Understanding Dual Battery Systems In Boats

Many boats have two battery switches, and there are several reasons for this. Firstly, it is essential to have a way to control the flow of electricity between the battery and the boat's systems, ensuring safe and efficient power distribution. The two switches likely consist of an ON-OFF switch and a 1-2-BOTH switch or an ON-OFF-COMBINE switch. The ON-OFF switch is straightforward and easy to use, while the 1-2-BOTH switch offers more flexibility by allowing the user to choose between batteries or combine their power. The ON-OFF-COMBINE switch provides even greater control and flexibility by allowing the user to choose which battery to use and when. Additionally, having two battery switches can prevent the engine batteries from being drained by house loads. This setup isolates the engine battery, leaving it with the sole purpose of starting and running the engine. It also ensures that the engine battery receives a charge from the alternator when the engine is running.

To isolate a battery bank in the event of failure

Having two battery switches on a boat is important for isolating a battery bank in the event of failure. The 1/2/ALL switch, also known as the 1/2/BOTH switch, is a common factory wiring configuration that allows boat owners to isolate a battery bank from both loads and charge sources in case of a failure. This means that if one battery fails, the other can be used as a backup to start the engine and power the boat's systems.

The 1/2/ALL switch has three terminals (Bank 1, Bank 2, and "C" Post) and four positions (1, 2, ALL, and OFF). In the OFF position, both battery banks are isolated and turned off. In positions 1 or 2, the corresponding battery bank is used to power the boat's systems and receive charge from the alternator. In the ALL position, both battery banks are connected in parallel and can be charged simultaneously.

It is important to note that even when the switch is in the OFF position, certain essential systems such as the bilge pump, propane sniffer, stereo memory, or VHF may still be directly wired to one of the battery banks. This ensures that critical functions remain operational even if the batteries are isolated.

To prevent battery failure, it is crucial to understand the proper use of the 1/2/ALL switch and to regularly maintain and charge the batteries. By following the manufacturer's recommendations and best practices, boat owners can minimize the risk of battery-related issues and ensure a reliable power source for their vessel.

To use either battery bank as the sole power source in an emergency

Having two battery banks on a boat is very common. This allows you to run lights, instruments, stereos, fridges, and so on, and still be able to start the engine the next morning. However, this requires a battery switching system.

The traditional way to switch between batteries is with a switch that allows you to select one or both batteries. This has been effective for decades, but it relies on the user remembering to switch it to one battery to start the engine and back to the house battery when the engine is turned off. Forgetting to switch it back can result in two flat batteries.

The 1/2/ALL switch is a very common factory wiring configuration. It is a versatile and redundant single-switch battery selector. It is also the number one selling multi-bank switch. However, it can be confusing for boat owners and lead to human error. Mishaps and human error can occur when an owner forgets to manually switch and charge one bank, or leaves it on BOTH, running both banks completely dead.

The 1/2/BOTH switch can be made easier and more foolproof by converting it to a USE switch. This makes it a simple ON/OFF scenario. This is a minimal-cost change and can be done by adding a Charge Management Device, such as an Automatic Charging Relay (ACR). This will provide for fully automated charging of both banks.

In an emergency, you can use the vessels' second bank for everything. This is done by pulling the house or start bank fuse. The removal of the fuse allows you to 100% isolate the bad bank yet keep alternator charging with the now "reserve" bank.

To prevent human error and misunderstandings

The typical main battery switch has four positions: OFF, 1, 2, and ALL (or BOTH). The 1/2/ALL switch is a very simple mechanism that, when used properly, can save you time, money, and stress. However, it can be confusing for some boaters, leading to human error and misunderstandings.

The 1/2/ALL switch allows the user to choose between one of two batteries or both batteries combined to power the boat's systems. In the OFF position, the switch is disconnected from both batteries, preventing any electrical current from flowing. Position 1 is usually assigned to the start battery, providing power to start the engine and recharging the battery while the engine is running. Position 2 is typically designated for the house bank, powering onboard appliances and charging the house bank through the alternator. The ALL (or BOTH) position enables the use or charging of all batteries simultaneously, but this setting can deplete the batteries entirely, leaving you unable to start the engine.

• Only use one battery at a time. Alternate between Battery #1 and Battery #2 on odd and even days to ensure both batteries are used equally and fully charged.
• NEVER use the "ALL" or "BOTH" position except to parallel both batteries if they are flat, similar to jump-starting your engine.
• When the engine is running, set the switch to position 1 (start battery) to recharge the start battery. After 20 minutes, shift to position 2 to recharge the house bank.
• Be cautious of the "ALL" or "BOTH" setting, especially when anchored and using amenities such as the stereo and lighting, as it can lead to depletion of all your batteries.
• Understand the functions of the battery switch and adopt best practices to enhance efficiency and safety.
• Regularly maintain your batteries by inspecting for corrosion, keeping them charged, monitoring electrolyte levels, securing them properly, and considering a battery monitor.
• When the boat is not in use, ensure the switch is in the OFF position to prevent battery drain and potential damage to electrical components.
• Consult a professional if you are unsure about any aspect of the installation or use of your boat battery switch.

By following these guidelines, you can prevent human error and misunderstandings related to the 1/2/ALL battery switch and ensure the safe and efficient operation of your boat's electrical systems.

To prevent alternator failure

Boats with two battery switches are often equipped with a Dual Circuit Plus battery switch (OFF-ON-COMBINE) or an OFF-1-2-BOTH switch. These switches allow the operator to control the flow of electrical power between multiple batteries, isolating the engine battery from the house battery bank. This isolation ensures that the engine battery is only used for starting and running the engine, while the house battery bank powers onboard appliances.

To mitigate this issue, some switches are designed with alternator field disconnect protection. This feature allows the switch to disconnect power to the alternator's field windings, cutting its output before the battery connection is lost. By preventing the alternator from being electrically disconnected from the batteries while the engine is running, this feature safeguards against irreversible alternator damage caused by the power having nowhere to go.

Additionally, some boats employ Automatic Charging Relays (ACRs) or Voltage Sense Relays (VSRs). These devices automatically connect two batteries in parallel when a charging voltage is detected, allowing for direct connection between the alternator and the house battery bank, which typically requires the most charge. This setup reduces the risk of alternator failure by optimising the charging process and minimising human error.

Furthermore, it is essential to familiarise yourself with the operation and wiring of your boat's battery switches. Understanding how your system works can help you make informed decisions and avoid accidental alternator disconnection, ultimately preventing alternator failure.

To isolate an engine battery from the house bank

The Dual Circuit Plus battery switch is designed to isolate an engine battery from the house bank. When the switch is turned to ON, the engine battery is connected to the engine and the house bank is connected to the house loads. The engine battery and house bank are completely separated so that all loads are connected to their respective batteries only.

The Dual Circuit Plus switch differs from the more common OFF-1-2-BOTH switch. With the OFF-1-2-BOTH switch, there is no separation between engine and house loads. All loads are always powered by whatever battery is selected, either 1, 2 or both. As a result, the operator cannot assign battery 1 to the engine only or battery 2 to the house only, as with the Dual Circuit Plus battery switch.

Using a Dual Circuit Plus battery switch prevents the possibility of draining the engine batteries with house loads. In effect, the engine battery is left with only one purpose, to start and run the engine. When the switch is in the ON position, the engine battery is connected to the starter and the house loads to the house battery bank. The most common method for connecting the alternator is to have it wired directly to the starter post. The alternator can be wired in a few other scenarios, but for now, let's assume it is wired directly to the starter post.

Whatever battery runs the engine will also receive the charge from the alternator. When the switch is turned to COMBINE, the alternators will charge both the engine and the house batteries. While underway, turn the switch to COMBINE. The moment the engine stops, make sure that the switch is turned to ON. If left on COMBINE while not running the engine, your house loads will drain both the house and the engine battery. Consequently, if the batteries are drained too much, it’s possible that neither engine nor house will be able to start the engine.

As a boat operator, you should be curious as to how your alternator is wired aboard your boat. The common practice of wiring the alternator directly to the starter is the easiest setup for the engine manufacturer but does provide two drawbacks.

The first drawback is the possibility of disconnecting your alternator from the batteries while the engine is running. This is a mistake we see when a boat owner attempts to switch battery banks and passes the battery switch through the OFF position while the engine is running and the batteries are charging from the alternator. With the alternator suddenly disconnected from the battery, the output of the alternator has nowhere to go and the alternator suffers from a catastrophic failure.

The second drawback is that the battery that requires the most charge on a boat is the house battery and yet the alternator is connected to the starter, which in turn is connected to the engine battery. The engine uses a lot of power to start, but once running only needs a modest amount of power to recharge and operate the engine. While underway, the engine battery is quickly topped off and the alternator limits its output to match the draw. Consequently, you have an alternator that is throttled and only able to output a modest amount of power.

To offset those drawbacks, boat electrical designers have introduced two different alternator set-ups.

Frequently asked questions