Powering Boats: How Many Batteries Are Required?

How many batteries does a boat need? This is a complex question, as it depends on several factors, including the size of the boat, the type of engine, and the number of electrical accessories. Boats require batteries for two types of power needs: the starting load (a large, short-term burst of electricity to turn over the engine) and the running load (powering accessories like lights, trolling motors, and other onboard tech).

Most boats require at least two types of batteries: a starting battery and a house or deep-cycle battery. The number of batteries needed will depend on the boat's size and its power requirements. Boats under 14 feet typically need one battery, while boats 15-23 feet with a single engine may need two or more batteries. Boats 24 feet and above with multiple engines will require three or more batteries, with additional batteries for each extra engine.

Additionally, the type of battery chosen is important. Common types include cranking or starting batteries, deep-cycle batteries, and dual-purpose batteries. Cranking batteries are designed to start the engine, while deep-cycle batteries power heavy electrical loads. Dual-purpose batteries can do both but are generally more expensive and have lower power capacity.

To determine the number of batteries needed, boat owners should consider their energy budget, including the power requirements of all electrical appliances and accessories, and select batteries that can meet those demands.

Boat size and number of engines

The number of batteries a boat requires depends on its size and the number of engines it has. Boats under 14 feet in length with a single engine will only need one battery. Boats that are 15-23 feet long with a single engine will require two batteries, unless they have a trolling motor, in which case they may need up to three more batteries.

Boats that are 24 feet or longer and have more than one engine will need at least three batteries, with an additional battery for every extra engine. These larger boats may also need extra house batteries to power electronic equipment.

The type of battery is also an important consideration. Starting batteries deliver a large burst of power to start the engine but are not suitable for long-term power needs. Deep cycle batteries, on the other hand, can provide long-lasting power for applications such as trolling motors, depth finders, and GPS trackers. Dual-purpose batteries can be used for both starting the engine and powering electronics, but they are more expensive and have lower power capacity.

When determining the number of batteries needed, it is crucial to consider the power requirements of the boat's engine and any additional electronic equipment on board. The size and weight of the boat, as well as the horsepower of the engine, will impact the number and type of batteries required.

Federal law mandates that powerboats less than 20 feet in length display a Capacity Plate that includes information on the maximum engine size in horsepower, as well as the maximum number of passengers and gross load. For boats without a Capacity Plate, the maximum engine size can be calculated using the boat's length and width.

Battery type

The type of battery you will need for your boat depends on the type of boating you plan to do. There are three general types of marine batteries: starting boat batteries, deep-cycle marine batteries, and multi-purpose boat batteries.

Starting boat batteries are similar to the starting battery in your car. They provide a short burst of power to start your boat motor. Draining them below 50% will damage them and shorten their lifespan. A 12-volt battery should be enough for most boat engines, but if you have a boat that is 15 feet or larger, you may require more than one starting battery.

Deep-cycle marine batteries can be drained slowly over long periods. Depending on the type of deep-cycle battery, you can repeatedly drain them to 80% or more of their capacity without causing damage. However, deep-cycle batteries cannot provide the cranking amps needed to start large motors. Deep-cycle batteries are typically used to power things like trolling motors, depth finders, GPS trackers, fish finders, and other onboard tech. The number of house batteries you use will depend on how many additional applications you have on your boat and how much power they require.

Multi-purpose boat batteries combine some of the benefits of a starting battery with a deep-cycle battery. They allow you to start small boat motors while also powering lights and gauges that don't draw a lot of power.

If you have a lot of applications on your boat, it's a good idea to pair a dual-purpose battery with several deep-cycle batteries. Use the dual-purpose battery to start your engine and run some of your electronics, and then use the deep-cycle batteries to power your trolling motor.

Lithium-ion deep-cycle batteries are also an option. They are more expensive but have many advantages, especially in boating applications. You can drain lithium-ion deep-cycle marine batteries to 80% or more of their capacity without damaging them, and they charge much faster than lead-acid batteries. They are also about half the weight of lead-acid batteries, which is a significant advantage in boating.

Power needs

The number of batteries a boat requires depends on its power needs. Boats require batteries for two types of power needs: the starting load and the running load. The starting load is the amount of power the boat needs to start up, and the running load is the amount of electrical power the boat needs while running.

The starting load requires a large, short-term burst of electricity to turn over the engine. The number of batteries needed for the starting load depends on the size of the boat. A 12-volt battery is typically enough for most boat engines, but boats that are 15 feet or larger may require more than one starting battery.

The running load refers to the power needed for accessories like lights, trolling motors, depth finders, and other onboard technology. This is often referred to as the "house battery" and needs to deliver smaller amounts of power over a longer period of time. The number of house batteries depends on the number of additional applications and how much power they require.

To determine the number of batteries needed, it is important to calculate the boat's energy budget by considering the power requirements of each electrical appliance and the anticipated length of use. A rule of thumb is to have 3-4 times the daily energy consumption in battery capacity. For example, boats that consume 120Ah of energy per day should consider having 360-480Ah of battery capacity.

Additionally, the type of battery used is an important consideration. There are three main types of boat batteries: starting batteries, deep-cycle batteries, and dual-purpose batteries.

Starting batteries provide a large burst of power for a short period, making them ideal for starting the engine. They are similar to car batteries and are sometimes referred to as cranking batteries.

Deep-cycle batteries have a heavy charge and discharge cycle, allowing them to provide long-lasting power. They are typically used to power accessories like trolling motors, depth finders, GPS trackers, and fish finders.

Dual-purpose batteries serve as both starting and deep-cycle batteries, providing the strong starting power needed to start the engine and enough cycling capacity to power additional accessories. However, they are generally more expensive and have lower power capacity than similarly-sized deep-cycle batteries.

In summary, the power needs of a boat depend on the starting load, running load, and the type of battery used. By calculating the energy budget and considering the different types of batteries available, boat owners can determine the number of batteries required to meet their specific power needs.

Charging

Choose the Right Charger

There is a wide variety of boat batteries available, and it is important to select a charger that matches the battery's chemistry and voltage. If you have multiple batteries on board, ensure the charger can accommodate all of them. Chargers can be either onboard or portable. Onboard chargers are more convenient as they are already connected to the battery system, while portable chargers are more affordable and can be used anywhere.

• Select the Right Time
• Clean the Battery Terminals

Before charging, it is important to clean the battery terminals to ensure efficient charging. Grimy terminals can disrupt the process and lead to longer charging times.

Connect the Battery Charger

To connect the charger, follow these steps:

• Connect the red (positive) cable to the red terminal.
• Connect the black (negative) cable to the black terminal.
• Plug in the charger and turn it on.

Smart chargers are designed to stop charging automatically, while lead-acid chargers require manual disconnection once the charging is complete.

Disconnect the Charger

Once the battery is fully charged, disconnect the charger by unplugging it and then removing the cables in the reverse order of connection, starting with the black cable.

Additional Charging Information

• Charging time: A marine battery typically charges from 0% to 80% in about four to six hours, depending on the charger and battery.
• Charging on the water: Portable chargers are ideal for charging your boat battery while it is in the water.
• Battery life: Boat batteries generally last three to four years and need to be replaced, with replacement costs ranging from $100 to $500.
• Charging sources: Batteries can be charged using portable generators, shore power, or renewable sources such as solar panels or wind turbines.
• Safety: When using a generator, ensure the exhaust is directed away from the boat's interior to prevent carbon monoxide poisoning.

Maintenance

Charging

To ensure your boat batteries are well maintained, it is important to keep them properly charged. All batteries will self-discharge if left idle. A lead-acid battery may lose 5% of its charge per month, so it is important to periodically charge batteries that are not in use.

It is critical to a battery's health and longevity that it is charged correctly. Both overcharging and undercharging will reduce the battery's lifespan and degrade its performance. A marine smart battery charger is recommended, as it is designed for permanent installation, can be left connected to the batteries, and will automatically reduce the charge to avoid overcharging.

Temperature

Marine batteries are susceptible to temperature fluctuations. During hot temperatures, batteries tend to expand and discharge energy at a higher rate, which can result in electrolyte loss and increased discharge. In cold temperatures, the battery has to work harder, which may result in a lower discharge rate. Keep your marine battery in a controlled environment where possible. If your boat is in dry dock during extreme weather, ensure the battery is housed in a moderate temperature environment.

Charging Profiles

Each battery has its own recharging specifications. Putting yours on overly high voltage for a quick charge is likely to cause an early end to its service life. High voltages cause sulfation, which displaces the elements that make the internal plates powerful. High voltages also speed up corrosion and increase discharge rates. Follow the charging guidelines provided by the manufacturer, adhering to the required voltage and avoiding overcharging the unit.

Storage

Storing batteries that are not clean can lead to unnecessary corrosion, and storing them when they are not fully charged can cause them to lose the ability to fully recharge later. When storing a battery over several months, ensure that it has been adequately cleaned and maintains a full charge. Store marine batteries in a dry, temperature-controlled environment. If you live in a cold climate, store your batteries indoors, hooked up to a battery minder to keep them topped off.

Battery Type

Using the wrong battery type for your boat's application can negatively impact its lifespan. For example, a deep cycle battery vs a cranking battery for a trolling motor. When choosing a marine battery, it is important to set up a new boating system with the best battery for each application. Do not mix AGM and flooded batteries in a parallel configuration, such as a trolling motor.

Installation

Poor installation can lead to acid leaks, fire, and even battery explosion. Loose cables or poorly maintained posts can also reduce a battery's life expectancy. Mount the batteries in a properly sized battery tray and use straps to prevent sliding around in the compartment. Use cable boots and nuts to secure the power leads to the battery terminals. All batteries should be secured in an acid-proof box or tray to contain spills and isolate the battery from physical harm.

Terminals

To get the most power from your battery, it is essential that the connections to the terminal posts are clean and well made. Poor connections will increase electrical resistance, causing a voltage drop which may result in electrical equipment not working as designed, or at all. The simplest thing you can do to maintain a battery is to clean away any corrosion with a solution of thick baking soda and water.

Frequently asked questions