The marine battery size is one of the key parameters that defines your journey on the water. Choosing a too-small marine battery will not be able to meet your power needs. On the other hand, investing in a too-big battery will waste your money. You may also find it difficult to fit it in the battery compartment.
The question that arises here is, "What does the size of the battery really mean, and how to choose the right fit for your boat?" This article talks about different marine battery sizes and their impacts. It also focuses on how group sizes work and how you can fulfil your energy requirements while travelling on the water.
What Does Marine Battery Size Mean?
The size of a marine battery refers to the Battery Council International (BCI) group size that defines three factors, including physical dimensions, runtime, and compatibility with the boat battery tray. BCI was founded in 1920 to empower the lead-acid battery industry. The BCI group size range is broad, including batteries for commercial purposes, electric vehicles, and passenger cars.
This guide highlights the common battery sizes used in marine applications, such as Group 24, 27, 31, and 8D. These group sizes define general battery dimensions. However, nowadays, battery manufacturers are trying to reduce the battery sizes to minimize the storage space. Simply put, the smaller the group size, the smaller the dimensions of the battery will be, and vice versa.
Common Marine Battery Sizes with Chart
Let's now look at the most common marine battery sizes and their uses. The table below shows them all.
| BCI Group | Length (inches) | Width (inches) | Height (inches) | Applications |
| 24 | 10.25 | 6.81 | 8.88 | Light trolling motors, small boats, and kayaks |
| 24F | 10.75 | 6.81 | 9.00 | |
| 24H | 10.25 | 6.81 | 9.38 | |
| 24R | 10.25 | 6.81 | 9.00 | |
| 24T | 10.25 | 6.81 | 9.75 | |
| 27 | 12.06 | 6.81 | 8.88 | Trolling, fishing electronics, medium boats |
| 27F | 12.50 | 6.81 | 8.94 | |
| 27H | 11.75 | 6.81 | 9.25 | |
| 31 | 13.00 | 6.72 | 9.44 | Longer runtime, large boats, and multiple accessories |
| 8D | 20.75 | 11.13 | 9.88 | Commercial vessels, yachts |
Let's get into a bit more detail about applications of each battery group size.
- Group 24: It consists of 70-85 amp-hour batteries that are considered ideal for small boats with minimal power requirements, such as running a fish finder or cabin lights.
- Group 27: Batteries belonging to this group have 85-100 amp-hour capacity. They can easily handle extra onboard systems and trolling motors.
- Group 31: This group contains batteries with 100-120 amp-hour capacity. They are perfect for extended trips on the water.
- Group 8D: Marine batteries in this group offer up to 300 amp-hour capacity, making them ideal for energy-intensive setups or larger vessels. From refrigerators to windlasses, Group 8D batteries can easily handle everything.
What to Consider When Choosing the Right Battery Size?
Now that you are familiar with the different group sizes, it is time to explore the factors (as given below) you should consider while choosing the right marine battery.
- Boat Type & Size: Consider the type of boat you have and its size. For example, a cabin cruiser needs a different battery compared to a kayak.
- Power Needs: Determine what you want to run, e.g., fish finders, trolling motor, live wells, and GPS.
- Runtime: Consider battery capacity based on the runtime you need: a few hours or all day.
- Space & Weight: Keep in mind your boat's compartment space and weight it can easily handle, as some compartments cannot accommodate heavy and large marine batteries.
- Budget & Performance: Smaller batteries are budget-friendly but deliver shorter runtime, whereas larger batteries can deliver longer runtime.
Starting vs. Deep Cycle vs. Dual Purpose Batteries & Size Impact
Starting, deep-cycle, and dual-purpose are three different types of batteries, each belonging to a unique group size. Let's look at them one by one.
- Starting Batteries: They usually belong to smaller group sizes, i.e., Group 24 and Group 27. A starting battery delivers a quick burst of energy to start your boat engine.
- Deep-Cycle Batteries: These batteries often belong to larger group sizes, e.g., Group 27 and Group 31, and are therefore ideal for electronics and trolling motors. Deep-cycle batteries are specifically designed for steady discharge over multiple hours.
- Dual Purpose Batteries: They are commonly available in two group sizes, i.e., Group 27 and Group 31. Dual-purpose batteries are good for both running accessories and starting the engine.
Important Note: Don't forget to check if your boat needs two batteries: one for cranking the engine and one for house power.
Lead-Acid vs. Lithium: Do Sizes Differ?
Lead-acid batteries have been a popular choice for marine applications for years. They include Gel, AGM, and Flooded batteries. The best part? These batteries are cost-effective and offer decent performance characteristics. At the same time, lead-acid batteries are bulkier and heavier. The weight of an AGM battery belonging to Group 31 is around 60 pounds.
Nowadays, lithium batteries have taken the industry by storm due to their unique features, such as higher usable capacity and lighter structures. The best thing is that the same group sizes are available for these batteries, too. A lithium battery belonging to Group 31 weighs around 30 pounds, half the weight of an AGM battery from the same group. Despite the half weight, lithium batteries offer double the usable runtime.
If lifespan, efficiency, and weight are the parameters that matter to you, upgrade to lithium batteries, as they are lighter, more efficient, and last longer than their lead-acid alternatives.
How to Measure and Confirm the Right Marine Battery Size?
Do you want to measure and confirm whether a specific battery fits perfectly into your boat? It involves several parameters to consider. Let's discuss them all one by one.
- Check the group number of your previously installed battery. You can find it on the battery label. If not, you can either review the owner's manual or use an online battery finder tool.
- Look at your battery compartment to get an idea of the amount of space available. Determining the available space will help you choose the battery that fits perfectly in your boat.
- Review the boat manual, in which manufacturers usually recommend compatible battery sizes that you can use without worrying about anything.
- Last but not least, match your power needs and choose the marine battery accordingly. Make sure the reserve capacity usage and amp-hour rating of the selected battery meet your energy requirements.
Actionable Tips for Choosing the Perfect Battery
When it comes to choosing the right boat battery, there is no concept like “bigger is better.” It is all about matching the available space, your usage pattern, and the overall setup. Below is a list of some expert tips you should follow.
- Avoid Undersizing: If a battery is too small, it will work hard to deliver the desired amount of power and drain quickly. As a result, its lifespan is greatly reduced.
- Don't Oversize: Choosing a larger battery than your power needs is not a wise decision. Though a big battery delivers more power, it is also heavier and costly.
- Match Battery Size to Your Charging System: Bigger marine batteries take more time to recharge. Make sure your onboard charger or alternator has enough power to charge the group size you choose.
- Balance Performance & Weight: You can get all-day power through a Group 31 deep cycle battery. Since it weighs around 60 pounds, it adds significant weight to your boat's structure. Weight affects different parameters, such as handling, speed, and efficiency, especially in small boats.
- Consider a Two-Battery Setup: Many boaters prefer using one starting battery to crank the engine and one deep cycle battery to run trolling motors and electronics. Having a two-battery setup ensures that you have enough power to start the engine, even if the deep cycle battery drains.
- Think of Upgrading: If you have limited space in the battery compartment but want to get more runtime, choosing lithium (LiFePO4) batteries will be worth it. The same group size of these batteries can deliver the required amount of energy for a longer time.
- Stick with Reputable Brands: Cheaper battery options may save money upfront, but fail sooner. On the other hand, trusted brands like Renogy are known for consistent and reliable quality across all group size batteries.
Pro Tip: Marine batteries within the same group may have slight differences in their dimensions (length, width, and height). Therefore, don't forget to measure your boat's battery compartment before you buy a new battery.
Final Thoughts
Selecting the right marine battery size is crucial to ensure safe travel on the water, as it allows you to keep a balance between your budget, power needs, weight, and space available. Batteries are generally categorized into four groups for marine applications: Group 24, Group 27, Group 31, and 8D.
Group 24 batteries can easily handle the energy requirements of smaller boats, whereas you need Group 27 batteries if you are a serious angler. Similarly, Group 31 batteries are used in boats where extended runtime is required. In contrast, commercial vessels and heavy-duty yachts need bigger batteries belonging to Group 8D.
The best marine battery is the one that powers you all day long, fits perfectly in your compartment, and matches your boat's needs. If you want to buy a premium-quality, long-lasting marine battery, explore the deep cycle batteries at Renogy.
FAQs
Which marine battery size is commonly used?
When it comes to marine batteries, Group 27 and Group 31 are the most common sizes used in recreational and fishing boats. These sizes are popular because they offer a great balance of power, reserve capacity, and physical dimensions, making them suitable for a wide range of vessels.
How do I know which marine battery size is perfect for my boat?
Choosing the right battery depends on your boat's specific needs. First, you need to check the battery compartment's dimensions to ensure a proper fit. Next, determine your boat's energy requirements, considering whether you need a deep-cycle battery for powering electronics, a starting battery for the engine, or a dual-purpose one. Matching the correct group size ensures optimal performance.
Can I upgrade to a larger marine battery?
Yes, you can upgrade to a larger marine battery, but only if your boat's electrical system can handle the increased capacity and the battery compartment has enough space. A larger battery will provide a longer runtime and more reserve capacity, which is especially beneficial for boats with a lot of accessories or for extended trips on the water.
What if I mistakenly choose the wrong battery size?
Selecting the wrong battery size can lead to several problems. A battery that's too small may not provide enough power to reliably start your engine or run your electronics, leading to poor performance and limited runtime. A battery that's too large might not fit correctly, creating safety risks and potentially damaging the battery or its compartment.
