With many types of boat batteries for sale, trying to choose the best can be confusing. To make buying decision I prefer to have as much information as possible. So to help consumers, I made this comprehensive guide. In this article you will learn everything about how to choose a boat battery in 2021.
To skip to a specific topic in this article there’s a clickable table of contents. Or read the whole article.
We updated this article in December 2020 to ensure the information in this guide is current for the upcoming year. Read on to learn about how to choose a boat battery in 2021.
A marine battery uses a special design specific for to maximize powering a boat and its accessories. Invented in 1859 by French physicist Gaston Planté the flooded lead-acid (FLA) battery was the first rechargeable unit for boats. Unlike the unit you use in a car, the marine version comes with heavier robust internal construction. This special design helps withstand the abrupt vibrations vessels experience out on the water.
Marine Versus Automotive
Boat power sources differ from cars in three distinct ways, the first difference is use. Unlike cars, most boats need two types of batteries to handle two different but simple tasks. One is for starting an engine and the second is for operating long term electrical loads, like lights.
A second difference, is the four available chemical types. The first is the traditional unsealed “Wet” Cell or flooded lead-acid (FLA). Next are a valve-regulated lead-acid batteries (VRLA battery), either Gel Cells, Absorbed Glass Mat (AGMs). For boaters where cost isn’t a factor the fourth chemical type, called Lithium is a premium pick.
The third difference is, their specific rating system using energy output (ampere hours) and recharging cycles. The amount of output and their lifespan ratings determine the price.
Which one you choose will depend on your use, chemical type and number of banks needed. Now, let me explain the basic materials and components of lead-acid power sources.
Lead-Acid Materials & Components:
Plastic Container – Made of polypropylene, with many internal sections. These sections or “cells” mimic one row in an ice cube tray. Metal connects each cell together to conduct electricity from one cell to the next.
Plates – These are grids made of lead or a lead alloy. Some plates have lead oxide paste painted on the grids; this makes a positive lead plate. Adding an expander material of powdered sulfate will make the plate negative. Each unit uses positive and negative plates to create electricity.
Plate Separators – Separate the positive and negative plates, used to prevent short circuits. Made of various synthetic materials, they look like thin porous sheets. Other purposes: help to change density, hardness, and porousness between the plates.
Electrolyte – Fluid made up of 35% sulfuric acid and 65% deionized/distilled water. Known as “battery acid.”
Terminals – Made of lead, located on the top of the container. They are a connection point for the battery to the load (ex: motor, lights, etc).
Important maintenance terms:
In order for your choice to be 100% reliable for your vessel it needs regular looking after. We can all agree every part of a boats needs regular maintenance and testing and for batteries it’s vital. Below are some important terms to know about typical maintenance.
This is a common cause of shortening life in sealed AGM and FLA batteries. It happens if the charge isn’t filled and/or maintained. During an insufficient charge, the sulfuric fluid can’t return to pure electrolyte solution.
In general, as the power discharges, soft lead sulfates form on the lead plates. The opposite is true if it gets a proper recharge. In this instance the sulfate lifts off the plates and recombining it into electrolyte solution.
Left in a state of discharge for more than 3 days, allows the lead sulfate to harden and crystallize. Inside, hardened lead sulfate crystals build up on the lead plates. Resulting in a thick insulating barrier forming. In fact as the barrier thickens the cells’ ability to accept a charge or deliver energy lowers. Due to lessening the exposed surface area of the lead plates.
Leaving lead-acid in a low state of charge for a long time invites early and hard sulfation. This results in awful performance and in the end the death of your battery.
Is a special controlled overcharge of a flooded deep-cycle boat battery. This overcharge can reverse the sulfation process by dissolving hardened sulfate crystals. Dissolved sulfates return into suspension within the liquid electrolyte.
The name explains the result of the process. “Equal” means a return to balanced voltage levels and electrolyte levels. Be mindful though, as this action creates violent vibration inside the battery. Be safe and follow the manufacturer’s guide for proper voltage use. Don’t try this on sealed cell batteries because irreparable damage can happen.
This occurs when a device continues to drain power from the battery when the boat isn’t running. Medium to large vessels have complex electronics that can suck up power when it isn’t in use. For example, bilge pumps, radios, or GPS systems can operate without the engine running.
It’s a problem for lead acid batteries. This type of boat battery can’t discharge for a long time or it will begin to sulfate and die out. Therefore, ensure you find and prevent parasitic drain on the boat.
How Does Lead-Acid Create Power?
Lead-acid develops voltage (power) from a chemical reaction. This reaction occurs from the electrolyte flowing between the unlike plates. The flowing chemical reaction between the lead plates and electrolyte generates power. Next, a load, like a boat engine, will request this power. This request creates a closed circuit between the unlike terminals. A closed circuit creates an uninterrupted path of electrical power to flow to the load.
Types: Starting, Deep-Cycle & Dual-Purpose
There are two main types of lead acid batteries, based on their use, along with three chemical types. Also, there is a third, a combination model that is becoming more popular due to its convenience. The two main types are starting (cranking), and deep cycle (onboard). The third is the dual-purpose, meaning it can crank start a motor and provide deep-cycle power.
Also known as SLI (starting lights ignition), this type delivers sudden short bursts of energy to start an engine. The inside of this type uses many thin plates with a different material composition. Since the plates are thinner, they are also more fragile. It is important to know that you don’t use a starting version for deep cycle applications. The reason for this is that the thinner plates can warp and pit when discharged. As a result, expect a short life expectancy.
A boat uses a deep cycle marine-battery for onboard power. For example, to power appliances you need long slow intervals of current. This type has fewer and thicker plates, hence they can handle more life cycles. Therefore, the name deep cycle, means a lower energy delivery and a long-term life expectancy.
Don’t try to start up your boat with a deep-cycling marine battery; it might be unable to do it. Besides, this action could damage the battery and connected equipment.
A dual-purpose battery is a combination of a starting and deep cycle battery. Most experienced boaters prefer separate and dedicated batteries for different purposes. Yet, dual-purpose batteries may be the sole solution for boats with space issues. Many small boats only have room for one battery, so combination marine batteries fix this issue.
When used for small boats this type has decent performance in both cranking and onboard power. Although for large boats, this isn’t a reliable solution. Big boats should use dedicated battery banks for cranking and onboard power.
Chemical Types Explained
After picking which type of marine battery you need, it is time to choose which chemical type. There are four chemical types of boat batteries to choose. These include:
Wet Cell (flooded lead acid)
Absorbed Glass Matt (AGM)
Although all AGM, GEL, and flooded batteries are in the lead-acid category. What makes them different is the internal construction of each battery.
What Is A Conventional Wet Cell Battery?
Wet cell or flooded batteries are the oldest type of rechargeable boat battery in use today. This chemical type is popular because it is cheap to buy and maintain. A wet cell battery will contain an electrolyte liquid in an unsealed container. Whereas AGM and Gel cell batteries, aren’t open or contain liquid.
How does a wet cell work?
The wet cell boat battery uses a rechargeable lead-acid reaction to store energy. The inside of the battery has reservoirs or cells between unlike lead plates. Between the plates is an electrolyte fluid made up of distilled water and sulfuric acid. As the fluid flows through the unlike plates, it creates a chemical reaction. The battery stores this chemical reaction until a load request power.
When a load connects to the battery terminals, the circuit closes, creating power. As well as producing hydrogen (explosive!) and oxygen, that must vent out. So it makes sense having an unsealed container with vented portions. The rechargeable action is that the electrolyte fluid is refillable (water part). Therefore, as long as the battery gets maintenance recharging can happen for a long time.
Pros of a Wet cell battery:
Cost – Cheaper to buy than the other types.
No overheating – Smaller risk of damage due to overheating and overcharging, unlike the Gel cell or the AGM batteries.
Durability – Premium flooded batteries are capable of as much as 1,000 deep cycles. Hands-on maintenance and complete proper charging is necessary.
Cons of Wet cell batteries:
Unsealed – Wet cell batteries have gases venting out of them they aren’t sealed. Any non-sealed flooded batteries carry the risk of spillage. With the fluid being hazardous to skin, you need to be extra careful around these batteries. Spillage can cause both corrosion and damage to a boat.
Vents on the battery need to be open, including the battery box that holds the batteries. Unsealed battery boxes need their own maintenance. You will need to buy and carry special safety gear in the event of an onboard battery leak or spill.
Vertical orientation only – Wet Cell boat batteries have fluid inside an unsealed container. So to prevent spillage these batteries should be in a vertical position. If installed in a horizontal position the fluid will leak out and damage the boat. This can create headaches for some boat owners because space comes at a premium.
Hands on maintenance required – This type of boat battery needs water top ups. One part of electrolyte fluid is water, but don’t use ordinary water. You should only use distilled water to refill. To refill you need to choose the right time; after charging the battery to full.
As well, you need to consider the right amount of distilled water. Ensure the fluid covers the plates when filling. If you don’t, expect the battery’s performance and longevity to suffer.
Poor vibration resistance – boat batteries have to handle vibrations due to unexpected waters. Unsealed boat batteries don’t resist vibrations as much as other chemical types. As a result, battery performance isn’t as high as Gel, AGM or Lithium. This can be a letdown for boat owners.
The Gel Cell Boat Battery Explained:
A modern Gel cell boat battery uses similar principles to the flooded battery. How it is differs is the gel cell is a sealed battery. Also, it has its electrolyte fluid gelled (hence the name) with a thickening agent. Since these batteries are seal units, they are spill proof and leak proof if cracked or broken.
The gel turns the electrolyte fluid in the interior of the cells into a semi-stiff paste. This gel is a combination of sulfuric acid, phosphoric acid, fumed silica and water. It converts the chemical energy into electrical energy the same way as wet cells do. The only difference is that the lead plates are calcium plates.
Unlike wet cells, gel cells aren’t rechargeable and operate under pressure. By operating under pressure the oxygen and hydrogen produced recombines back into water. As a bonus, a Gel cell doesn’t need venting either. This means that boaters can install and operate Gel cell batteries in the bilge. If this type of battery had a hole drilled in it, it will operate, even under water! Another benefit of having Gel cells in bilges is this area is cold.
Batteries that are subject to heat over 77-degree F, will degrade (no matter the chemical type).
The Gel cell battery pro’s:
“Deep Cycle” applications – Gel cell batteries are excellent for deep cycle applications (defined in a section further in this guide). They have a life expectancy of around 500 to 5000 cycles range.
Vertical or horizontal orientation – Gel cells can operate in any orientation, although they may lose some capacity. They can even operate when a container cracks since these batteries don’t leak. This feature is important to blue water sailors who experience survival storms. During this type of violent storm, a boat could roll upside-down so battery operation is critical. For these sailors heavy-duty battery restraints are necessary.
No hands-on maintenance – For Gel cells, there is no maintenance after installation of the boat charging system. The sealed container means no electrolyte refilling is necessary. Which is great, as you don’t need special safety gear on board to protect yourself.
High vibration resistance – With a sealed container and the Gel technology, this type of boat battery is excellent in high vibrations. As well, they are spill proof.
Minimal corrosion – Unlike unsealed batteries that are at risk of spills and damage, Gel cells stand up to corrosion. You can install one close to sensitive electronics since they won’t damage or degrade like a Wet cell. A Gel cell is safe because there is less risk of sulfuric acid burns for unsealed batteries.
Cons of Gel cell batteries:
High initial cost – The gel cell battery is a specialty boat battery that costs twice as much as the best-flooded batteries. Although this cost lessens over time, because of its long-lasting deep-cycle uses. Anyways, entry costs are a factor for most buyers.
Needs special chargers – You must use the correct charger on a gel cell battery or it will lead to poor performance and early failure. This means more money to spend for the boat owner.
Overcharging sensitivity – This type of boat battery uses a special charger plus it charges at a lower voltage than other types. So, some boaters have a higher chance of high-voltage overcharging. The result is damage to the battery and is annoying to the owner.
Poor heat resistance – Hot temperatures can create adverse effects to the electrolyte. As a result, this can turn the gel hard allowing it to shrink off from the plates. Exposing the plates will shorten the life expectancy of this boat battery.
What Is An Absorbed Glass Mat (AGM) Boat Battery?
An Absorbed Glass Mat (AGM) boat battery is like a wet cell. In fact, it is the latest type of lead-acid battery.
What is different about an AGM battery?
The difference between a flooded or Gel cell is inside the plate separator. An AGM battery uses glass fibers woven into sheets as the plate separator. Next the plates and separators soak in electrolyte. The soaked plates and separator absorb and suspend the solution. The result is constant electrolyte contact with the plates at all times.
An AGM battery produces electricity the same as a wet cell. Yet, it is spill-proof and the most vibration resistant lead-acid batteries available today.
Six advantages of an AGM boat battery:
Direct replacement for flooded battery – An AGM uses near the same voltage set points as flooded batteries. Therefore, it is a direct drop-in replacement for flooded cells. Ergo it is a way to spend less money for upgrading your boat.
Vertical or horizontal orientation – Like the Gel cell, the AGM battery is a sealed battery. As a result, you can use it and store it in any orientation. This can be a great advantage in tight spaces.
Maintenance free – AGM batteries have much less terminal corrosion, compared to flooded batteries. In turn, this means less maintenance. Also like Gel cells, the container is spill proof. So with an AGM, you don’t need special safety gear onboard for hazardous acid spills.
No venting needed – An AGM battery doesn’t produce or expel hydrogen/oxygen gases. So these batteries are easy to store and you avoid having to worry about electrolyte loss.
Larger plate surface area – When compared to batteries of the same size, an AGM has a larger plate surface area. A larger plate surface area equals higher ratings. This is vital for cold cranking amps (CCA) and reserve capacity (RC) measurements.
Fast recharge – The AGM battery has a lower internal resistance than a flooded lead-acid battery. This allows for better starting power, quicker recharge and slower discharge. Thus, this battery lasts much longer than other types.
Four disadvantages of an AGM battery:
Higher cost than flooded batteries – Like Gel cells, AGM batteries have a high initial cost. However, the longevity of the battery tends to outweigh this cost for most boaters.
Overcharging Sensitivity – AGM batteries are even more sensitive to overcharging than Gel cells. A lot of damage occurs when overcharging a battery. This damage can “fry” the battery to a point of uselessness.
Capacity Decline (Gel has a performance dome) – While an AGM has a reliable lifecycle, it will lose capacity over time. Capacity is the batteries’ ability to hold a charge. Therefore, as an AGM discharges and recharges, its ability to hold future charges lessens. Gel cells are better at holding capacity over time.
Poor Heat Threshold – AGM batteries hate the heat, so don’t store or use this type of boat battery near engine bays.
Lithium-Ion Boat Batteries Explained:
Lithium (ion) batteries are a newer invention. In fact, John Goodenough and Stanley Whittingham among others helped to develop it in the 1970s to the 1980s. They have been available to buy since the 1980′s and are expensive for boating use. However, the advantages of lithium-ion and LiFePO4 (lithium-iron) are compelling. I have included some of them in my 2021 boat battery guide.
Seven advantages of a Lithium-Ion boat battery:
Usable capacity – “Lead-acid” batteries cannot compare with lithium batteries for capacity. Lithium allows for 85% rated capacity whereas lead-acid allows for about 30-50%. For example, using 100-amp hour battery. Lead-acid equals 30 to 50 amp hours of usable juice, but lithium provides 85 amp hours or more!
Long cycle life (discharge/charge cycle) – The best deep cycle lead-acid batteries provide 500 – 1500 lifecycles. However, a well-cared for lithium can provide 2000 -3000 lifecycles. The results speak for themselves.
Ultra Fast charging – Lithium batteries charge “fast” to 100% of capacity. Unlike lead acid which needs to slow down during an absorption phase to get the final 20%. Contrary to lead-acid batteries, lithium is usable without a full charge. Lithium is a great choice for boaters using solar recharging systems on cloudy days.
Built-in overcharge protection – Lithium batteries have a built-in protection system (BPS). This protects the battery from low charge, overcharging, short-circuiting and reverse polarity. This extends battery life and lowers premature replacement costs.
Lightweight – Lithium is feather-light, much more than compared to lead-acid. In fact, lithium is four times lighter! As a result, installing and replacements are much easier for lithium.
No placement problems – Like AGM or Gel, Lithium can be place in any orientation. This is useful if you have an existing power bay with limited space. Or when upgrading from the capacity of older lead acid units.
Cold temperature resistance – Lithium-ion is much more efficient at low temperatures. At -20 °C, a Lithium battery can deliver around 80% of its energy capacity, whereas an AGM will deliver only 30%. For harsh environments, lithium is a superior choice.
The current disadvantage of Lithium-ion:
Massive initial cost – The major disadvantage of lithium is the initial cost. Lithium is much more expensive than lead-acid.
Capacity Ratings Explained
The standards that most marine companies use for rating the output and capacity of a battery are CCA, AH, CA/MCA, and RC. Below are explanations of each rating.
Cold cranking amps (CCA)
Defined as a measurement of amps a battery delivers at 0 degrees Fahrenheit (-17.78 °C) for 30 seconds. Without dropping below 7.2 volts. High CCA ratings are crucial for a starting battery as well as cold weather applications. However, for deep-cycle batteries, don’t expect to see this rating. Which is funny because this is the most common battery measurement.
Amp Hour (AH)
This is a rating which measures the function standards for deep-cycle batteries. The standard rating is an amp rating taken for 20 hours.
For example, using a 100 AH rated battery, connect and draw from the battery for 20 hours. This will discharge 5 amps per hour through the 20-hour cycle.
In a math equation, it looks like this:
5 (amp per hour) X 20 (hours) = 100 total amp hours
Simple right? Nope, it’s important to understand that total amp hours and load type don’t match in every instance. So, you can’t use a 100 amp rated load on a 100 AH battery and expect 20 hours. To reduce problems you need to account for load type when sizing this type of battery.
Cranking Amps (CA) or Marine Cranking Amps (MCA)
This is a rating of the amount of amps produced while starting the engine at 32 degrees Fahrenheit. The greater MCA rating the better. Look for an equal or greater MCA to what the manufacturer recommends for your boat’s engine.
Hot cranking amps (HCA) is an old rating system that you likely won’t see any longer. However, if you do see this rating it measures the amps produced at 80 °F.
Reserve Capacity (RC)
An important time measurement rating for some boat batteries. It is a valuation standard for deep-cycle and dual-purpose batteries. It shows in minutes the time a full charged battery at 80 °F will discharge 25 amps of current. This time stops when the battery drops below 10.5 volts.
The higher the RC rating is, the longer the battery will keep the boat powered up before dying. An average battery will have a 90-minute RC. Although good, it is in your best interests to choose the highest RC rating you can afford.
How To Choose A Boat Battery: 12 Considerations
Now that you understand the basics about a boat battery, it is time to choose what is best for your boat. Below are some basic things to consider before buying.
Choose what application your boat needs. Remember that “Starting” batteries deliver short bursts of juice to power the starter. Under no circumstances should you discharge these types to empty. Whereas “Deep-cycle”, batteries tolerate deep discharges to power accessories. They can discharge and recharge many times before dying. Then there’s “dual-purpose” batteries which combine some qualities of both. This type is a compromise of each.
Next, choose which type of chemical technology you want. One choice is an old standby, the Wet cell battery. It provides reasonable performance at a low cost. Yet, as an unsealed battery, it can spill hazardous acid. As well, it needs hands on maintenance in the form of checking and filling the electrolyte (aka “acid”).
You could upgrade to sealed battery, the gel cell is more money, but spill proof. Alternatively, for even more money try the “Absorbed glass mat” (AGM) batteries. These offer a slow self-discharge rate and more power in a smaller footprint.
If initial cost is not a worry, try a lithium battery. These are superior to all previous types, but expensive.
For starting batteries, you want to pay attention to the “Cold cranking amps” (CCA). This is the number of deliverable amps at 0 degrees F for 30 seconds. Or look at the “Marine cranking amps” (MCA), which is the same as (CCA) but determined at 32 degrees F.
Next for deep-cycle and dual-purpose batteries, consider the “Ampere-hour” (Ah) rating. A measurement based on a 20-hours long draw of a full charged battery.
Another important consideration for battery selection is “Reserve Capacity” (RC). This is the number of minutes a full charged battery at 80 degrees F will discharge 25 amps. Until it drops below 10.5 volts and “dies.”
Batteries come with volts; most boat batteries are either 12-volts or 24-volts. The amount of volts will depend on the size of the boat and the type of equipment that needs power. Larger boats use 24-volt batteries because this type will cause less of a voltage drop on long wire runs. Pay close attention to available space on your boat. In many instances you can have a 24-volt system with ‘droppers’, instead of many 12-volt batteries. These ‘droppers’ lower the voltage to supply 12-volts for certain electronics (VHF, or GPS, etc.).
A “group” (example: 24, 27, 31, 34, 6D, 8D, etc.) refers to a battery’s physical size, all manufacturers build to group sizes. Despite some having differences in dimensions, due to handles or post height. Match the group size on the label to the physical space on your boat. Finally, keep in mind about if the battery requires a ventilation system.
Battery labels state the date of manufacture, the label denotes the month and year. Months are letters (A-Jan., B-Feb. C-Mar., etc.) and the year by numbers (ex: 1= 2011, 2=2012, etc.). For example, “C1” denotes a battery manufactured in March 2011. Its in your best interest to buy the newest built battery.
Weight is an important factor for boater who intend to move the battery in and out of the boat every season. It makes no sense to choose a battery that pose an injury risk when handling. Choose the battery you are comfortable carrying.
Boats operate on motors so the battery feels constant and violent vibrations. Vibration is a common cause of damage to the internal parts of most boat batteries. Vibration can cause the inner cells to dislocate which deteriorates the battery. Thus, vibration resistance batteries will last longer than the standard ones.
Before buying the battery, check the position of your battery compartment. Certain chemical types like Wet cells need to be vertical, since it’s at risk of acid spilling. Whereas, the Gel cell, AGM, and Lithium work in any position. Except upside down, as it might damage the internal parts of the battery. For battery compartments that accept positions other than horizontal it’s best to choose a sealed battery.
Flooded marine batteries need hands on maintenance. This includes checks for spills or terminal corrosion and refilling of distilled water. Many casual boaters find this type of maintenance task a nuisance. Sealed batteries (Gel, AGM, and lithium) only need external checks and cleaning. So consider how much maintenance appeals to you.
All the boat batteries offer some kind of warranty period. Some batteries come with full-rated warranty period and some come with pro-rated period. In full-rated warranty, free replacement is an option if it fails before the warranty ends. Whereas for a pro-rated, you will only get some amount refunded for the unused period of warranty. Be mindful when you review any warranty.
Many batteries are available at a wide range of budgets. If you have a budget in mind, go for the best battery that is affordable to you. Lower budget ones can be as powerful as higher budget ones but will need more maintenance.
Cautions About Life Expectancy:
Most average battery life expectancies are shorter these days due to increased energy demands onboard. Here’s a sad fact; only 27% of batteries sold today reach the 48-month mark. What’s worse is that sulfation build-up is the reason for 80% of all battery failures. There’re many causes of sulfation, such as:
Batteries sit too long between charges will sulfate. As little as 24 hours in hot weather and several days in cooler weather.
Improper storage without some type of charger connected to it.
Over discharging an engine-starting battery. These types of batteries cannot stand deep discharge.
Undercharging of a battery to only 90% of capacity. Sulfation deposits don’t break down, so they increase in size, damaging the lead plates.
Exposing a battery to high external temperatures (100+ degrees) for long periods. Doing this will increase the natural internal discharge. A full charged battery left sitting for 24 hours a day at 105 °F for a month will result in a dead battery.
Low electrolyte level. Flooded batteries need refilling of electrolyte. Any lead plates exposed to oxygen will start to sulfate.
Incorrect charging levels and settings. Most cheap battery chargers do more harm than good.
Cold weather is hard on the battery. The chemistry will not create the same amount of energy as a warmer battery. A near empty battery can freeze solid in subzero temperatures.
Parasitic drain, this is a load put on a battery with the power off. Ongoing parasitic draining makes for poor battery performance and short life expectancy.
As you can read buying the right boat battery for you depends on many factors. It’s my goal for this 2021 guide of “how to choose a boat battery” to be the ultimate boat battery guide. Gaining this knowledge will help you to make confident and informed buying decisions.
Thank you for allowing Outdoors Informed to help you in your research so you can spend more time “energized” on the water.