Choosing a Marine Battery

What kind of battery do you need?

First, there are three types of 12-volt marine batteries: starting (also known as cranking), deep cycle, and dual purpose batteries.  Starting batteries are designed to start the main engine. Deep cycle batteries are used to power accessories and trolling motors. Dual-purpose batteries, as you would expect, are a combination of both starting and deep cycle batteries.
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Sealed Maintenance Free Battery

Sealed batteries are known as maintenance free batteries.  They are made with vents that (usually) cannot be removed.  A standard auto or marine maintenance free battery is sealed, but not fully leak proof.  Sealed batteries are not totally sealed since all batteries must allow gas to vent during charging.  There are sealed lead acid (SLA) batteries that are non-spillable.

 AGM or Absorbed Glass Mat Battery

The newer type of sealed non-spillable maintenance free valve regulated battery uses “Absorbed Glass Mats”, or AGM separators between the plates.  This is a very fine fiber Boron-Silicate glass mat.  These type of batteries have all the advantages of gelled, but can take much more abuse.  These are also called “starved electrolyte.”  Just like the Gel batteries, the AGM Battery will not leak acid if broken.

Advantages of the AGM battery

The advantages of AGM batteries are no maintenance, sealed against fumes, hydrogen, leakage, or non-spilling even if they are broken, and can survive most freezes.  AGM batteries are “recombinant” – which means the Oxygen and Hydrogen recombine inside the battery.  These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis.  The recombining is typically 99+% efficient, so almost no water is lost. 

Charging voltages for most AGM batteries are the same as for a standard type battery so there is no need for special charging adjustments or problems with incompatible chargers or charge controls.  Since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents.  

AGM batteries have a very low self-discharge rate (from 1% to 3% per month).  So they can sit in storage for much longer periods without charging.  The plates in AGM’s are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery.
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Gel Cell Battery

A gel battery design is typically a modification of the standard lead acid automotive or marine battery.  A gelling agent is added to the electrolyte to reduce movement inside the battery case. 

Many gel batteries also use one way valves in place of open vents, this helps the normal internal gasses to recombine back into water in the battery, reducing gassing.  “Gel Cell” batteries are non-spillable even if they are broken. 

Gel cells must be charged at a lower voltage (C/20) than flooded or AGM to prevent excess gas from damaging the cells.  Fast charging them on a conventional automotive charger may permanently damage a Gel Battery.
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Marine starting

Marine starting batteries enable a faster power delivery. Starting batteries provide power for accessories while your engine is running, and the battery’s power is replenished by the alternator.
When shopping for marine starting batteries, check the engine manual for its recommended rating. Choose a battery of equal or greater power than the recommended value designated in the engine manual.

Deep-cycle

Deep-cycle marine batteries, contrary to cranking batteries, discharge power at a slower rate for an extended period of time. They have fewer, yet thicker lead plates. These batteries are primarily built to provide power for your trolling motor and other accessories, like fish-finders, whether or not your engine is running.

Dual-purpose

Dual-purpose marine batteries can be used both for starting and deep-cycling, though only to an extent. Dual-purpose batteries don’t have as much cranking power as a dedicated starting battery. They’re also unable to endure as much deep discharge and recharge cycles as a dedicated deep-cycle battery. Dual-purpose marine batteries are best for small boats that have weight restrictions or room for just one battery. Otherwise, you should install two separate batteries, one for starting and one for trolling and accessories.
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