in a liquid sulfuric acid solution.
Wet-cell batteries either have removable caps, so owners can add water to the cells lost during the charge/discharge cycle, or are of the sealed, maintenance-free variety (see Maintenance-free? sidebar). Despite being old-school technology, wet-cell batteries offer some attractive advantages over gel-cell and AGM batteries. They’re cheaper, have an excellent cost-to-life-cycle ratio (provided they’re properly maintained) and are more tolerant to abuses such as over- or undercharging.
As to wet-cell cons, they don’t hold a charge as long as gel-cells and AGMs, are more prone to internal shorting and vibration damage, and can leak electrolyte if placed at odd angles or punctured. The fact they need regular maintenance, mainly the addition of water, is considered by some to be a disadvantage, as is the fact they generate explosive gases during charging, a phenomenon known as gassing. Gassing and the corrosive acid mist that accompanies it mean proper ventilation is an important consideration for any wet-cell installation.
Many of the negative aspects associated with wet-cell batteries involve electrolyte leakage or loss. The electrolyte in gelled acid or gel-cell batteries is immobilized by adding silica gel to the sulfuric acid solution, creating a more or less solid, gelatinous goo that is then placed within a pressurized, sealed battery that utilizes special valves for venting needs. Both gel-cells and AGMs are recombinant batteries, meaning water lost during operation is reclaimed internally.
The result is a truly maintenance-free battery in which the electrolyte can’t be spilled due to the case suffering damage or being tipped over. Gel-cells operate equally well in most any position, except upside-down, and even underwater.
Other advantages include a virtual lack of gassing and the ability to hold a charge longer than wet-cells. They can also discharge a lot of current and are less susceptible to damage if left in a discharged state.
Gel-cell disadvantages start with cost, which is significantly more than wet-cell batteries. They also recharge inefficiently after they’ve been deeply discharged, as most of the charge current applied during recharging produces heat rather than the chemical process necessary for recharging. This means they have to be charged at a lower voltage — no fast charges — than wet-cell or AGM batteries or the heat produced during overcharging can create permanent voids in the gel, which reduces battery capacity.
They can also lose water if battery temperature is excessive, such as occurs during improper charging or during use in hotter climates. That’s the purpose of those special valves mentioned previously, which are essentially one-way vents designed to release gas in just such situations. This lost water can’t be replaced, so extreme cases of excess venting can result in premature battery failure.
Absorbed Glass Mat (AGM)
The next evolutionary step for lead-acid batteries is AGM, which utilizes a fiberglass mat rather than gelling material to hold the electrolyte in place. This design is also known as “starved electrolyte” construction, because the fiberglass mat is only 95 percent saturated with electrolyte, to ensure there’s no excess acid to leak, even if the case is damaged.
AGM batteries have all the advantages of gel-cells with virtually none of their shortcomings. Their plates are more securely packed than wet- or gel-cells, and their construction is so robust they’ll survive installations that would literally shake a standard battery to pieces.
Since there’s no liquid that can expand, AGM batteries can survive freezing, and unlike gel-cells their internal resistance is extremely low, meaning almost no battery heating occurs even during heavy charge and discharge currents. They also have the highest charge acceptance rate, efficiency and life expectancy. Their self-discharge rate is also lower, meaning they can be stored longer without charging than standard batteries, which is a plus for use on boats that may be left unattended for months at a time.
The primary disadvantage associated with AGM batteries is cost, which can be two to three times that of a comparable wet-cell battery. Another would be intolerance to overcharging.
Gel-cell and AGM batteries are both VRLA (valve-regulated lead-acid) batteries, which simply means a tiny valve keeps the battery under pressure but allows venting when necessary. It’s one of the features that makes these batteries recombinant — oxygen and hydrogen generated during use are recombined inside the battery — and truly maintenance-free.
Most of the cheaper “no-maintenance” batteries offered at the megamart are simply sealed wet-cell batteries that contain an internal reservoir of electrolyte, which is used to replace whatever water is lost during recharging. The problem is this reservoir is eventually used up and there’s no way to replenish it, so the battery will likely succumb to an early death simply so the owner doesn’t have to add a little water.
STARTING VS. STORAGE
Starting batteries are constructed of thinner plates that are more numerous, a design that maximizes surface area and provides the highest burst of current possible, which is exactly what’s needed for cranking an engine.
Deep-cycle batteries have fewer plates, but they’re thicker, which is a requirement to survive prolonged discharges. This reduced surface area provides less cranking power, and that’s one reason deep-cycle batteries have to be oversized when used as a starting battery.
What starting batteries are not designed for are deep discharges. This isn’t a problem under normal use, since only a small amount of the battery’s actual capacity is used during cranking, and that amount is quickly replaced by the alternator once the engine starts running. The problems start when they’re forced to work as a deep-cycle battery and are subjected to the deep discharges associated with them, which causes their thinner plates to buckle and fail rapidly.
Deep cycle is a chronically abused buzzword touted by many battery manufacturers to imply a more robust, heavy-duty product. While any battery can technically be termed “deep cycle” — all can be deeply discharged and recharged — only a true deep-cycle battery is designed to withstand such discharges time and again without premature failure.
Cheaper substitutes, such as those that utilize lead sponge plates, can suffer irreversible damage after only a few such deep discharge cycles. A true top-of-the-line deep-cycle battery has solid lead plates that allow hundreds of deep discharges of up to 80 percent with no significant reduction in projected service life.
Section E-10 of the American Boat & Yacht Council’s “Standards And Technical Information Reports For Small Craft” covers battery installation requirements in great detail, but we’ll touch on the basics here, which are generally the same for wet-cell, gel-cell or AGM.
Install batteries in liquid-tight/acidproof battery trays or boxes. Boat owners can purchase these or make their own as long as they meet the ABYC requirements. DIYers need to make sure the mounting hardware (e.g., bolts, screws) doesn’t compromise the leak-proof quality of the containers.
Batteries must be secured against movement. The requirement is 1 inch maximum in any direction for at least one minute when exposed to 90 pounds of pull or twice the weight of the battery, whichever is less.
All positive terminals must be covered to prevent accidental shorting from things such as dropped tools. This requirement can be satisfied by the use of rubber or plastic terminal caps or boots, non-conductive covers or by the lid of a battery box.
They must be installed in a cool, well-ventilated area well above the normal accumulation of bilge water. Adequate ventilation is critical to remove fumes and gases that are generated during charging. Chargers or other electronics should never be installed directly above a battery or bank, as they could be damaged by these corrosive vapors.
Never install batteries above or below fuel tanks, fuel filters, fuel-line fittings or similar fuel system components. Installation above or below an uninterrupted (one-piece), non-metallic fuel line is OK, but any metallic part of the fuel system within 12 inches of a battery terminal must be shielded to prevent sparks.
Battery terminal connectors must provide secure mechanical and electrical connections, meaning spring clips and alligator clamps are not acceptable. The use of wing nuts is also prohibited for battery cables and other conductors size 6 AWG and greater, because they’re difficult to properly torque and can work loose due to vessel movement. If provided by the manufacturer for use in attaching primary leads, replace them with marine-grade lock nuts.