What Size Marine Battery Do I Need for My Boat?
Like other vehicles, a battery is one of the key components of your boat because it delivers power to its electrical system, ranging from the trolling motor to GPS navigation. To make sure all the boat's energy requirements are met, you need to choose the right battery.
Marine battery sizing is more complicated than sizing a car battery. The reason behind this fact is that boats usually have multiple electrical systems, each with different power requirements. Therefore, many boaters often search for “What size battery do I need for my boat?”. Are you one of them?
If so, your search ends here. This article helps you understand the basics of marine batteries and choose the correct size battery. Let's get deeper into this discussion.
Understanding Marine Battery Basics
A battery installed in boats to power all electrical systems and deal with a strong marine environment is commonly called a marine battery. Whether you want to run boat lights, an oven, a GPS system, a refrigerator, or any other electrical equipment, power comes straight from a marine battery. Additionally, this battery also runs a stern drive motor or a trolling motor.
There are different types of marine batteries, including starter, deep cycle, and dual-purpose batteries. Let's look at them one by one.
- Starting Batteries: They are perfect for starting the boat's engine. They can deliver high current (75-400 amps) for a short time (5-15 seconds). Then, the alternator starts charging them. Starting batteries don't support deep discharges.
- Deep Cycle Batteries: When there is no source of charge, these batteries power up the entire electrical system of your boat. Overnight use of deep cycle batteries may discharge 50-70% of the total capacity, depending on the total load.
- Dual-Purpose Batteries: Unlike starting batteries, they can bear deep discharges. However, dual-purpose batteries have lower storage capacity than their deep-cycle counterparts. They are good for small boats that use one battery to crank the engine and run electrical loads.
How Are Marine Batteries Different from Standard Batteries?
Though marine batteries and standard batteries appear similar, they differ from each other in various aspects, such as design, construction, discharge tolerance, size, and cost.
Marine batteries crank the boat's engine and run multiple onboard systems for extended periods, e.g., radios, fish finders, and lights. Therefore, they are designed for both purposes: starting and deep cycling. In contrast, a car battery delivers a quick energy burst to start the engine.
With thicker internal plates, marine batteries are designed to resist shock and vibration. They also resist corrosion from saltwater and moisture. On the other hand, standard car batteries may not survive in these conditions.
Moreover, marine batteries are developed to be recharged and discharged repeatedly, making them a perfect choice for powering electronic equipment. Meanwhile, deep discharge may damage standard car batteries.
Furthermore, the dual-function ability and rugged design of marine batteries make them bigger and more costly than car batteries.
What Size Battery Do I Need for My Boat?
Choosing the right size marine battery mainly depends on what kind of electrical systems you want to run and how you use the boat. Two batteries, starting and deep cycle, are frequently used in most boats. The starting battery cranks the engine, whereas a deep cycle (or house) battery runs electronic equipment. Both have different sizing considerations, as given below.
1. Starting Batteries
The primary job of these batteries is to start the boat's engine by delivering a quick burst of high current. The size of starting batteries is rated in Marine Cranking Amps (MCA). The MCA measures how much current a battery can deliver at 32 degrees Fahrenheit for about 30 seconds, thereby maintaining at least 1.2V per cell.
When it comes to sizing a starting battery, always check the manufacturer's guidelines for the desired MCA. It ensures good performance. Different engine types may have different cranking amps per cubic inch requirements. For example, having 2 cranking amps per cubic inch for diesel engines and 1 cranking amp per cubic inch for gasoline engines is a good rule of thumb.
Generally, gasoline engines need 800 to 1,000 MCA depending on their size, whereas 1,000 to 1,400 MCA are required for diesel engines.
2. Deep Cycle Batteries
They are also known as house batteries. Deep cycle batteries are ideal for delivering steady current for extended periods. They are typically rated in Amp-Hours (Ah) and Reserve Capacity (RC). To determine the total power consumption of your boat, you need to add the Ah rating of each appliance. For example, if the light uses 2A and you run it for 6 hours, it becomes 12Ah.
Moreover, the depth of discharge directly impacts the Ah capacity of deep cycle batteries. Lithium batteries can handle deeper discharges (80-90%), which means they allow you to use a greater percentage of their capacity. A 120-130 Ah lithium battery is enough to meet 100Ah daily use.
In contrast, discharging lithium batteries below 50% greatly reduces their lifespan. It means that you need at least a 200Ah lead-acid battery to fulfill your 100Ah daily requirements.
It is advisable to add a 20-30% buffer as extra capacity to the calculated amp-hour rating for less than ideal charging conditions or unexpected power needs. For example, if you calculate a 250 amp-hour lead-acid requirement, choose a 300-325 Ah battery bank.
Reserve Capacity (RC) measures how long a marine battery can supply sustained power, which is required during emergencies when charging sources are unavailable or fail. Simply put, RC is the measure of the minutes your battery can supply 25A at 80 degrees Fahrenheit before its voltage drops below 10.5V.
Marine Battery Group Sizes and Dimensions
The group size of a marine battery refers to its physical dimensions. For marine applications, batteries are divided into four standard group sizes, as given in the table below.
| Group | Size |
| 24 | 10.25 x 6.81 x 8.88 |
| 27 | 12.06 x 6.81 x 8.88 |
| 31 | 13 x 6.72 x 9.44 |
| 8D | 20.75 x 11.13 x 9.88 |
The group size correlates with the battery capacity; you should not consider it as the main factor in calculating your power requirements. More important specifications to determine your power needs include Marine Cranking Amps (MCA), Reserve Capacity (RC), and Amp Hours (Ah).
Note: Never forget to measure the battery compartment or box to ensure the newly selected battery perfectly fits in it.
Can I Use a Regular Charger to Charge a Marine Battery?
If your boat has a standard lead-acid battery, a regular battery charger can charge it effectively. At the same time, this charger can also charge modern lithium batteries in many cases.
However, using a dedicated charger for lithium batteries will speed up the charging process, and the batteries will charge fully more quickly than with the standard charger. Though replacing your old-school batteries with the newer ones is a significant investment, many boat owners prefer to invest in getting the maximum out of their batteries.
When it comes to charging your lithium batteries via an alternator, an alternator controller or a DC-DC charger will be worth using. Lithium batteries charge faster than their lead-acid counterparts, which places excessive strain on an alternator and leads to damage. The use of a controller manages the charging process and protects your system from damage.
When to Wire Marine Batteries in Series or Parallel?
Marine battery configuration can be both series and parallel. Let's understand when to use which configuration.
Series
If you are aiming for a higher voltage with unchanged battery capacity, establish a series connection between your marine batteries. For example, if you want to increase the voltage level from 12V to 24V, 36V, or 48V, you can connect up to four Renogy 12V 100Ah Trolling Motor LiFePO4 batteries in series. The series combination is used in boats where increased power output is required to run certain onboard equipment or larger electric motors.
Parallel
If you want to increase the runtime of your batteries by increasing their capacity while keeping the voltage the same, you should go for a parallel connection. You can connect up to four Renogy batteries (the same as mentioned above) in parallel. The parallel connection is needed to run various devices that consume a moderate amount of energy for extended periods. The batteries should be connected in parallel in boats where the energy demand is high.
Renogy Trolling Motor Battery
Renogy is a global brand of off-grid and renewable energy solutions, specializing in high-quality, user-friendly solar products designed for both mobile and stationary applications. Their extensive product lineup includes solar panels, deep-cycle batteries, charge controllers, inverters, portable power stations, and complete solar kits - all engineered to support energy independence for RVs, boats, cabins, homes, and outdoor adventures.
Renogy 12V 100Ah Trolling Motor Lithium Battery with Bluetooth
The 12V trolling motor lithium battery delivers superior power reliability for marine enthusiasts and outdoor adventurers. With its high-capacity lithium iron phosphate technology, this premium battery ensures extended runtime for trolling motors, fish finders, marine electronics, and other vessel systems. The built-in Bluetooth connectivity allows seamless monitoring of battery status through your smartphone, providing real-time insights into power consumption and battery health.
Engineered for exceptional durability in marine environments, this battery offers 10x longer lifespan than traditional lead-acid alternatives while weighing 60% less. Whether you're facing choppy waters or planning an extended fishing expedition, this battery ensures reliable power throughout your journey. The compact, waterproof design with integrated carrying handle allows for simple installation and transport anywhere on your vessel.
Marine Battery FAQs
How many cranking amps do I need for my boat?
The cranking amps of a boat depends on its engine size. Generally, outboards need 350-600 MCA. In contrast, 800-1,000+ MCA are required for diesel or gas inboards.
How to calculate battery size for a boat?
First, calculate the Ah requirements of the appliances you use and multiply by the number of hours you want to use them. Once done, choose a marine battery with 20-30% extra capacity.
How many amps should a boat battery have?
Starting boat batteries need high current (75-400 amps) to crank the engine, whereas deep cycle batteries require low current (like 80 amps) to run electrical equipment for longer periods.
Final Thoughts
If you are searching for “What size battery for my boat?”, remember that the selection of the right marine battery depends on the purpose, e.g., starting or deep cycling. To select the deep cycle marine battery, you should first calculate the Ah rating of all appliances and add 20-30% to the overall Ah rating.
Marine batteries are available in different groups, each of which has different physical dimensions. Therefore, always make sure that the battery you choose can perfectly fit into the battery box or compartment of your boat. When it comes to choosing a battery charger for your lithium batteries, try using a dedicated charger that best matches the battery's requirements.
