The Ultimate 12-Volt Power Guide

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Nothing is more frustrating in boating than twisting the starter key and getting a useless click, or switching on the stereo and getting the Ode to Silence. And to many, there is nothing more confusing than the myriad battery choices, both types and sizes, all requiring different maintenance, offering varying benefits and posing unique liabilities. To help unravel some of the voodoo, here is Boating’s primer on making sure you always have 12-volt power and the equipment to refresh it.

Wet Cell/Flooded Electrolyte(left)
How It's Made: Flooded cells feature thicker lead and antimony plates surrounded by liquid electrolyte. Marine units are built with thicker plates and extra antimony mixed with the lead, to make them more resistant to shedding and other damage from vibration.

Pros and Cons: Lowest initial investment cost for cold cranking and deep-cycle use, but their construction makes them prone to sulfation. One deep discharge can drastically reduce the battery’s ability to store and give power.

Maintenance: Flooded-cell batteries release a gas that causes the electrolyte to evaporate, leaving plates dry. Monthly additions of distilled water are necessary for keeping them up to snuff. Annually remove cables, clean posts and clean or replace terminals.

Charging: Simple automatic chargers are recommended. Overcharging with a manual charger is destructive to the plates. Batteries discharge rapidly with no load over a few months, so in storage, charging power should be available.

Best Choice When: Low initial investment cost is primary.

Cost and Cost to Own: The West Marine SeaVolt costs from $200. Regular charging and more frequent maintenance add to the cost. Rapid dormant discharge can add to frustration and towing or jumpstarting costs.

Gel Cell(center)
How It's Made: Many thin plates of lead and calcium are coated with a waxy or gel-like electrolyte and packed tightly together within a sealed case.

Pros and Cons: Ideal as pure deep-cycle batteries, but less than ideal as starting batteries (thin plates may bend under heavy draw) and not as rugged as AGMs.

Maintenance: As in AGM batteries, gels have sealed cells and no monthly electrolyte monitoring is needed. Annually remove terminals and clean and lube connections.

Charging: Gel cells have very specific charging voltage requirements and, while some recommend AGM profiles, they are apt to last longer with charging profiles designed for AGM batteries, often requiring a special charger.

Best Choice When: Superior cycling ability is a plus for deep-cycle use, but low resistance to vibration and lower cranking voltage make them less useful in all but slower moving powerboats.

Cost and Cost to Own: Cost is comparable to that of AGMs without the advantages. The West Marine SeaVolt Gel is $300.

AGM(Absorbed Glass Mat, right)
How It's Made: A pasty electrolyte bonded to plates pressed between an electrolyte-impregnated glass mat in a sealed box means these batteries can’t spill acid and won’t shed needed plate material from vibration of boating.

Pros and Cons: Maintenance-free sealed cells can’t gas off electrolyte , so you never need to check and top off with water. Fast to recharge, they are kept up to snuff by a short run of the engine. Offers more power in smaller footprints and more versatile installation options.

Maintenance: AGMs are sealed so electrolyte can’t escape and won’t need to be replaced in cells. They need no charging during storage if isolated from current requirement. AGM battery terminals don’t “cold creep,” reducing terminal dimensions, so that annual removal and cleaning may not be necessary.

Charging: AGMs require more voltage and a special charging profile to properly recharge.

Best Choice When: Both cold-cranking and deep-cycle power are demanded from same batteries, since these can be heavily drawn down without damage.

Cost and Cost to Own: EnerSys, Odyssey and Energizer make AGMs ranging from $250 per battery. Ownership costs often negate initial sticker shock, thanks to long life (three to five years) and ability to recover from repeated deep discharges.