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Know How: Batteries

Given the ongoing proliferation of electronic equipment and gadgets like watermakers, autopilots and electric winches on modern sailboats, it’s little surprise that batteries have become increasingly important. Typically speaking, marine batteries are 6- or 12-volts and are used as cranking batteries to start an engine, as deep-cycle batteries for house loads or as dual-purpose cells.

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Cranking batteries are typically lead-acid and have many thin lead plates to expose maximum surface area and produce maximum power in short bursts in order to start a boat’s auxiliary engine. These batteries are recharged via an alternator once the engine is running.

Unlike cranking batteries, deep-cycle batteries are designed to service smaller loads over a much longer period. These “house batteries” are built using a variety of technologies. The most common lead-acid batteries are wet cell (or flooded), gel cell and absorbed glass mat (AGM).

State-of-the-art lithium-ion batteries can also be used to handle large capacities and are capable of withstanding many deep-discharge cycles. House batteries typically have fewer (but much thicker) lead plates than starting batteries and are available in capacities of 25AH (amp hours) and 225AH. They are also often wired together to create a bank of batteries with much greater capacities. Look for batteries that meet your anticipated power needs and offer the most charge cycles at 50 percent discharge. Also, be sure that your entire house bank is comprised of a single battery type—never mix battery types (or old and new batteries), as they will have different charge and discharge rates.

Dual-purpose batteries used to start an engine and service house loads are targeted toward small boats that only have space/weight accommodation for a single battery. These provide a reasonable amount of starting power and cycling capacity relative to their size, however, they are typically not suitable for extended cruises. 

Photos courtesy of (from left) Optima, Mastervolt, West Marine

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