Buying Guide: How to Buy the Right Marine Battery for Your Boat
There are many types of marine batteries available, each for a different purpose with various advantages and disadvantages; here's what you need to know to make a good decision
It’s that time of year when many anglers are preparing boatsand fishing gear for the spring season soon to come. If you’re among them,perhaps one of the items you need to purchase is a new 12-volt battery for yourboat. This can be a daunting task, especially if you’re buying a battery forthe first time.
With this guide, however, you’ll have easily understandableinformation so you can choose a battery ideal for the tasks at hand.
Marine Battery Types
There are two basic types of 12-volt batteries: 1) cranking,or starting, batteries, which are designed to start your main engine, and 2)deep-cycle batteries, which are used to power electrical accessories such as trollingmotors, fish-finders and radios. Dual-purpose batteries that can perform boththese functions to some extent also are available.
Engine cranking requires lots of power in a short burst. A battery with more surface area on the leadplates inside it delivers more fast power than one with less plate surface. Forthis reason, cranking batteries are made with thinner, more numerous lead plates.When the engine is running, the battery is quickly replenished by the alternator.
The marine cranking amp (MCA), or just cranking amp (CA),rating found on a battery’slabel measures a battery’s starting power. And if you’re using a newer modeloutboard with sophisticated computers, pumps and sensors that don’t take kindlyto being underpowered, you certainly don’t want to scrimp on starting power.Check your engine’s manual for its recommended MCA/CA rating before shoppingfor a battery, and always choose a battery with a rating equal to or greaterthan the recommended value.
Trolling motors and other accessories sip power at a slowerrate for extended periods. Batteries that power them usually aren’t rechargeduntil the end of the day. These deep discharges are hard on battery plates, so deep-cyclebatteries have fewer yet thicker lead plates than cranking batteries and arebuilt to withstand deep cycling.
A deep-cycle battery’sreserve capacity (RC) rating indicates how long it can carry a specific loadbefore falling into the dead zone. The higher the RC number, the longer the battery will power your accessories.Remember this when choosing a battery.Typically, a deep-cycle battery will have two or three times the RC ofa cranking battery. A deep-cycle battery also can withstand several hundreddischarge/recharge cycles, while a cranking battery is not designed to betotally discharged.
The key thing to remember when purchasing cranking anddeep-cycle battery types is not to substitute one for the other. If you use a crankingbattery to power your trolling motor, for instance, the battery will soon overheatand fail, leaving you without power and requiring you to buy a new battery. Adeep-cycle battery substituted for a cranking battery, on the other hand, maynot provide the power needed to start your outboard. You could get strandedwith an engine that won’t run. The inherent design strengths of each batterytype also are their weaknesses in opposite applications.
Marine Dual-Purpose Batteries
It’s usually bestto install separate cranking and deep-cycle batteries. If your boat is small,however, and there’s only room for one battery due to space or weightrestrictions, consider buying a dual-purpose battery that handles starting andcycling. Bear in mind, however, most dual-purpose batteries won’t start an engine quiteas well as a true cranking batteryand won’t endure as many deep discharge/recharge cycles as a dedicateddeep-cycle model.
Wet Cell, Gel or AGM?
Marine batteries can be further categorized as wet cell, gelor AGM (absorbed glass mat), depending on the configuration of the electrolyte,or conducting medium, inside the battery. Each type has pros and cons youshould know before making a purchase.
The traditional lead-acid marine batteries we all arefamiliar with are wet-cell or “flooded-cell” batteries. Each contains a liquid mixtureof sulfuric acid and distilled water (usually referred to as “battery acid”).This is the most popular battery type, primarily because the price is generallyless than other types, but also because these batteries have a number ofinherent advantages. For example, a properly charged and maintained, premiumwet-cell battery is capable of as many as 1,000 discharge/recharge cycles. Thiscan translate to many years of dependable service at an initial costsubstantially less than comparable capacity gel or AGM batteries. Wet-cellbatteries also are less likely to be damaged by overcharging and tend to weighless than comparable gel or AGM batteries.
One disadvantage of wet-cell batteries is the fact most havevented, interior accessible designs. This requires they be regularly inspectedand the cells topped off with distilled water. Venting also releases hydrogengas, which means the battery compartment must be well ventilated. Otherdrawbacks include possible spilling of corrosive battery acid, a higher rate ofself-discharge (6% to 7% per month) and the fact that wet-cell batteries aremore fragile in high-vibration environments such as boats.
AGM batteries feature a dense filling of absorbent glassmatting packed tightly between the battery’s plates. The matting is saturatedwith acid electrolyte. This allows oxygen to recombine with hydrogen gas toreplenish the battery’s water content and alleviate the need for refilling. Nomaintenance is required, except periodic external cleaning. And because thesebatteries are sealed, acid inside cannot spill and flammable gases aren’treleased.
AGM batteries also can be installed at any angle. They areshock and vibration resistant, submersible without damage unlike wet-cellbatteries, and have a low self-discharge rate (around 3% per month at 77degrees F). Primarily drawbacks are higher initial cost, greater weight thancomparable wet-cell batteries and the fact water cannot be replaced if thebattery is accidentally overcharged.
Gel batteries are filled with liquid electrolyte that isgelled with silicates before the batteryis sealed. Like AGM batteries, they use recombinant technology that eliminatesthe need for adding water. They are maintenance free, sealed, low-temperaturetolerant, shock/vibration resistant and have long cycle life.
Their most notable advantage is resistance to over-dischargethat can damage other batterytypes. Gels have an internal self-discharge rate less than 1 percent per month,so they can be stored for long periods without being recharged. And becausethey aren’t prone to develop life-shortening plate sulfation when leftuncharged, they are a good choice for boaters who often forget to rechargebatteries promptly after use.
Gel batteries show their dark side when it comes to priceand recharging. They generally cost more than other battery types with the sameRC and MCA ratings. Most manufacturers recommend charging them to at least 13.8volts, but no more than 14.1 volts. Standard marine battery chargers routinelyreach higher voltages, so chargers designed to include gel batteries or thosewith a gel setting must be used.
Understanding the differences between the various types ofbatteries, and the pros and cons of each, should no doubt prove useful whenyou’re trying to decide which battery will work best to crank your boat andpower all your accessories. Making the right choice eliminates many worrieswhen you are on the water, allowing you to concentrate on the most importantreason you bought your boat in the first place—having fun on the water.