Boat fishing is a hugely popular pastime and can be done from various boat sizes. Unlike cars, boats have no means of recharging batteries while out on the water. Thus, with many things in use, such as water pumps, lights, stereos, trolling motors, and fish finders, you need a battery that can last the day and face repeat charges.
There are many things to consider besides just physical size and output when looking at batteries. The best battery with a larger group size will be a recommendation if you want to spend a day of fishing rather than cutting your trip short.
In our guide, you can learn more from our marine battery size chart besides just your new marine battery dimensions. By the end, you’ll learn about maintenance-free batteries that hold enough reserve amps and also the group sizes.
You’ll learn a lot, but most of all, which is the right marine battery type for your fishing style and needs. (Read Side Imaging Vs Down Imaging)
What Are The Sizes Of Marine Batteries?
Marine Battery Group Size Chart
| Group Size | Length | Width | Height | Estimated capacity (Ah) | Estimated CCA |
|---|---|---|---|---|---|
| 22NF | 9.43’’ | 5.5’’ | 8.94’’ | 40-56 | 300-550 |
| 24M | 10.25’’ | 6.81’’ | 9.75’’ | 75-85 | 500-840 |
| 25 | 9.06’’ | 6.88’’ | 8.88’’ | ||
| 27M | 12.5’’ | 6.81’’ | 9.75’’ | 66-110 | 640-870 |
| 31M | 13’’ | 6.81’’ | 9.44’’ | 75-125 | 600-1150 |
| 34M | 10.25’’ | 6.81’’ | 9.44’’ | 50-75 | 750-880 |
| 35 | 9.06’’ | 6.88’’ | 8.88’’ | ||
| 65 | 12.06’’ | 7.56’’ | 7.56’’ |
Types of Boat Batteries
Boat owners can typically select between two types of 12-volt batteries.
The first is the cranking or starting battery, which is tailored to start the main engine.
On the other hand, the deep-cycle battery is used to power electrical devices such as a fish finder, trolling motors, and radios for long periods.
Dual-purpose batteries are also available, which serve both purposes.
The following are the different types of batteries based on their structure and conducting medium:
- Wet cell battery (aka flooded battery)
- Gel battery
- Absorbent glass mat (AGM) battery
It would help if you used only AGM batteries on boats since they are spill-proof, maintenance-free, offer a longer service life, and are shock-resistant, leading to fewer accidents than lead-acid batteries.
What Are The Different Sizes Of Deep Cycle Batteries?
Marine batteries are made specifically for use aboard boats.
These batteries deliver a higher capacity and currents with many cranking amps, making them a reliable power source for electric motors in many applications.
They make batteries with thicker plates and casings that can withstand the common hammering and vibrations on boats. (Find Interstate Trolling Motor Battery)
There are various types of marine boat batteries available for varied marine applications.
1. Marine Starting Batteries
A marine starting battery, often known as a marine cranking battery, is the most frequent alternative for boats using alternator engines. This battery produces a short burst of cranking amps to start the boat’s engine.
The marine cranking battery has a more extensive surface area with thinner plates to produce large currents. Batteries of modest capacity are used to start the boat’s engine. The boat battery is fully charged when the engine alternator starts.
While the design allows for high cranking amps, it limits capacity and depth of discharge. As a result, starting marine batteries should never be utilized as a trolling motor battery or to power appliances.
2. Marine Deep-Cycle Batteries
Deep-cycle or deep-cell marine batteries are used in boats with trolling motors or other electric motors.This battery can be deeply depleted and has a high capacity, like RV and golf cart batteries.
Deep-cycle marine battery includes fewer but thicker plates, allowing for longer discharge times and deeper discharge depths. These batteries’ thicker plates allow them to endure high temperatures while delivering high currents. Unlike starter batteries, you can recharge a deep-cell marine battery numerous times. They are used to power electric motors and other devices.
3. Marine Dual-Purpose Batteries
The dual-purpose marine battery is comparable to the motorbike battery in design. Inside, dual-purpose batteries are designed to be deeply drained and used as starter batteries.
Replace the deep-cycle and starter batteries with a dual-purpose marine battery that can power the engine and appliances simultaneously. Manufacturers’ sizes vary by only a fraction of an inch. (Find the Best Crappie Fish Finder)
However, they cannot deliver the same depth of discharge as a deep cycle battery or the same cranking amps as a starter battery.
What Group Size Battery Do I Need For My Boat?
Many boats feature illumination, speakers, fish finders, and other gadgets. While some batteries may not have enough power to handle that, others may readily power all the gadgets simultaneously.
The marine battery group sizes categorize all boat battery sizes options, making selection easier.
It is straightforward to learn about marine battery group sizes and how to interpret a marine battery size chart. You can quickly see which marine battery group size suits your boat’s requirements.
It’s used to categorize batteries’ capacity, C-rate, and cranking amps. The Battery Council International (BCI) devised these categories to help boaters find the right battery.
The guideline with deep-cycle Gel and AGM marine batteries is that the larger battery has more capacity.
Although, this criterion doesn’t apply to the starter or dual-purpose batteries, making marine group sizes an essential factor in battery selection.
What Does Group Size Mean?
Marine battery group size is one of many boat battery specs. In short, the BCI defines the group size as the battery housing’s physical dimensions (Battery Council International).
It should specify the group size of your present battery on the labels. Notate the group size for the charts below.
Group Size for New Batteries
The larger the current battery’s group size, the higher the Amp Hour capacity of deep cycle marine batteries. If your trolling motor is weak after a long day of fishing in the wind, you may benefit from updating your deep cycle trolling motor batteries.
Cranking batteries and group size vary. A battery that has enough cold-cranking amps to start your outboard engine and enough reserve amps to power your fish finders, pumps, and other accessories. Deep cycle circuits should only be used for trolling motors.
Here are some tips when buying fresh batteries:
- For marine and RV use, look for starting batteries with a M in the name, which is the BCI classification.
- Measure the available space in your boat battery compartment before buying batteries.
- You may need to stay within the size of the battery tray or install a larger tray or battery housing for a bigger group size.
- If you continue to have issues powering your electronics and accessories over a full day of fishing, you could have a wiring problem.
- Check that your boat wiring has sufficient wire gage to support the current draw of your electronics.
- Consider adding a dedicated dual-purpose battery to power only your fish finders, leaving the less demanding parts on the regular cranking battery.
What Is Difference In Group Size Of Marine Battery?
Marine Battery Group Size Chart
| Battery Group Size | Length (inch) | Width (inch) | Height (inch) |
|---|---|---|---|
| U1 | 7 3/4 | 5 3/16 | 7 1/8 |
| 21 (R) | 8 3/16 | 6 13/16 | 8 3/4 |
| 22F | 9 1/2 | 6 7/8 | 8 5/16 |
| 22HF | 9 1/2 | 6 7/8 | 9 |
| 22NF | 9 7/16 | 5 1/2 | 8 15/16 |
| 22R | 9 | 6 7/8 | 8 5/16 |
| 24 | 10 1/4 | 6 13/16 | 8 7/8 |
| 24F | 10 1/4 | 6 13/16 | 9 |
| 24H | 10 1/4 | 6 13/16 | 9 3/8 |
| 24R | 10 1/4 | 6 13/16 | 9 |
| 24M(T) | 10 1/4 | 6 13/16 | 9 3/4 |
| 25 | 9 1/16 | 6 7/8 | 8 7/8 |
| 26 (R) | 8 3/16 | 6 13/16 | 7 3/4 |
| 27 | 12 1/16 | 6 13/16 | 8 7/8 |
| 27F | 12 1/2 | 6 13/16 | 8 15/16 |
| 27H | 11 3/4 | 6 13/16 | 9 1/4 |
| 27M | 12 1/2 | 6 13/16 | 9 3/4 |
| 29NF | 13 | 5 1/2 | 8 15/16 |
| 31M | 13 | 6 13/16 | 9 7/16 |
| 33 | 13 5/16 | 6 13/16 | 9 3/8 |
| 34 (R) | 10 1/4 | 6 13/16 | 7 7/8 |
| 34M | 10 1/4 | 6 13/16 | 9 7/16 |
| 35 | 9 1/16 | 6 7/8 | 8 7/8 |
| 36R | 10 3/8 | 7 1/4 | 8 1/8 |
| 40R | 10 15/16 | 6 7/8 | 6 7/8 |
| 41 | 11 3/16 | 6 7/8 | 6 7/8 |
| 42 | 9 5/16 | 6 13/16 | 6 13/16 |
| 43 | 13 1/8 | 12 1/16 | 8 1/16 |
| 45 | 9 7/16 | 5 1/2 | 8 15/16 |
| 46 | 10 3/4 | 6 13/16 | 9 |
| 47 | 9 11/16 | 6 7/8 | 7 1/2 |
| 48 | 12 1/16 | 6 7/8 | 7 9/16 |
| 49 | 15 | 6 7/8 | 7 3/16 |
| 50 | 13 1/2 | 5 | 10 |
| 51 | 9 3/8 | 5 1/16 | 8 13/16 |
| 51R | 9 3/8 | 5 1/16 | 8 13/16 |
| 52 | 7 5/16 | 5 13/16 | 8 1/4 |
| 53 | 13 | 4 11/16 | 8 1/4 |
| 54 | 7 5/16 | 6 1/16 | 8 3/8 |
| 55 | 8 5/8 | 6 1/16 | 8 3/8 |
| 56 | 10 | 6 1/16 | 8 3/8 |
| 57 | 8 1/16 | 7 3/16 | 6 15/16 |
| 58 | 10 1/16 | 7 3/16 | 6 15/16 |
| 58R | 10 1/16 | 7 3/16 | 6 15/16 |
| 59 | 10 1/16 | 7 5/8 | 7 3/4 |
| 60 | 13 1/16 | 6 5/16 | 8 7/8 |
| 61 | 7 9/16 | 6 3/8 | 8 7/8 |
| 62 | 8 7/8 | 6 3/8 | 8 7/8 |
| 63 | 10 3/16 | 6 3/8 | 8 7/8 |
| 64 | 11 11/16 | 6 3/8 | 8 7/8 |
| 65 | 12 1/16 | 7 1/2 | 7 9/16 |
| 70 | 8 3/16 | 7 1/16 | 7 11/16 |
| 71 | 8 3/16 | 7 1/16 | 8 1/2 |
| 72 | 9 1/16 | 7 1/16 | 8 1/4 |
| 73 | 9 1/16 | 7 1/16 | 8 1/2 |
| 74 | 10 1/4 | 7 1/4 | 8 3/4 |
| 75 | 9 1/16 | 7 1/16 | 7 11/16 |
| 76 | 13 1/8 | 7 1/16 | 8 1/2 |
| 78 | 10 1/4 | 7 1/16 | 7 11/16 |
| 85 | 9 1/16 | 6 13/16 | 8 |
| 86 | 9 1/16 | 6 13/16 | 8 |
| 90 | 9 11/16 | 6 7/8 | 6 7/8 |
| 91 | 11 | 6 7/8 | 6 7/8 |
| 92 | 12 1/2 | 6 7/8 | 6 7/8 |
| 93 | 15 | 6 7/8 | 6 7/8 |
| 95R | 15 9/16 | 6 7/8 | 7 1/2 |
| 96R | 9 9/16 | 6 13/16 | 6 7/8 |
| 97R | 9 15/16 | 6 7/8 | 7 1/2 |
| 98R | 11 3/16 | 6 7/8 | 7 1/2 |
Buying new batteries: group size considerations
A greater group size means a higher Amp Hour (Ah) capacity for deep cycle marine batteries.
If you conclude a day of fishing in windy circumstances and your trolling motor is weak, update to a larger Ah battery. The replacement battery will therefore have a different group size. (Learn How To Read a Fish Finder)
Many battery group sizes are only fractions of an inch apart. you can swap these batteries if:
- The replacement battery fits the old battery tray.
- The new battery charges and self-discharge characteristics of the old battery.
The following are special considerations help make shopping for a new battery easier:
- Marine batteries are built differently from car batteries, yet you’ll find lithium batteries are equally expensive.
- A traditional size battery must fit the battery tray. If your group size is huge, you can add a larger tray.
- You may also add a dual-purpose battery with reserve capacity solely for your boat’s fish finders, leaving the less demanding components to the conventional cranking battery.
- Install a competent onboard marine battery charger on conventional boats to protect your battery investments.
