How To

    Understanding Boat Batteries: Part One

    Boat Batteries – The Technical Details

    Although your boat uses a gasoline engine for its primary propulsion, without ample electricity, that outboard is pretty much out of business.

    Let’s tackle a fistful of the most common boat battery-related topics. To keep things simple, we’ll discuss a single engine boat; however, the principles are similar for multi-engine installations.

    Boat Battery Applications and Ratings

    On most boats, especially those with lots of demanding electrical equipment (trolling motors, aerated/recirculating livewells, washdown pumps, downriggers, full electronics, powerful sound systems), you’ll need multiple batteries – a starting battery, and at least one storage battery – or a combination starting/storage battery.

    If your boat doesn’t have enough room for more than one battery, consider a dual-purpose cranking/deep-cycle battery. Make sure to check the label to make sure it meets your outboard manufacturer’s specs.

    Marine Cranking Battery (Starting Battery)

    As the name implies, the cranking battery’s sole purpose is to start the engine.

    When you initiate the starting sequence (turn the key, press a button), the starting/cranking battery sends an enormous surge of electricity to the engine’s starting motor. After the outboard is running, the engine’s charging system sends electricity back to the cranking battery to recharge it.

    The starting battery expends a great deal of energy for a few seconds at a time. A cranking battery is exceptionally strong in the short term; however, it doesn’t have an excess of power in reserve, and will fail prematurely if deeply discharged repeatedly.

    According to the American Boat & Yacht Council (ABYC®), a marine battery must have at least the cold cranking amperage (CCA) required by the engine manufacturer. Cold Cranking Amps is a rating that defines a cranking battery’s ability to start an engine at 0°F (-17.8°C). This can range from about 250 CCA for an electric-start 25 hp kicker to 750 CCA for a 350 hp mega-outboard.

    You should find cranking battery specs in the outboard’s owner’s manual – Cold Cranking Amps (CCA), Marine Cranking Amps (MCA) and Reserve Capacity (RC). At least one of these three ratings – CCA, MCA and RC – should be clearly marked on the battery’s label.

    Prudence dictates installing a dedicated starting battery connected only to the outboard’s ignition/starting circuit. If all the boat’s electrical gadgets use the starting battery, there is a very strong possibility that you’ll hear a sickening “click” when you try to crank the engine after a long day on the water.

    Deep-Cycle/Storage Batteries

    A storage battery (or banks of multiple batteries) provides the juice to energize all the boat’s electrical systems – without the engine running. Another name for a storage battery is the “house battery,” because, like in a house, it delivers electricity for the lights, TV, air conditioning and other components. On a fishing boat, you can substitute the term “trolling motor” batteries for “house” batteries.

    You’ll want to use deep-cycle batteries for your boat’s non-engine electrical needs. Deep-cycle batteries (or banks of batteries) provide large volumes of power over extended periods of time, can endure extreme discharges – and bounce back as good as new after a recharging session.

    A deep-cycle battery has several ratings on its label, but the two you ought to take note of are Amp-Hours (AH) and Reserve Capacity minutes (RC).

    Amp-Hours (AH) quantifies the number of amps a new 12 volt battery can deliver at 80°F (27°C) by multiplying the electrical system’s current draw in amps by the time it takes (industry standard is 20 hours) to discharge the battery to 10.5 volts. For example, a battery that can successfully energize a 7 amp load for 20 hours (7 amps x 20 hours = 140 AH) would display a 140 AH rating on its label.

    Reserve Capacity (RC) is the number of minutes at 80°F (27°C) that a new, fully charged battery can be discharged under a 25 amp load while maintaining a voltage of at least 10.5 volts.

    Dual-Purpose Batteries

    What if your boat doesn’t have enough space for more than one battery? Then you may consider a dual-purpose cranking/deep-cycle battery for your boat.

    Dual-purpose batteries excel in the engine-starting arena, and have sufficient reserves for low-demand accessories.

    Make sure to check out the label to ensure the battery meets your outboard manufacturer’s Cold Cranking Amps specs, as well as the Amp-Hours, to make sure the battery has the intestinal fortitude to feed the electrical requirements of your boat’s nav lights, bilge pump, and electronics.

    Sample warning label provided. Please refer to the specific warning label that accompanies your specific battery for complete details and warnings.

    Marine Battery Construction

    Boat batteries are available in three fundamental designs: flooded-cell lead-acid, gel-cell, and Absorbed Glass Mat (AGM).

    Flooded-cell lead-acid batteries contain lead plates (and other components), within a battery case filled with an electrolyte solution. These batteries can be maintenance free or fully maintainable (removable caps to check fluid levels). Install flooded-cell batteries upright, to keep the liquid electrolyte in the battery case where it belongs.

    Gel-cell and AGM batteries use Valve-Regulated-Lead-Acid (VLRA) technology for safer, more durable electrical storage. VLRA batteries are sealed lead-acid batteries, except for a small one-way valve that allows internal gases to exit the battery housing when/if these gas pressures exceed the manufacturer’s specified levels.

    A VRLA (gel-cell/AGM) battery is often called a “recombinant” battery, meaning that the negative plates capture the oxygen emitting from the positive plates. This suppresses hydrogen production at the negative plates, creating water instead, thus keeping the battery well hydrated.

    A gel-cell is a lead-acid battery in which the electrolyte solution is suspended in a thick jelly-like mixture contained in a sealed case (with a VRLA valve). Gel-cell batteries are spill-proof and maintenance-free. Due to the way the acid/gel mix functions, these types of batteries can be well suited for deep-discharge applications. The properties of gelled electrolyte that make a gel-cell so desirable are also a potential weakness; a gel-cell battery’s power drops rapidly below 32°F (0°C).

    In an AGM battery, absorbent glass-fiber mat separators soak up the electrolyte like a sponge. Encased in a VRLA-valved sealed housing, an AGM battery is maintenance-free and spill-proof; you could conceivably install this battery at any angle, except upside-down. AGM batteries are noteworthy for their consistently high current and power output, even in bitterly cold conditions.

    VRLA batteries – AGM and Gel-cell – are particularly sensitive to overcharging; therefore, use a good constant potential, temperature-compensated, voltage-regulated charger.

    Never use a constant current charger on VRLA batteries.

    Other Battery Thoughts

    The final word on what type/size/rating of battery you must use is in your outboard’s owner’s manual – for example, Yamaha Marine Group specifies flooded-cell batteries to avert potential overcharging gel-cell or AGM during extend periods of high-speed engine operation.


    American Boat & Yacht Council (ABYC®) Standards and Technical Information Reports for Small Craft

    U.S. Coast Guard Boatbuilder’s Handbook

    Yamaha Motor Corporation, U.S.A.

    Image Credits

    Ranger® Boats

    How Much Battery Do I Need?

    Grab your calculator. How much electricity does your boat require? You’ll need enough power to start the engine – very important – and sufficient electrical reserves (storage batteries) to operate the navigation lights, bilge pumps, electronics, livewell pumps, trolling motor, and assorted accessories.

    Battery Warnings

    • Batteries contain sulfuric acid. When handling batteries, use extreme caution
    • Wear rubber gloves, safety glasses, and protective clothing while working with batteries
    • Electrolyte/sulfuric acid will burn your skin and can destroy your eye
    • If acid contacts skin, wash skin thoroughly with soap and water
    • If acid contacts eye(s), flush aggressively with fresh water and get medical help immediately
    • Batteries produce explosive gases
    • Ventilate when charging or using in an enclosed space
    • Keep sparks, flames and cigarettes away
    • Do not allow any electrically conductive material to contact the positive (+) and negative (-) terminals at the same time (i.e. a wrench accidentally touching both terminals when connecting battery cables) This will short-circuit the battery, creating dangerous sparks and/or causing the battery to explode, resulting in severe personal injury
    • Keep batteries out of reach of children

    List provided as a sample only. See the instructions that accompany your specific battery for complete list and warnings.

    To continue to part two of this guide, click here.

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