What Size Battery Do You Need For Your Boat: 14 Tips to Make the Best Decision

A marine battery is a critical piece of equipment for any boat. It powers every electrical device in a boat, from GPS navigation to trolling motor. Therefore, choosing the right marine battery for your boat is vital.

However, deciding ‘what size battery do I need for my boat?’ can be tough. There are several different sizes of battery options available in the market. Additionally, every size comes in a variety of battery chemistries. You need to evaluate what you need before you make your choice.

This article will help in easy decision-making for a marine battery. You will know about the factors that need priority when buying a marine battery.

What is a Marine Battery?

What is a Marine Battery?

A marine battery is a deep-cycle battery installed in a boat. It powers the various electrical appliances and devices in a boat. For instance, gadgets like GPS navigation, oven, refrigerator, boat lights, etc., use these batteries. Heavy boat equipment like a trolling motor and sterndrive motor also use a marine battery.

A marine battery is different from a boat starting battery. A boat cranking battery provides a short burst of current to start the boat batteries. On the other hand, the marine battery is designed to provide current for long periods of time. These two types of batteries should not be used interchangeably.

Does Marine Battery Size Matter?

Yes, marine battery size is very important. Every boat is compatible with only a particular marine battery size. Choosing the wrong battery can cause critical damage to the boat appliances, the battery, and even the boat itself.

How Does a Battery Size Affect a Boat’s Performance?

The boat battery size will have a deep influence on its performance. The engine alternator provides the power to the battery. Therefore, choosing an improper battery size can strain the engine significantly. The engine alternator pushes the engine more to provide power to the battery. This results in higher gas consumption and slows down the overall performance of the boat.

What happens if you use the wrong size battery?

A wrong-size battery can be detrimental to every component of the boat. Oversized or undersized batteries can stress the boat electrical system. This results in appliance failure and irreversible damage. Choosing the wrong size can also damage the battery itself. Damages to the battery result in fluid spills, sparks, overheating, and property damage. In worst cases, it can even lead to fire hazards.

How to Choose the Right Marine Battery Size?

How to Choose the Right Marine Battery Size?

Having the right size battery requires finding some important specifications for your boat. Here are the factors that you should evaluate:

Battery Power

Battery power size is the first criterion that you should evaluate when deciding on the best battery for your boat. The battery power should, at the least, meet the minimum power requirements of the boat. After that, the battery power can go over it based on the duration for which you want the boat appliances to run.

How to Estimate Your Boat’s Power Needs?

Evaluating your boat’s power requirements is easy. Follow the steps below to accomplish this.

  1. Start by making a checklist of all the equipment on your boat. This should include devices like lights, bilge pumps, trolling motors, and other electrical gadgets.
  2. List the electric consumption of each electrical gadget. For convenience and calculation, this value should be listed in amps. The boat’s manufacturer manual usually comes with these values. If you cannot find them, you can use a multimeter to measure them.
  3. List the running time for each electrical device. The running time is based on how long you will require to run each device during one charge cycle. For instance, if you want to run a light for six hours a day and intend to charge the battery once a day, the running time for the light would be 6 hours.
  4. Determine the Amp hour rating of each electrical device. Multiply the Ampere consumption and running duration to achieve this. For instance, if a light bulb runs for 6 hours a day and consumes 1 A will have a power draw of 1 Ah.
  5. Add the Amp-hours rating of each electrical device to find the total power draw for the boat.
  6. There are certain losses in each electrical system. Additionally, you need to account for some higher electrical requirements at times. Therefore, add a 20% to 30% margin to the total electrical consumption.
  7. Now you can know the Amp hour battery rating that will be sufficient for your requirement. You can buy the battery with this Ah rating. Since the batteries in the market are listed by their Amp-hours, no further calculations are required.


An alternative method to measure the total wattage of the load. This is useful when you know the watt rating of an appliance. The wattage divided by the voltage gives the Amp-hours rating of the boat electronics.

For instance, suppose you have a 20 W light, a 10 W navigation system, an autopilot system of 40 W, radar of 10 W, and navigation lights of 40 W, and a watermaker of 30 W, the total wattage becomes:

Wattage = 20 + 10 +40 + 10 + 40 + 30 = 150 W

If you intend to use it for six hours on a single charge,

Watt hours = 150 Watts x 6 hours = 900 Wh

Now dividing it by the voltage will give the Ah rating. For a 12V boat system,

Ah = 900 ÷ 12 = 75 Ah

Battery Chemistry

Battery chemistry is an important factor when it comes to battery size. It might seem that batteries of the same power will provide the same output regardless of the chemistry. However, chemistry can affect the output significantly.

The important parameter to focus on here is the Depth of Discharge (DoD). A battery with 50% DoD will only provide half of the stored power at any time. A battery with an 80% depth of discharge will provide almost all of its power.

Let us study the different battery chemistry one by one:

Flooded Lead Acid Batteries

Flooded lead acid batteries are the most primitive solution. They contain two heavy lead plates dipped in an electrolyte solution. The internal reactions of the battery turn the electrolyte to gas slowly. This results in the requirement of frequent electrolyte top-ups. The Depth of Discharge of these batteries is only around 40% to 50%. Therefore, for a requirement of 50 Ah, you will need 100 Ah flooded batteries.

Absorbed Glass Mat Batteries

Absorbed Glass Mat batteries have a thin glass mat that absorbs the electrolyte solution. These are sealed batteries, so you don’t have to top-up the electrolyte. However, their lifespan is not as good as lithium batteries. They have a Depth of Discharge of 60% to 80%. These batteries are quite heavy.

Gel Batteries

Gel batteries contain a silica-based electrolyte gel. These batteries, like AGM batteries, do not need electrolyte top-up. However, they require a valve for the release of built-up gases. Gel batteries have a DoD of around 70%. They have a high cost and low value for money.

Lithium Batteries

A lithium battery is a great improvement over the other options. Lithium batteries have a lithium-based electrode and a graphite electrode. These batteries have a high cycle life and a better depth of discharge than lead acid batteries. The high options for lithium-based electrode has led to the emergence of different types of lithium batteries.

Physical Size

The battery’s dimensions are important to ensure that they will fit into your battery compartment. Battery dimensions are divided into various classes for buying convenience. You can buy a new one of the same class when replacing current batteries.

What is a Standard Size Marine Battery?

There are four standard marine battery sizes- Group 24, Group 27, Group 31, and Group 8D. Each of these standard group sizes has various sub-groups.

Here is a size chart for different marine battery classes:

Group Size (inches)
24 10.25 x 6.81 x 8.88
24F 10.75 x 6.81 x 8.88
24H 10.25 x 6.81 x 9.38
24R 10.25 x 6.81 x 9
24T 10.25 x 6.81 x 9.75
27 12.06 x 6.81 x 8.88
27F 12.5 x 6.81 x 8.94
27H 11.75 x 6.81 x 9.25
31 13 x 6.72 x 9.44
8D 20.75 x 11.13 x 9.88

Battery Weight

The weight of the battery is important for marine applications. Every pound of weight is important on the boat. The weight mainly depends on the battery chemistry. Let us study the weights of different types of marine batteries:

  • Flooded Batteries: Flooded batteries weigh 60 lbs (27 kg) for smaller group sizes 24 and group size 31; up to 130 lbs (59 kg) for larger group sizes.
  • AGM Batteries: AGM batteries weigh 50 lbs (23 kg) for smaller group sizes; up to 85 lbs (39 kg) for larger sizes.
  • Gel Batteries: Gel batteries weigh 70 lbs (32 kg) for smaller group sizes; up to 120 lbs (55 kg) for larger group sizes.
  • Lithium Batteries: Lithium batteries weigh 30 lbs (14 kg) for smaller group sizes; up to 80 lbs (36 kg) for larger group sizes.

Battery Voltage

Battery Voltage

The battery voltage should be compatible with the boat’s electrical system. Generally, boat electricals have a 6 V, 12 V, or 24 V system. Therefore, your battery will fall into one of these voltage ratings. The 12 V battery voltage is the most common among these.

Charging System

Marine batteries are charged with the engine alternator. Therefore, the battery you buy should be compatible with engine alternator charging. Boats have an inbuilt charger as well.

Cold Cranking Amps

Cold Cranking Amps (CCA) is the amount of current that a battery can provide at 0 F (-18 C) for 30 seconds. CCA rating reflects the engine starting ability of the battery in cold conditions. You need to check with the boat’s manufacturer manual to know the CCA rating requirement of the engine.

Does CCA Rating Matter For a Marine Battery?

Yes, CCA rating is quite crucial for a marine battery. It is common for boats to travel in cold conditions at the sea. A battery with an insufficient CCA rating will lead to engine starting issues. This can be quite troublesome when the boat is in operation far away from any available help.

Reserve Capacity

Reserve capacity is the power that deep cycle batteries can provide when the engine is switched off. A higher reserve capacity will mean that a battery can provide greater power. The battery’s reserve capacity should be enough to meet your requirements when the boat is docked or the engine is not in operation.

Charging Time

Charging Time

Charging time is important for users who frequently use their boats. A shorter charging time is preferable if you do not have a long waiting period between trips.

Type of Motor

Different types of motors are present in a boat. Each motor has its own specific requirements for the marine battery.

Trolling Motor

The battery requirement for a trolling motor varies depending on the thrust produced. Trolling motors with a thrust up to 55 lbs require one battery of 12 V. Thrusts between 55 lbs to 80 lbs require 2 12 V batteries. E-drive motors can require 4 12V batteries.

Additionally, for boat batteries with trolling motors, a lightweight option is preferred. Lithium batteries are the best solution in this regard.

Outboard Motor

The outboard motor battery is usually 12 V with a minimum rating of 110 Ah. For the outboard motor, the user should focus on the Cold Cranking Amps (CCA) rating of the battery. This will allow the engine to start in cold conditions.

Additionally, outboard motors are prone to high vibration. Therefore, a vibration-resistant battery will provide better performance and a longer lifespan.

Inboard Motor

Inboard motors also require a high CCA rating similar to outboard motors. Usually, the CCA rating of battery for inboard motor should be between 400 to 500 A. It is better to go above 500 A than below it.

Additionally, inboard motors should have a lower maintenance requirement due to the location of the motor. Lithium batteries again tend to be the better choice for this purpose. Traditional lead acid batteries are not a good choice for indoor motors.

Sterndrive Motor

Sterndrive is also known as an inboard/outboard drive. The battery for the sterndrive motor should be compatible with the current boat electrical system. Additionally, a starting battery might be required with a deep-cycle battery option. You may prefer dual purpose batteries that can start and run the motors.

Jet Drive Motor

Jet drive motors need a battery that can provide bursts of current. Therefore, rapid discharge battery types fit better for this application. The battery should be completely spill proof. This eliminates using flooded lead acid marine batteries.

Jet drive motors require a very rugged battery. Choose a branded option instead of a cheaper alternative.

Solar PV System

Many boats have a Solar Photovoltaic (PV) system as a secondary power source. This system provides electricity to the boat’s electronics. It also provides trickle charging to the battery. If your boat has a solar PV system, choose a battery compatible with this system.



Budget is an important parameter for every user. There are always bigger and better options when it comes to marine batteries. Find a price range so you can limit the maximum capacity of the battery you will buy.

In-built Heater

Advanced batteries like those provided by Eco Tree Lithium come with an in-built heater. This heater warms the battery in cold conditions. It eliminates the problem of cold engine starting. It also improves battery performance significantly.

Reputed Brand

The brand reputation is important when it comes to marine battery. Branded batteries can cost fractionally more, but last for many more years.

For instance, Eco Tree Lithium batteries easily last for over 10 years. In fact, the brand provides a warranty period of 6 years. This is more than any other alternative you will find in the market. The longer warranty also reflects the brand’s faith in its products.


The right battery can lead to a pleasant experience on your boat. On the other hand, choosing the wrong battery size can leave you stranded on the water in the middle of nowhere. Therefore, when you need to choose a marine battery, refer to the information provided above. It will help you make the best choice when it comes to marine deep-cycle batteries.

Frequently Asked Questions (FAQs)

Here are the answers to some common questions regarding marine deep cycle batteries:

1. Can you mix battery sizes on the boat?

Yes, you can mix battery sizes on a boat, but it’s recommended to use batteries of the same type, size, and age for optimal performance. When using multiple batteries, ensure that the batteries have the same voltage. Additionally, batteries with the same capacity will provide better charge and discharge performance.

2. What is a standard-size marine battery?

12 V is the standard marine battery size. Some boats also have the 6 V and 24 V requirements. 6 V is very rare and 24 V deep cycle battery is used in larger boats.

3. What is good CCA for a marine battery?

A CCA rating of more than 500 A is generally considered good for a marine battery. Choose a higher CCA rating instead of a lower option for better cold-starting performance.

4. How does the boat’s Amp hour rating affect the battery size choice?

The Amp hours (Ah) rating describes the amount of current (Amps) the battery can provide for one hour, or the number of hours the battery can provide 1 A current. It provides details on the amount of electrical load you can power and the duration for which you can power it. Higher Ah ratings are suitable for boats with more demanding electrical systems or longer periods of power usage, while lower Ah ratings may suffice for smaller boats with fewer power needs.

5. What role does a boat’s engine size play in choosing the right battery?

Bigger engines produce more thrust, so they require a bigger battery. Additionally, the engines also require more cranking amps. For bigger engines, choose a battery with a higher CCA rating.

6. How can I optimize my boat’s electrical system to reduce the required battery size while still meeting my power needs?

Use energy-efficient electrical devices to optimize the boat’s electrical system for a smaller battery. Additionally, reducing the load and limiting the number of appliances can help in this as well. Another option is to have better time management for the boat electricals. Following proper battery maintenance can also reduce the battery capacity you require by prolonging its discharge capacity.

7. How does the duration of boating trips affect the choice of boat battery size?

Longer boating trips require a deep cycle battery with a higher Amp-hour capacity. A higher reserve capacity is also important in this situation. If there is less time period between the boat trips, choosing a battery with a short charging time is useful.