Selecting Batteries for your Boat
Updated: Mar 6
We love batteries. Without them our lives aboard boats would be diminished. Batteries run the lighting, refrigeration, fans, depth sounders, chart plotters, electric winches, windlass, bow thruster, propane distribution, VHF, engine start motor, computers. The list goes on and on.
There are still a few purists who don’t believe in comfort, technology, and cold drinks... but that isn’t us.
If you want certain amenities away from the dock, the right type of battery and battery bank will make your time on the water a joy.
Everyone has different needs and budgets, and there is no single perfect solution to a battery bank. But to help you determine the best solution to your vessel, we compiled a list of the most popular batteries types used in boats today, with the positive and negatives of each, to help you with your selection process.
As always, if you have any additional question, need help with the process, or need to purchase or install a new battery bank, give Sea & Land Services a call!
Before going any further, there are a few keys to understand. These terms include stratification, sulfation, and equalizations.
Stratification: This is the tendency for denser sulfuric acid to concentrate at the bottom of a battery, allowing the water to float to the top. Stratification occurs when a battery is not fully charged (around 80%-85%). In a fully charged battery, stratification is prevented by gassing in the electrolyte solution, causing a mixing effect in tern producing a homogeneous solution. Stratification itself does not cause damage to battery on its own, but the lack of acid at the top of the cell causes high corrosion and less plate activation.
Sulfation: When a battery discharges, small sulfate crystals form on the lead plates. These crystals dissolve back into the electrolyte solution when the battery is recharge, and is normal process in the way lead acid batteries operate. Problems occur when a battery is not fully charged after a discharge cycle, or stored for long periods of time drained or with a partial charge. When this happens, only a portion of the sulfate crystals dissolve, and the remaining crystals can harden and become permanent. With further partial charging cycles, the condition becomes worse, and referred is to as sulfation. When sulfation occurs, the amount of sulfuric acid in the electrolyte solution and surface area of the plate decreases, decreasing battery capacity.
Equalization: This is the process of overcharging a lead acid battery to counter sulfation and stratification. Equalization is done by applying a 10% higher voltage to the battery, which frees sulfur ions back into the electrolyte solution, and forces the sulfuric acid at the bottom of the battery to rise and remix with the water.
Flooded Lead Acid
Flooded lead acid (FLA) batteries have been around for many years, and are still the most common battery found on boats due to the lowest up front cost . Flooded lead acid come in several versions, including start batteries (which use thick cells to provide an instantaneous high amperage output), deep cell batteries (which use thinner plates to provide a high output continuous output). Treated properly, these batteries can last quite a long time, but they do require periodic maintenance to maximize the lifespan of the battery.
FLA batteries are widely available and relatively inexpensive, and are used in a wide range of applications.
FLA batteries have the lowest life cycle of available battery chemistries, usually between 250-500.
Discharging FLA batteries discharged past 80% capacity can suffer physically damage to these batteries cells, which will severely impact overall life and usable capacity, leading to frequent replacement. Some manufactures give a rated discharge of 50%, but this value will exponentially decrease the life span of a battery bank.
At a safe discharge of 80%, these batteries only give roughly 1/3rd of usable capacity, and require additional batteries to give the same usable capacity of other battery chemistries.
FLA batteries produce oxygen and hydrogen when being charged, so they require adequate ventilation as these gases are highly flammable, explosive and corrosive. When determining installation location, they cannot be installed below other electrical equipment such as battery chargers or electrical panels as the gases the corrosive nature of this gas can cause failure of this equipment.
FLA batteries can spill, which is why they are required to be housed in a battery box (that do not have mounting holes drilled in the bottom for mounting). Since FLA batteries have liquid electrolyte, these batteries must be checked frequently for properly fluid levels and topped off as need with distilled water. Failure to do this can cause rapid failure destroying capacity and output.
FLA batteries are the heaviest of any battery chemistry.
Sulfation is a big problem with FLA batteries, and can lead to a very early death.
Efficiency diminishes with age and use.
6v Golf Cart (GC2)
Flooded 6V golf cart batteries are very popular option for marine application. They are very similar to 12V Deep Cycle FLA batteries, but differ in that they are designed to meet the rigors of frequent deep discharge, and as such are a good option. They are generally not sealed, so require the same maintenance and specialized tools as FLA batteries. Sealed AGM golf cart batteries are also available, and are maintenance free.
Because they are 6 volts, they must be used in pairs in order to make usable power, and each pair wired in series to achieve 12 volts.
Higher cycle life than 12V FLA batteries (2000 cycles @ 80%, 550 cycles at 50%)
Lead acid Golf Cart have the same disadvantage as 12V FLA batteries, with the exception of increased cycle and depth of discharge.
Efficiency diminishes with age and use.
AGM (Absorbed Glass Mat) batteries are second in popularity to FLA. They rely on the same chemical reaction as traditional FLA batteries, but instead of lead acid plates being submerged in liquid acid solution, they are separated by a thin fiberglass mat saturated in the acid, and hence are fully sealed. These batteries are sealed, so are more expensive, but have a longer lifespan, are maintenance free, and have higher useable capacity. AGMs can be discharged to 30% capacity without damaging the battery.
AGM batteries have a lower safe depth of discharge (80% maximum) in comparison to FLA batteries (50% maximum), which allows for a battery bank of the same capacity to be built with less batteries saving weight, space and cost.
AGM batteries have a lower internal resistance, which allows them to charge as much as five times faster than FLAs.
As AGMs utilize glass mat and are sealed, they can be mounted in any orientation. Note: Even though these batteries are sealed, and venting is less of an issue, they still have the same installation location requirements as FLAs as they will vent corrosive gas if overcharged.
Besides equalization, which is performed automatically by most newer battery and solar chargers, AGMs are maintenance free.
Since AGM batteries utilize glass mat, they are not susceptible to stratification as seen in FLA batteries, and can be left unattended for longer periods of time before a charge becomes necessary.
Just as with FLA batteries, AGMs are built with lead and as such are heavy.
AGM batteries are particularly susceptible to sulfation if not charged to 100% within several days after a deep discharge. In comparison to FLA batteries, AGMs do not respond as well to equalization, and as such can make them particular vulnerable to excessive discharge events in regard to the lifespan of the batteries. Note: With AGM installations, we often recommend the installation of a low voltage protection device to prevent this from occurring.
In comparison to FLA batteries, AGM batteries do come at a higher cost, but at a higher usable capacity this generally becomes irrelevant.
Efficiency diminishes with age and use.
GEL (Gelled Electrolyte)
Gel batteries are built similarly to FLA batteries, but with the addition of silica added to the battery acid, which causes it to become thick and gelatinous. Gel batteries have fallen somewhat out of favor due to them being less robust than AGMs. We rarely recommend these batteries, but they do have their place in specific instances, such as a small battery bank that may be cycled deeply frequently, with no ability to recharge in a timely manner.
Gel batteries are more resistant to the harm caused by deep discharge, and as such can tolerate a higher number of deep discharge cycles in comparison to FLAs or AGMs.
Gel batteries are completely sealed, and as with AGMs, can be mounted in any orientation.
Although efficiency will still diminish with age and use, these effects will not be seen until the end of the batteries life and will appear rapidly, as opposed to the gradually decline as seen in AGMs and FLAs.
Gel batteries require a slower charge rate than AGM to ensure maximum performance.
Since Gel batteries are lead based, like AGM and FLA, they are susceptible to sulfation. Unlike AGM and FLAs though, Gel batteries are not able to equalize and as such are less able to recover from sulfation.
Gel batteries are more expensive than FLAs.
Firefly (Carbon Foam AGM)
Carbon Foam AGM batteries are a new battery technology patented and produced by Firefly International Energy. Firefly batteries are similar in construction to AGM batteries, with the exception that the lead plates are replaced with carbon foam, eliminating many of the issues seen with AGMs, FLAs, and Gels.
Sea & Land Services is proud to be one of the limited distributors for this revolutionary battery.
Firefly carbon foam design resists sulfation and corrosion (which are the two leading causes of failure in lead acid batteries), and provides a greater surface area than other battery designs. Since these batteries are immune to sulfation and corrosion, they are able to be stored at a low start of charge with no internal damage, which is often a death sentence for other battery types.
Since these batteries are built with carbon foam as opposed to lead, the internal plates have a much larger surface area within the battery, which allows for a much larger capacity and a faster recharging rate compared even to AGMs.
Firefly batteries offer a up to 4 times the cycle life compared to AGM, FLA, and Gel batteries at 50% depth of discharge (3600-4200 cycles). That means if you were to drain the battery to 50% and recharge it every single day of the year, they would last 10-12 years. In the real world, the majority of vessels are discharging batteries far less, meaning these batteries could last 20+ years.
Although depth of discharge still has an impact on total battery cycles, Firefly batteries can be discharged to 20% capacity without damaging the cells.
Since Firefly batteries can tolerate a greater depth of discharge, and are not damaged by a low state of charge, a battery bank can be built with significantly fewer batteries which allows for less weight, space requirements, and total cost compared to other AGMs.
Firefly batteries are not as susceptible to stratification.
Firefly batteries have a higher upfront cost than other lead acid battery types. Fortunately, with considerations towards cycle life, usable capacity, and immunity to the issues that typically cause other batteries to fail, they are the most cost efficient option if you plan on keeping your boat for more than 5 years.
Compared to Lithium technologies, they do take more space per capacity and are heavier.
Lithium Iron Phosphate (LifePO4)
There are many Lithium battery chemistries on the market, but LifePO4 is the chemistry that has become the standard in the marine industry as it has a thermal and chemical stability that is better than any other.
LifePO4 batteries have an extremely low weight compared to other battery chemistries.
LifePO4 batteries have an incredible cycle life. They are not damaged by being left at partial states of discharge (they actually like it!), and recharge very quickly due to a low internal resistance.
Due to usable capacity, a battery bank can be sized to roughly 1/3rd of what an AGM bank would require in terms of amp hours. Not only does this further reduce weight, but it also allows for a smaller footprint if space is at an absolute premium.
Price - LifePO4 batteries alone will cost 4 times the price of a premium lead acid battery.
Installation - Converting from a lead acid battery chemistry to LifePO4 requires a complete system redesign. Due to LifePO4 low internal resistance, battery cables, and alternators, will all require to be significantly upsized as these batteries will try to pull in as much current as possible. Battery chargers and solar controllers will also have to be compatible with LifePO4 chemistry, which is will require them to replaced and appropriately sized. Failure to do so will only invite the possible of a catastrophic failure of your boats electrical system, or complete destruction of your new LifePO4 battery bank. Certain LifePO4 manufactures do offer built in BMS systems to
LifePO4 batteries rely on a battery management system (BMS) to control input current and battery output, balancing cells, and monitoring battery temperature, since even a slight imbalance between cells can lead to catastrophic overheating issues. If the BMS is damaged in any way, or if the voltage drops below a set BMS threshold, the BMS will stop any output and your boat will no longer have any battery power.
LifePO4 are particularly susceptible to cold weather charging, and only a few charging cycles at near freezing temperatures can completely destroy a battery bank. There are several manufacturers that build in heating elements to allow for cold weather use, but if you plan to use your boat in cold weather, or you leave your boat in the water during the winter, the risk of loosing your entire house battery is often not worth the benefits lithium provides.
With a plethora of battery options available for you boat, with advantages and disadvantage to each, the decision can be daunting. Hopefully this article will be helpful when trying to make the correct selection decision for your boat and wallet.