Understanding Dual Battery Systems In Boats

Boats often have two batteries to ensure they have a reserve bank to start the engine, or to have a large house bank to run bigger DC loads for longer time periods. The primary reason to use two batteries is to have the ability to hold one in reserve in case of difficulties on the water. Many boats are set up with two battery systems that use a starting battery and another battery sometimes called a house battery, or the electronics battery. The starting battery is used to start the boat's engine and is not designed for extended, continuous use. The house battery is used to power electronics and accessories.

The two types of boat power requirements

Every boat has two types of power requirements. The first is the starting load, which is the amount of power required to start the boat's engine. The second is the running load, also referred to as the house load, which is the amount of electrical power the boat requires to run its electronics while on the water.

Starting Load

The starting load requires a battery that can deliver large amounts of current (75 to 400 Amperes) in a short time (less than 15 seconds) to start the boat's engine. These batteries are known as starting batteries and are specifically designed for this purpose. They can be damaged if used for longer periods.

Running Load

The running load is the power required to run the boat's electronics, such as GPS, trim tab motors, lighting, and other electrical equipment. This type of power requirement is typically served by a deep-cycle battery, also known as a house battery. Deep-cycle batteries can withstand deep discharge cycles without compromising their lifespan, making them ideal for powering electronics over extended periods.

Dual-Purpose Batteries

In some cases, boats may use dual-purpose batteries, which can serve both the starting and running load requirements. These batteries have active lead paste chemistry and thicker, larger plates containing more antimony than other battery types. While dual-purpose batteries offer space and cost savings, they are generally more expensive and have lower power capacity than similarly-sized deep-cycle batteries.

Multiple Battery Banks

Larger boats may have two or more house batteries or battery banks. This setup helps reduce voltage drop, which occurs when power has to travel over long distances. Multiple battery banks can be structured and placed strategically to power specific components in different parts of the boat.

Trolling Motors

Boats with trolling motors typically require two to three deep-cycle batteries. Unlike gasoline-powered engines, trolling motors rely solely on battery power, and therefore, require a larger number of batteries to ensure sufficient power throughout the journey.

The role of a starting battery

Starting batteries have very thin lead plates inside with a large surface area, and the plates are very close together. This design allows for a large but short burst of amperage to start the engine. When discharged, starter batteries will begin to build up lead sulphate on the plates, which crystallizes and acts as a barrier to charging and discharging. This is why they are not suitable for deep-cycle applications.

The number of batteries a boat requires depends on its size and energy requirements. Boats under 14 feet require just one battery, while boats 15-23 feet with a single engine will require two batteries. Boats with two or more engines will require a separate starting battery for each engine, plus an additional house battery to run the electronics.

It is important to keep the starting battery and the house battery separate. If the starting battery is used to power electronics, it can be drained, leaving you stranded and unable to start the engine.

The role of a house battery

Deep-cycle batteries are ideal for electronics, trolling motors, and appliances. They are used for all other battery-operated needs when on the water. These include GPS systems, fish finders, and onboard appliances that require a continuous power supply over long periods.

House batteries are typically marine deep-cycle batteries or even golf cart batteries, which are also rugged and deep-cycle. They can be discharged to a greater extent than starter batteries and recharged multiple times, making them more suitable for long-term power needs.

A boat's house battery bank may consist of multiple batteries or banks, especially on larger boats. This setup helps to reduce voltage drop and ensures that items near the bow and stern are adequately powered.

When selecting a house battery, it is important to consider the boat's energy requirements and size. A good rule of thumb is to have 3-4 times your daily energy consumption in battery capacity to avoid deep discharges and reduce recharging time.

The benefits of dual batteries

Boats typically have two or three batteries to serve as separate "house" and "starter" batteries. A dual-battery system offers several advantages for boat owners. Firstly, it provides redundant starting capability. In other words, if one battery fails or gets drained, the other can be used to start the engine. This backup ensures that you're not left stranded somewhere on the water, unable to start your boat's engine.

Secondly, dual batteries allow for longer periods of running electronics, lights, and stereos. This is particularly beneficial for boats with high accessory demands, such as those used for fishing, which may require additional power for equipment like a trolling motor, a DC mini-fridge, or a fan.

Additionally, having separate batteries for starting the engine and powering the boat's electronics helps to prevent overloading the motors. It also improves energy efficiency, especially if you have multiple batteries to power your boat.

Another benefit of dual batteries is the ability to isolate the starting battery from the house battery. This means that even if you turn off the engine and continue to use the electronics, you won't accidentally drain the starting battery. This ensures that you'll always be able to start the engine and get back home safely.

Lastly, dual-purpose batteries can save space and weight on smaller boats. Instead of having two separate batteries, a dual-purpose battery can be used for both starting the engine and powering electronics, making it a versatile and space-saving option for boats with limited room.

Charging multiple batteries

Onboard vs Portable Chargers

Firstly, you can choose between onboard and portable chargers. Onboard chargers are permanently installed and connected directly to the boat's electrical system, offering automatic charging by monitoring battery conditions and adjusting the charging rate. Portable chargers, on the other hand, are versatile and can be carried on board or used on land. They are preferred when flexibility and mobility are crucial, such as when boating far from shore power.

Charging Methods for Onboard Systems

For onboard charging, there are several methods:

• Shore Power Charging: This is the most common method when docked at a marina. It involves using a battery charger designed for marine use and a shore power connection.
• Offshore Charging: This method uses onboard generators or alternative power sources like solar power systems to charge batteries when away from the mainland.
• Alternator Charging: The boat's engine alternator can charge the batteries while on the water. However, it primarily maintains the starting battery's charge and may not fully recharge a deeply discharged battery.

Battery Configurations

When charging multiple batteries, the configuration matters:

• Series Connection: In this setup, batteries are connected in series, adding their voltages together. For example, two 12-volt batteries in series result in a total voltage of 24 volts. In this case, a charger compatible with the higher voltage is required.
• Parallel Connection: Batteries connected in parallel have the same voltage but increased capacity. For instance, two 12-volt batteries in parallel will have double the capacity. The charger should handle the combined amperage needs in this case.

When it comes to charging multiple batteries with a dual battery switch, there are a few considerations:

• Using an External Charger: It is recommended to charge the batteries one at a time with the switch in the "off" position to avoid potential damage to electrical components. Alternatively, a dual bank marine battery charger can be installed permanently, allowing simultaneous charging of both batteries when docked.
• Charging with the Engine Alternator: The alternator charges the batteries according to the switch position. To charge them with the engine, charge one battery at a time or put the switch in the "both" position in emergencies.

Other Options for Charging Multiple Batteries

There are also other options for charging multiple battery banks:

• Battery Switches: This traditional method uses a switch to select one or both batteries. However, it relies on the user remembering to switch between batteries.
• Split-Charge Diode: This method uses diodes to split the charge and deliver it to two isolated batteries. While a 'fit and forget' solution, it may result in a volt drop, preventing batteries from being fully charged.
• Twin Alternators: Installing an additional alternator provides a dedicated means of charging the start battery and redundancy in case of battery or alternator failure.
• Voltage-Sensitive Relays (VSRs): VSRs keep battery banks isolated when not charging but use a high-current relay to connect them when charge is available. They can combine batteries for charging and isolate them when the engine is off.

Frequently asked questions