Choosing The Right Boat Battery: A Comprehensive Guide

Choosing the right battery for your boat is a complex task. There are many factors to consider, and many types of batteries to choose from. The first thing to consider is the purpose of the battery. Will it be used for starting the boat, powering onboard electronics, or both? This will determine whether you need a starting, deep cycle, or dual-purpose battery. Starting batteries deliver high bursts of energy for short periods to crank the engine, while deep cycle batteries provide a smaller amount of energy for a longer period to power onboard electronics and can handle long discharge cycles while the engine is off. Dual-purpose batteries can be used for both starting and deep cycle functions but may not perform as well as dedicated batteries.

Battery type: cranking, deep cycle, or dual-purpose

Marine batteries come in three main types: cranking, deep cycle, and dual-purpose. Each type serves a different purpose.

Cranking Batteries

Cranking batteries, also known as starting batteries, are designed to deliver quick bursts of energy to start a boat's engine. They are not intended for extended, continuous use. Cranking batteries have many thin plates, maximising the plate surface area inside the battery. This design facilitates the delivery of the necessary power to start a boat engine. However, it also makes them less suitable for continuous discharge or "cyclic" use.

Deep-Cycle Batteries

Deep-cycle batteries are designed to provide a steady and sustained power output over an extended period, making them ideal for applications that require continuous power delivery. They are commonly used to power various electrical systems and appliances on boats, such as lights, trolling motors, fish finders, radios, and refrigerators. Deep-cycle batteries have fewer but thicker plates, allowing them to provide continuous power output over long periods. They can be completely drained and recharged frequently, and they are less prone to overheating.

Dual-Purpose Batteries

Dual-purpose batteries, as the name suggests, combine the functions of both cranking and deep-cycle batteries. They deliver the high burst of power needed to start an engine and provide moderate sustained power for running onboard electronics and accessories. While dual-purpose batteries offer convenience by serving both purposes, they are less efficient than specialised batteries. They may not stand up well to total discharges and can be more prone to overheating in harsh conditions. Therefore, they are more suitable for short trips if used and maintained correctly.

Battery capacity (Ah rating)

Battery capacity, or Amp-hours (Ah), is a crucial specification to consider when choosing a boat battery. It refers to the total amount of energy that a battery can deliver over time at a constant rate of discharge before the voltage drops to a certain level, usually 10.5 volts. This is important for understanding how long your battery will last when powering electrical equipment without the engine or generator running.

For example, a 100 Ah battery can theoretically deliver 5 amps for 20 hours or 100 amps for 1 hour. However, it is important to note that no battery should be discharged at a rate greater than 50% of its rated total capacity to avoid permanent damage. Therefore, a fully charged 100 Ah battery in perfect condition will have a useful capacity of 50 Ah, and this will reduce slightly as the battery ages.

When considering battery capacity, it is also worth noting the concept of "cycles". A cycle refers to the process of a battery discharging through use and then being recharged back to full capacity. A light- to medium-duty battery will typically be good for about 200 to 300 cycles, while a heavy-duty battery can often double this number with no ill effects.

When choosing a battery, it is important to match the battery capacity to the expected power demands of your boat's electrical systems. It is generally recommended to size deep cycle batteries to store three to four times the expected amount of energy to be used between recharge cycles.

Battery size

When it comes to battery size, bigger isn't always better. A larger battery may not be suitable for your boat's layout or weight distribution. Choose a battery with an average weight to make it easier to move in and out. Additionally, consider the battery's output, which is indicated by its marine cranking amp (MCA) rating, reserve capacity rating, and cold cranking amp (CCA) rating. CCA refers to the number of amps a battery can deliver for 30 seconds at 0°F while maintaining a voltage above 7.2 volts. MCA, on the other hand, is measured at 32°F.

Another factor to keep in mind is the battery's capacity, typically given in amp-hours (Ah). This rating indicates the total amount of energy the battery can deliver over a specific period. For example, a 100 Ah battery should be able to deliver 5 amps for 20 hours or 100 amps for one hour. However, it's important not to discharge a battery beyond 50% of its rated total capacity to avoid permanent damage.

When selecting a battery size, also consider the type of battery. Starting or cranking batteries have thinner and more numerous plates, providing the high amperage bursts needed to start an engine. On the other hand, deep cycle batteries have thicker plates and are designed to deliver a steady output over a long period, making them ideal for powering electronics when the engine is off.

In summary, choosing the right battery size for your boat involves considering the physical dimensions, weight, output, and capacity of the battery, as well as the specific needs of your boat and engine.

Battery weight

Flooded batteries, which are the most traditional type, weigh around 60 lbs (27 kg) for smaller group sizes and up to 130 lbs (59 kg) for larger group sizes. Absorbed Glass Mat (AGM) batteries are slightly lighter, with smaller group sizes weighing around 50 lbs (23 kg) and larger ones up to 85 lbs (39 kg). Gel batteries are heavier, with small group sizes weighing 70 lbs (32 kg) and larger group sizes weighing up to 120 lbs (55 kg).

Lithium batteries are the lightest option, with small group sizes weighing around 30 lbs (14 kg) and larger group sizes up to 80 lbs (36 kg). Lithium batteries are a good choice if you want to reduce the load on your engine. They also have a long life cycle, so you won't have to replace them as often.

When choosing a marine battery, it's important to consider not just the weight but also the size, voltage, and power requirements of your boat. The battery should fit comfortably in your battery compartment and have the right voltage for your boat's electrical system (usually 6V, 12V, or 24V).

Additionally, you'll want to make sure the battery has enough power to meet the minimum requirements of your boat's electrical devices and engine. You can estimate your boat's power needs by making a list of all the electrical equipment, their electric consumption, and the desired running time for each device. This will help you determine the total power draw and choose a battery with the appropriate amp-hour rating.

Remember that heavier batteries can increase the weight of your boat, affecting its speed and fuel consumption. So, when selecting a marine battery, consider the weight and how it will impact your boat's performance.

Battery output (MCA, RC, CCA)

When selecting a boat battery, it is crucial to understand the different battery output ratings, such as MCA, RC, and CCA. These ratings provide essential information about the battery's performance and suitability for your boat's specific needs. Here's a detailed explanation of each:

Marine Cranking Amps (MCA):

MCA, also known as Cranking Amps (CA), measures a battery's ability to deliver current at a higher temperature of 32°F (0°C). This rating is similar to CCA but is more relevant for marine environments as they typically don't experience extreme cold conditions. MCA values are generally 20-25% higher than CCA due to the improved performance of batteries at higher temperatures. When choosing a starting battery, ensure it meets or exceeds the MCA rating recommended by your boat engine's manufacturer.

Reserve Capacity (RC):

RC is a crucial factor for applications requiring consistent power over time. It measures how long a fully charged battery can deliver a constant current of 25 amps before dropping below 10.5 volts. A high RC is advantageous for trolling motors and RVs, as it allows for more time on the water or extended trips without the need for frequent recharging.

Cold Cranking Amps (CCA):

CCA is a critical metric for starting batteries, especially in cold climates. It measures a battery's ability to deliver current in cold temperatures, specifically at 0°F (-18°C). CCA indicates the number of amperes a 12-volt battery can provide for 30 seconds while maintaining a voltage of at least 7.2 volts. Opt for a higher CCA rating if you plan to use your boat in colder environments to ensure reliable engine starting.

When choosing a boat battery, consider the MCA, RC, and CCA ratings to ensure the battery meets your specific needs. MCA is more relevant for marine environments, RC is crucial for consistent power, and CCA is vital for reliable engine starting in cold conditions. Additionally, pay attention to the manufacturer's guidelines and the specific requirements of your boat's engine and electrical system.

Frequently asked questions