It happens to the best of us. You work all week, anxiously awaiting the weekend so you can get out on the water and leave all land-based aggravation in your wake. Then you turn the key and hear the dreaded clicking sound that means your trip is over before it begins. Sure, you could get a jump start, but you have no way of knowing if you have a more serious problem than just a run-down battery. And as bad as it is having a dead boat at the dock, experiencing the same problem 25 miles offshore is infinitely worse and could involve a four-figure tow charge. Choosing the correct batteries is essential to boating happiness, and the first decision tree has three branches: cranking, deep-cycle or dual-purpose batteries.
Cranking Batteries
As the name implies, these batteries start your engine and need to deliver a lot of energy in a short amount of time. For wet cell, lead acid batteries this is accomplished by having many, thinner plates. The rating number that's important with these is the marine cranking amps, or simply cranking amps (MCA/CA). This number indicates the discharge load in amperes that a new, fully charged battery can continuously deliver for 30 seconds while maintaining 1.2 volts per cell at 32 degrees Fahrenheit. For a 225 hp outboard engine, the manufacturers' recommended MCA varies from a low of 675 for a Yamaha F225 to a high of 1,025 for a Honda BF225, with most other engines falling between 800–1,000 MCA.
Deep-Cycle Batteries
These batteries power the electronics, lights and trolling motors for a boat and deliver power at a slower, steadier rate than cranking batteries. They also have to withstand numerous charge/discharge cycles, so a wet cell battery has fewer, thicker lead plates. Although these can be used for starting an engine, their cranking amp rating isn't as important as their reserve capacity (RC), which is how long a battery can carry a load before being rendered ineffective, and is expressed by its amp hours (AH). Since you should never discharge a battery more than 50 percent, divide the AH in half before making your calculation (amp load x hours) to find a battery that will meet your power needs. Models like the premium Trojan SCS200 are rated for 130 AH at a 20-hour rate, so they can discharge 6.5 amps per hour for 20 hours (10 hours if you don't want to fall below a 50 percent charge).
Dual-Purpose Batteries
Most marine batteries are really dual-purpose batteries and closer to deep-cycle batteries than cranking batteries because their lead sponge plate designs are far thicker. This gives them the ability to perform a wider range of tasks, and they are especially appropriate for small boats that can only handle the weight of one battery. Batteries such as Exide's Stowaway group 27 give you 730 cranking amps and 95 amp hours.
Battery Types
Wet Cell
Although this battery type has been around for 150 years, they are still a great source of power at a reasonable price and come in a wide variety of sizes for any application - deep cycle, cranking or dual purpose. Lead plates are submerged into a sulfuric acid/electrolyte solution and generate electricity from the chemical reaction.
Gel Cell
These lead acid batteries have the electrolyte solution suspended in gel form with fumed silica and are sealed to avoid spilling. Great care must be taken to avoid overcharging because the gel cannot be replaced.
Absorbed Glass Mat (AGM)
These are the most advanced, robust and expensive lead acid batteries on the market. These sealed units have fiberglass mats that are impregnated with the electrolyte, so there's no liquid to slosh around and they need no ventilation. Lead plates are sandwiched in between the mats, making them very strong and stable. Batteries such as the EnerSys Odyssey use Thin Plate Pure Lead (TPPL) technology and have an extremely high charge acceptance rate for faster charging, can be installed in any orientation and are rugged enough to handle the roughest seas. Another variant is the spiral-wound Optima Blue Top, which can withstand up to three times more recharges than a standard wet cell.
Cranking Batteries
See More PhotosLead's Not Dead
It was 151 years ago that Gaston Planté invented the wet cell lead acid battery using the same basic principle that standard batteries still rely on today. It has generally been assumed that this ancient technology would be replaced by something more exotic. Today, the popularity of hybrid cars is the driving force in the exploration of new battery technology, and the overwhelming choice is the nickel metal hydride (NiMH) battery. Lithium ion batteries are also gaining in popularity. Now there's a new technology emerging: the lead acid battery, with a twist.
This venerable technology is getting a boost, thanks to the efforts of Axion Power International, with an assist from Exide, the fourth largest battery maker in the world. Conventional lead cranking batteries can't withstand the numerous charge/discharge cycles that hybrid batteries experience, but in these new batteries, the lead negative electrode is replaced with activated carbon, a material used in superconductors. It not only lightens the battery but lasts up to four times longer and costs far less than standard hybrid car batteries.
How to Kill a Battery
Over-discharge it: The deeper you discharge a battery before recharging it, the quicker you will kill it. If you only discharge a battery 10 percent before recharging, it will last five times longer than one cycled down 50 percent, the maximum DOD (depth of discharge) recommended by most manufacturers.
Improperly use it: Using a cranking battery to power a trolling motor or other electronic device will result in early retirement. Deep or total discharge damages the thinner plates, leading to their failure.
Overcharge it: When you attempt to charge a battery faster than it can accept or longer than you should, gassing occurs when the electrolyte solution literally boils, leading to a variety of damaging effects, such as a warped grid, active material shedding, or even thermal runaway, which can cause a fire or explosion in enclosed areas. To avoid overcharging, use chargers with a three-stage process: bulk, absorption and float.
Store it without charging it: A lead acid battery loses up to 5 percent or more of its charge in a month, so if it sits for six months unattended, the charge drops to 70 percent or less. This causes serious problems such as permanent plate damage from sulfation. If it's stored in a warm location, the process is accelerated. When buying a battery, make sure you aren't buying an old battery by looking for a one-letter, one-number code. The letter is the month (A=January), and the number is the last number of the year it was made.
Expose it to extreme temperatures: At temperatures less than 41 degrees, totally discharging a battery can result in failure. At less than 32 degrees, the electrolyte solution in an undercharged lead acid battery can freeze. Hot temperatures can reduce the life of a battery dramatically. At 95 degrees, the battery life is about half that of a battery used in a more temperate climate. For boats that stay in the water, placing them in the bilge keeps the temperature down because of the ambient water underneath the hull.