How to Choose A Boat Battery (2020 Guide)

Green and white outline diagram of a boat battery
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With many different 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, Outdoors Informed made this comprehensive guide.

In this article you will learn everything you need to know to choose a boat battery in 2020. If you want to skip to a specific topic in this article there is a clickable table of contents. Or do what I recommend and read the whole article.

We updated this article in January 2020 to ensure the information in this guide is current. Read on to learn about how to choose a boat battery in 2020

What Type Of Battery Is a Marine Battery?

Boat batteries are lead-acid batteries and are different from a car battery. Invented in 1859 by French physicist Gaston Planté. The lead-acid battery is first type of rechargeable battery. A marine battery has three vital differences from an automotive battery.

The first difference is battery use. Unlike cars, most boats need two types of batteries to handle two different but simple tasks. One type is for starting an engine and the other is for operating long term electrical loads, like lights.

A second difference, is that boat specific batteries are available in four chemical types. The first is the traditional unsealed Wet Cell. Next are a valve-regulated lead-acid battery (VRLA battery), either a Gel Cell, Absorbed Glass Mat (AGM). For boaters who feel cost is not a factor there is Lithium.

Third, they’re rated by energy output (ampere hours) and recharging cycles. The amount of output and lifespan ratings determines the price.

Which battery you choose will depend on use, chemical type and number of batteries needed.

The Parts Of A Lead-Acid Boat Battery:

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 battery uses positive and negative plates to create electricity.

Plate Separators – Separate the positive and negative plates, preventing 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.

Battery Terminals – Made of lead, located on the top of the battery. They are a connection point for the battery to the load (ex: motor, lights, etc).

Diagram showing the internal parts of a lead acid battery
Outdoors Informed - 12V Lead Acid Battery Parts

Important marine battery maintenance terms:

In order for your marine battery to be 100% reliable for your vessel they need a 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 battery maintenance.


This is a common cause of shortening battery life in sealed AGM and flooded boat batteries. It happens if a lead-acid battery doesn’t charge up to full. During this insufficient charge, the sulfuric fluid cannot return to pure electrolyte solution.

In general, as a battery discharges, soft lead sulfates form on the lead plates. When a battery gets a proper recharge, this sulfate lifts off the plates and recombining it into electrolyte solution.

A battery left in a state of discharge for more than 3 days, will allow the lead sulfate to harden and crystallize. Inside the battery, hardened lead sulfate crystals build up on the lead plates. Resulting in a thick insulating barrier will forming. In fact as the barrier thickens the battery’s ability to accept a charge or deliver energy lowers. Due to lessening the exposed surface area of the lead plates.

Leaving lead-acid batteries in a low state of charge for a long time invites early and hard sulfation. This results in 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 AGM or Gel cell batteries because irreparable damage can happen.

Parasitic drain:

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 sulfating and die out. Therefore, it is important to find and prevent parasitic drain on your boat.

Closeup photo of the cockpit of a power boat

How Does A Lead-Acid Marine Battery Work?

A lead-acid battery 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 battery terminals. A closed circuit creates an uninterrupted path of electrical power to flow to the load.

Types Of Boat Batteries

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 battery that is becoming more popular due to its convenience. The two main types are starting (cranking), and deep cycle (onboard).

Diagram of a speed boat with arrows showing three boat battery types
Outdoors Informed - 2020 Types Of Boat Batteries

Starting battery:

Also known as SLI (starting lights ignition), this battery 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 battery 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 battery for onboard power. For example, to power appliances a battery needs 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:

  1. Wet Cell (flooded)
  2. Gel cell
  3. Absorbed Glass Matt (AGM)
  4. Lithium

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 boat battery work?​

Blue diagram of a battery with a pro and three cons of Wet Cell
Outdoors Informed - Advantages & Disadvantages Of Wet Cell

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 sulphuric 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 formulation of today’s Gel cells is from Otto Jache’s and Heinz Schroeder’s U.S. Patent 4,414,302.

What is different inside a Gel cell battery?

Green diagram of a battery with pros and cons of Gel cell
Outdoors Informed - Advantages & Disadvantages Of Gel Cell

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, the 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.

When batteries are subject to heat over 77-degree F they 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 spill 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?

Orange diagram of a battery with pro and cons of AGM
Outdoors Informed - Advantages & Disadvantages Of AGM

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 are compelling. We have included some of them in our 2020 boat battery guide.

Yellow diagram of a battery with pro"s and cons of Lithium
Outdoors Informed - Advantages & Disadvantages of Lithium-Ion

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 speaks for itself.

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 batteries are feather light, much more than compared to lead-acid. In fact, lithium is four times lighter! As a result, installing and replacement 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 battery bay with limited space. Or when upgrading from the capacity of older lead acid batteries.

Cold temperature resistance – Lithium-ion batteries are much more efficient at low temperatures. At -20°C, a Lithium battery can deliver around 80% of its energy capacity, whereas an AGM battery will deliver only 30%. For harsh environments, Lithium is a superior choice.

The disadvantage of a Lithium-ion battery:

Massive initial cost – The major disadvantage of lithium is the initial cost. Lithium is much more expensive than lead-acid.

How Are Boat Batteries Rated?

The standards that most battery companies use to rate the output and capacity of a battery are CCA, AH, CA/MCA, and RC. Below are explanations of each rating.

Closeup photo of a black boat battery with a white ratings label
Labels help to inform you if the battery is right for the job.

Cold cranking amps (CCA)

Defined as a measurement of amps a battery delivers at 0 degrees Fahrenheit 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 is 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.

Closeup photo of the view from a speeding boat on a lake

How To Test Boat Battery Charge:

Battery testing is an integral part of any periodic maintenance routine. For best battery life, perform testing regardless if a starting problem has occurred. Many times little warning occurs before battery failure.

Before testing a battery, it is important to charge the battery to full. Notwithstanding, a small discharge from full can give a false reading. You can test the charge of a battery in more than one way. The first thing to do is a physical check of the battery for damage.

Physical battery tests:

Broken or loose terminals – These are dangerous because they can cause a short circuit. You can identify short circuits by finding burnt or melting spots. A short circuit happens when a burst of the power unloads in an instant. This produces extreme heat and even causes explosions.

Damaged container – If there is a bulge in the case, this indicates a possible overcharging the battery. Or it could be from poor handling. If the container has cracks, splits, and holes the battery can operate. But for safety reasons, label it unsafe and stop using the battery.

(Flooded batteries only)

Low Electrolyte  – With flooded batteries you will need to maintain the water levels inside. When low, you can refill the battery with distilled water. Leaving a battery low or dry for an extended period causes a problem. Low fluid levels will expose the lead plates to oxygen. This allows for sulfation build up. This is the number one cause of early battery failure. The other issue is that dry charging a battery will cause it to overheat and burn it up.

Discolored Electrolyte – If there are plenty of fluid in the cells, but the fluid color is dark brown this indicates a poor battery. What if a just single cell is dark brown, would that be a problem? Yes, it does matter; even one discolored cell hurts the entire battery. The battery is useless because the discoloration can flow from cell to cell.

Battery charge tests:

Next, you will need to obtain the state of the charge in the battery. The most accurate method is a measuring electrolyte specific gravity and battery voltage. Below is an explanation of what to look for and how to complete charge tests.

How Do I Measure Specific gravity (acidic concentration)?

Diagram of a chart which shows specific gravity and state of discharge of boat batteries

To measure specific gravity, use a temperature compensating hydrometer. This tool measures the amount of sulfuric acid in the electrolyte. If a reading is low, this means the battery is suffering from sulfation. Remember sulfation is a byproduct of when a battery discharges. However, if a battery doesn’t recharge to full or you leave it near empty for a long time, problems arise. Hard sulfur crystals will accumulate on the lead plates. Excess sulfation leads to battery death. Keep in mind you cannot use a hydrometer on sealed batteries, only flooded batteries.

Follow the directions below to measure specific gravity in a flooded battery. Remember don’t add water during this test and wear protective safety gear when near electrolyte fluid.

  1. Pull off vent caps to battery and remove top seal.
  2. Fill and drain the hydrometer 3 times before pulling out a sample.
  3. Pull a sample from one cell. Remember to ensure the amount of electrolyte in the hydrometer supports the float.
  4. Record the reading and return the electrolyte back into the cell*.
  5. Proceed to check every cell in the battery by repeating the three steps above.
  6. Replace the vent caps and top seal. Next wipe off any spilled electrolyte.
  7. Compare the recorded readings to the table below to measure the state of charge in the battery.

* Readings should not vary more than .05 differences between cells.

How do I test the voltage on a marine battery?

To measure voltage, use a digital D.C. Voltmeter. A quality load tester is a good investment for testing sealed batteries. To test follow the directions below.

  1. Attach the voltmeter’s leads to cables. One is black (negative) and the other red (positive).
  2. Set the dial on the voltmeter on V to measure volts.
  3. Touch the black lead to the negative (-) terminal on the battery. Next, touch the red lead to the positive (+) terminal on the battery.
  4. Read the voltage on the screen of the voltmeter. Compare it to the table below.
Multi-colored diagram of a chart showing voltage range of a boat battery

Methods for load testing (capacity) a boat battery

To load test a boat battery you can choose from two methods depending on your battery. One tests measures the starting or crank capacity of a battery. The other tests the capacity of deep-cycle batteries.

Adjustable Load Test

Use an adjustable load tester to test for the starting capacity of a battery. This tests for a real load too, for example, when cranking the engine. Be careful not to test on loose or corroded terminals, as sparking risks can occur.

The tester will load the battery to fifty percent of its cold cranking amperes (CCA) for 15 seconds. As long as voltage readings are above 9.6V*, the battery is okay. If the result is under 9.6V, the battery is not good and may not start the engine.

*Check the individual manufacturer’s specifications for smallest acceptable CCA rating.

Constant Rate Discharge Test

This test uses a constant rate discharge tester. It checks the amp hour capacity of a battery for deep-cycle batteries.

The test works by discharging the battery at a preset current (Amps). It finishes when the amps drop to a preset disconnect voltage. Most boaters consider this test a nuisance due to the time it takes to perform the test.

For example:

You test a 100 Ah (Ampere Hour) battery with a preset of 5 Amps, it could be 20 hours to complete the test!

How To Charge A Boat Battery:

Remember; when it comes to batteries, you have to return any energy you use in an instant. If you don’t the consequence is sulfating which degrading the battery. Some times you can use the boat engine’s alternator as a battery charger. An unfortunate consequence of doing this is a slow and incomplete charge. Poor charging develops sulfate deposit build-up, excessive gassing and reduces battery life. In fact, if you do recharge a starting battery with an alternator, you may only get 10 re-chargeable life cycles.

Therefore, it is in your best interest to recharge batteries using a multi-step method. Which is critical if you are prone to discharging battery to low or near empty. This method uses a smart charger. It has three steps of regulated charges, known as multi-stage charging. An explanation of the three steps are below.

Bulk charging stage:

Diagram depicting eighty percent charge for a boat battery

During this stage, the charger will use its max voltage and current amps. The purpose of this stage is to replace up to 80% capacity of a near empty battery.

NOTE: One reoccurring term mentioned in recharging is a battery’s natural absorption rate. Defined as the amount of charge applied that will not overheat the battery. Although, during the bulk stage the battery is near empty, so there is no risk of overcharging.

For a typical 12-volt lead acid battery, the charging voltage going into a battery will reach 14.1-14.4 volts. For 24-volt systems, double these figures. The bulk charge voltage for flooded lead-acid batteries should be about 14.4 volts.

For AGM batteries about 14.2 Volts and gel cell batteries about 14.1 volts. If charged to a lesser voltage, sulfates that form during discharge will remain on the plates. Over time, sulfates cause a 250 amp-hour battery to perform more like a 125 amp-hour battery.

Absorption Stage:

Diagram depicting an eighteen percent bulk charge for a boat battery

When the battery reaches 80% of capacity, the charger changes to the absorption stage. This stage is to allow the battery to absorb the final 18 percent of its charge at its natural absorption rate. It uses its natural absorption rate to prevent undue gassing or overheating. To achieve this, the voltage holds steady while the current (amps) begin to decline. The incoming current declines as the battery comes to near full capacity. The nuisance of this stage is that it takes more time to finish.

Float Stage:

Diagram depicting an two percent final trickle charge for a boat battery

The final stage takes place when the battery reaches around 98% full charge. Although some chargers enter float mode as early as 85% state of charge. Either way, in this stage the charger will bring the battery to 100% charged or close to it. This stage will lower the voltage to 13.2-13.4 volts and decrease the amps to a point or a “trickle”. Which explains the term “trickle charger.” Since it is a trickle, the float stage does not boil or overheat the battery.

A float stage is when there is charge going into the battery at all times. While at the same time charging at a safe rate to ensure a full state of charge and nothing more. One purpose of this stage is to maintain the battery’s readiness. Another is to aid in preventing cycling issues from long-term inactivity. It is common to leave a battery in float mode for months to even years at a time.

Note: Some Gel cell and AGM batteries need special settings or chargers for this type of charging. Remember to check your battery manufacturer’s charging specs before buying your chargers.

How Do I Maintain My Boat Battery?

Proper maintenance of a boat battery is important for its service life. How much maintenance you do will depend the battery you choose. Ranging from physical checks on sealed batteries to full maintenance for flooded batteries. Consider each of the points below on a regular basis to extend the life of your boat batteries.

Physical maintenance:

  • Clean the battery case at regular intervals. Use a baking soda and water solution. Combine two of tablespoons of baking soda to a pint of water. Wipe down the entire battery, and let it dry.
  • Next clean all cables and tighten any loose cable connections. Boaters tend to have problems arise from dirty and loose connections.
  • Do your best to try to prevent cable connection corrosion on “top-post” batteries. An unprotected metal post connection corrodes from contact with acidic vented battery gases. Protect the post by applying a small bead of silicone sealer around the base of the post. After place a “coated” felt battery washer over the post and tighten it. To coat the washer, use high temperature grease or (Vaseline) petroleum jelly.
  • For flooded batteries, test and check fluids, refill as needed. Only use distilled or deionized water to refill. This type of water is best because it doesn’t have contaminates which damage the cells. Make sure not to fill with sulfuric acid!
  • Do not overfill battery cells. This is important in warmer climates, since heated weather creates thermal expansion. Thermal expansion can push excess electrolytes out of the battery.

Sealed Maintenance Free (SMF)

  • If using this special type of boat battery, check the State of Charge Indicator. This window on the battery will give you a snap shot of the battery’s condition. It lets you know if the battery needs charging or replacement. If the State of Charge Indicator advises ‘Replace Battery’, do it as soon as possible. Not remedying this leads to irreversible sulfating or even to an internal explosion.

Battery storage:

If your boat batteries are seasonal or you store them long term. It is important that you make sure to recharge the battery to full before storing. If using a “SMF” battery, do regular checks of the state of charge or voltage. If the voltage drops below 12.5V, recharge the battery. Remember to check your battery charge before reconnecting to electrical loads.

Photo of two women and two childern sitting on rear of a boat in water

12 Factors For Selecting The Right Boat Battery For You:

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. Do not discharge these types to empty. “Deep-cycle”, batteries tolerate deep discharges to power accessories. They can discharge and recharge many times before dying. While “dual-purpose” batteries combine the qualities of both. This type is a compromise of each.

Chemical technology

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 ** batteries and dual-purpose batteries, consider the “Ampere-hour” (Ah) rating. A measurement based on a 20-hour 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.).


Diagram of battery showing group sizes, voltage and dimensions

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.

Date codes

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 with carrying.

Vibration resistance

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.

Installation position

Before buying the battery, check the position of your battery compartment. Certain chemical types like Wet cells need to be vertical, since it is 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. If the battery compartment can 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 these maintenance tasks 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.

The Cautions About Battery Life Expectancy:

The average battery life expectancy has become shorter as energy demands have increased. Here’s a sad fact; only 27% of batteries sold today reach the 48-month mark. What is worse is that sulfation build-up is the reason for 80% of all battery failures. There are 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.

Final Words

As you can read buying the right boat battery for you depends on many factors. It is our goal for our 2020 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 on the water.

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