Battery 101 for the Marine Enthusiast

Choosing the right battery and charger system can help save money in the long run

Batteries are the lifeline of modern boats. Besides performing their primary function of starting the motor, they are responsible for running everything from fish finders, navigation lights, stereos and aerator pumps, among other on-board electronic accessories. However, buying new boat batteries can be a daunting task because each boat is unique and requires different power amenities depending on size, engine capacity, construction and electrical features. Before making a purchase, it’s important to evaluate the pros and cons of each battery option to determine which is best for your boat based on its specific power needs. This article will provide an overview of the various types of marine battery applications and chemical structures available on the market today.

Types of Marine Battery Applications
Deep Cycle

A boat’s trolling motor relies on deep cycle batteries. These batteries are the marathon runners of the boat’s storage system, as they power the electrical loads on the boat when no charge source (shore power charger, engine alternator, wind generator or solar panel) is available. They are intended to discharge energy over long periods of time, using approximately 50-80 percent of their capacity, and are able to recover completely afterwards. They are built with thicker plates than starting batteries and are significantly heavier than a starting battery of similar dimensions.


Also known as a cranking battery, a starting battery powers the starter motor, enabling the engine in the boat to start. Starting batteries are easily depleted and rely on an engine’s alternator to restore the charge. They house negatively and positively charged plates that are separated by insulation. One of the main differences between a starting battery and a deep cycle battery is the thickness and number of charged plates. Along with engine starts, starting batteries are commonly used for bilge pumps, navigation, courtesy lights, aerators and general marine electronics.

Dual Purpose

Simply put, dual purpose marine batteries can perform the function of both traditional deep cycle and starting batteries. With larger, thicker plates than starting batteries, dual purpose batteries are ideal for tolerating deep discharges that would ruin a typical starting battery. This class of batteries contains a small amount of liquid, just enough to moisten the internal plates so nothing can break or leak out. This design makes these types of batteries durable, stable and able to start the engine while powering the electronics over the course of a full day on the water.

Marine Battery Basics: Chemistry, Benefits and Features

Marine batteries are available in four chemical types for different applications: flooded, gel, Absorbed Glass Mat (AGM) and lithium. Choosing the right battery depends on the boat’s power needs (deep cycle vs. starting), the capacity, the lifespan and the cost. This article focuses on the variations between the most commonly used marine batteries, which include flooded, gel and AGM chemistries.

Flooded Batteries

Flooded batteries, also referred to as wet batteries, use a reservoir of liquid sulfuric acid and produce hydrogen and oxygen when the battery is being charged. Vented wet cells allow the gases to escape into the atmosphere, unlike gel and AGM batteries, which recombine the gases and reintroduce them to the system. Flooded batteries require significant maintenance, including periodic inspection and topping off with distilled water when the electrolyte level falls. They are known to self-discharge at a rate of six to seven percent per month, which means they cannot be stored for extended periods of time without maintenance charging. In addition, flooded batteries are not well suited to marine applications since they have minimal vibration resistance and must be kept upright at all times. 

Gel Batteries

Sealed, valve regulated gelled-electrolyte batteries offer several unique advantages over regular flooded batteries. They provide up to 20 percent more cycles than AGM batteries, sustain charging losses of four percent and a maximum continuous discharge rate at 25 percent of their capacity, are virtually maintenance-free, spill proof and leakproof. These batteries are also referred to as recombinant batteries because the hydrogen and oxygen produced during charging is converted back into water by the pressure inside the batteries.

AGM Batteries

AGM batteries offer premium dual-purpose performance. They feature thin, highly absorbent microfiber glass separators compressed tightly between the battery’s positive and negative plates, which are saturated with just the right amount of electrolyte to activate the battery. In addition to providing equal saturation across the entire surface of the battery’s positive and negative plates, the fibers in the separators embed themselves securely into the plates’ surface, providing better vibration protection than in conventional flooded lead acid batteries. AGM batteries excel in both high-rate applications, such as engine starting, as well as in lower but longer discharge rates typical in applications such as trolling motors and powering house loads.

The ODYSSEY® Battery Power Solution

The advanced line of ODYSSEY® Marine Dual Purpose batteries and chargers is ideal for trolling, starting and powering the motor, as well as the many on-board electronic accessories common in today’s marine applications. Engineered with TPPL technology and AGM construction, ODYSSEY® batteries provide deep cycling capabilities up to 400 cycles at 80 percent Depth of Discharge (DOD). Battery sizes range from as small as 6.70 inches long by 3.90 inches wide by 6.95 inches high for ODYSSEY® model PC-625 to as large as 22.87 inches long by 4.92 inches wide by 12.46 inches high for model PC1800-FT and offer as much as 1,300 cold cranking amps (CCA).

With twice the overall power and three times the life of conventional marine batteries, ODYSSEY® batteries will get the boat out on the water reliably, even after laying idle for long periods of time during winter storage. Its unique design and internal construction enable it to withstand even the harshest weather conditions and can be stored for two years at temperatures of 77°F (25°C) or lower. They are vibration resistant, can be mounted on its side without spilling, and include a limited two- to four-year full replacement warranty.

However, even the best battery is only as good as the charger needed for proper maintenance. ODYSSEY® battery chargers are designed for AGM batteries and provide automatic six-stage charging and a wide range of built-in safety features. ODYSSEY® battery chargers facilitate full and proper charging, and are available in 6-amp, 12-amp and 20-amp models. They are backed by a limited two-year full replacement warranty.

Technological advancements in battery chemistries, like the ODYSSEY® battery portfolio for marine applications, and charger systems can drastically save time and money, while ensuring maximum performance and peace of mind during every open water excursion.


EnerSys, the global leader in stored energy solutions for industrial applications, manufactures and distributes reserve power and motive power batteries, battery chargers, power equipment, battery accessories and outdoor equipment enclosure solutions to customers worldwide. Motive power batteries and chargers are utilized in electric forklift trucks and other commercial electric powered vehicles. Reserve power batteries are used in the telecommunication and utility industries, uninterruptible power supplies, and numerous applications requiring stored energy solutions including medical, aerospace and defense systems. With the recent Alpha acquisition, EnerSys provides highly integrated power solutions and services to broadband, telecom, renewable and industrial customers. Outdoor equipment enclosure products are utilized in the telecommunication, cable, utility, transportation industries and by government and defense customers. The company also provides aftermarket and customer support services to its customers in over 100 countries through its sales and manufacturing locations around the world.