Tips for Battery CareMany boaters take boat battery systems for granted, in much the same way as vehicle owners do with car batteries. Understanding the essentials of boat charging systems and basic maintenance will help to ensure that the boat you own or rent will provide the “get up and go” when you need it and will power auxiliary systems without affecting your ability to move about. Neglecting a marine battery is certain to shorten its life and can strand you on the water. 

Following these vital tips will help ensure that your boat has the power to function properly when you need it to.

Tips to Evaluate a Boat's Battery

  • Ask the boat owner for a description and demonstration of the boat’s battery, charging system, and isolator switch when considering a boat rental. 
  • Determine how long it has been since the boat was last underway and have the owner crank (start) the engine and operate all auxiliary systems prior to departure. Ask about the maintenance schedule.  
  • Recreational boats of medium to larger size can be equipped with two batteries and a battery switch (or isolator), allowing one or both to be used at the same time, depending on power needs at any point during a voyage. 
  • Remember that a battery can lose up to 30% of its charge per month, depending on how and where it is stored and how often it has been used. Other variables (temperature, humidity, state of discharge, and battery age) can also affect the level of charge.  

Basic Marine Battery Types

Boat battery technology has come a long way since the 1960s. Now, a variety of batteries are available and all have at least one thing in common: they all need care – even the maintenance-free types. The type of battery installed on a boat is based on need, capacity, expected lifespan, and budget. The three basic types of marine batteries are described below.  

  • Cranking (Starting) Batteries: Designed to start the engine. They are made with numerous lead plates inside, providing more surface area to provide the massive amount of energy required to start the engine quickly. They are appropriate as a single battery for runabouts, personal watercraft, and other boats with minimal direct current loads when the engine is always running and the need for auxiliary power (like a marine radio) are minimal or have their own power source.  
  • Deep-Cycle Batteries: Designed to power on-board electronic equipment like trolling motors, fish-finding and Global Positioning System (GPS) units, marine radios, and stereos. These batteries use energy at a much slower rate and can withstand the rigors of several hundred discharge/recharge cycles compared to cranking batteries.
  • Dual-Purpose Batteries: Work well in some applications and can handle the deep discharges that would drain a starting battery. They have a lower storage capacity than comparable deep cycle types are good for boats with two identical batteries used interchangeably for starting and electrical loads. Most boats with dual-battery systems use a starting battery and a deep-cycle battery.

Electrolyte and Interior Battery Plates

Batteries can be further categorized by the configuration of the electrolyte and interior battery plates, described below:  

  • Wet (or Flooded) Cell Batteries: Generally the most popular, these batteries contain cells filled with sulfuric acid and distilled water. These deliver a high number of discharge/recharge cycles, are less susceptible to damage from overcharging, and weigh less than others. Most wet cells most have access ports to allow inspection and topping off with distilled water. Drawbacks include the potential for battery acid spillage, a higher rate of self-discharge (6% to 7% per month) and susceptibility to high-vibration environments, like boats.
  • Absorbed Glass Mat (AGM) Batteries: These batteries are filled with absorbent fiberglass matting, packed tightly between the battery plates and saturated with liquid acid/electrolyte. The design allows oxygen to recombine with hydrogen gas, which replenishes the battery fluid and eliminates the need for refilling. AGM batteries are more expensive, heavier, and easy to overcharge.
  • Gel Batteries: Like wet cells, gel batteries are filled with electrolyte, but the electrolyte is gelled with silicates before the battery is sealed, so adding water is not necessary. They are low-temperature tolerant, vibration resistant and have a long cycle life and a good choice for boaters who forget to recharge batteries promptly after use. Gel batteries are pricey and require special charging equipment.
  • Lithium-ion Batteries: The “super battery” of the marine world, with high-energy density and excellent deep-cycle capacity. Compared to wet cells, lithiums deliver a savings of up to 70% in weight and provide triple the charging cycles. Typical uses include high-performance racing sailboats and others where weight savings and cutting-edge performance is important.

How to Store Boat Batteries

Regardless of where and how a boat is stored during the off-season, the best bet is to remove all batteries and store them in a heated garage, basement or other climate-controlled space. A fully charged battery with a perfect electrolyte level will likely withstand temperatures down to zero degrees without freezing. Follow these recommendations for longer battery life:

  • Keep batteries off concrete floors and cover terminals to help prevent discharge.
  • Regulate charge voltages based on battery temperature and acceptance (manually or with sensing devices) to maximize life and reduce charge time.
  • Keep batteries clean, cool, and dry.
  • Check terminal connectors regularly to avoid loss of conductivity.
  • Check fluid levels frequently during the off-season and add distilled water as needed.
  • Keep batteries charged, even when in storage.
  • Clean corrosion off the battery case and terminals with a paste of baking soda and water. 

Developing a basic understanding of a boat’s battery and charging system, the type and categories of batteries on board, and how the systems have been maintained will provide the peace of mind you need to confidently embark on a voyage, whether on a boat you own or one that is rented. Keeping a battery healthy by regularly performing charging, cleaning, related maintenance, and off-season care will ensure that the boat’s engine, the safety equipment and creature comforts operate as they should when you need them.