Wiring Dual Battery Switches: A Boat Owner's Guide

Wiring dual batteries on a boat can be done in several ways, depending on the boat owner's preferences and requirements. One common method is to use a 1/2/BOTH battery switch, which allows for bank isolation, cross-connection use, and ease of use. This type of switch enables boat owners to select between battery banks and direct charging to a specific bank. However, it is essential to understand the system to avoid issues such as unnecessary switching, human error, and voltage transients. Another approach is to use multiple on-off switches, which provide more control over battery charging, especially in boats with multiple engines and batteries. In any case, consulting a certified marine electrician is recommended to ensure safe and proper wiring.

Understanding your boat's electrical system

The electrical system of a boat is what holds the entire vessel together. Almost every system on the boat depends on the electrical system to function. Therefore, it is important to understand how it works to be able to troubleshoot common problems and ensure safety.

The Basics

Your boat is designed to run on direct current (DC). Most boats operate on 12 Volts direct current, or 12VDC. However, some boats may operate on 32VDC or 24VDC, especially those manufactured outside the US. The 12VDC battery delivers power to the DC load of the boat, which includes lights, electronic equipment, pumps, and any other system requiring DC power. The boat motor has an alternator attached to it that recharges the DC battery. This is the basic system found on most small boats.

Variations

Some boats have variations of this simple system, including one or more batteries selected by a battery switch. One battery can be used to start and run the boat, called the engine battery, while the second battery, or the "house battery," provides all the DC requirements when the motor is off, such as the anchor light or galley lights.

Using the System

When the engine is off, you can turn the battery switch to the house battery position and operate the DC systems from that battery. When you're ready to start the engine, switch to the engine battery position and start the engine. Once the engine has warmed up, you can switch the battery switch to "both," allowing the engine to charge both batteries as needed.

Recharging Batteries

Recharging batteries is a major concern for boaters, especially those on longer cruises. While some boaters add wind generators and solar panels, others opt for a battery charger to recharge the batteries from shore power.

AC Equipment

Many modern conveniences, such as TVs, microwaves, and computers, operate on alternating current (AC). In the US, the standard is 60 Hz (cycles per second) 120 Volt single-phase power. Some larger vessels with bigger AC appliances also require 240VAC two-phase power. Foreign countries may use different standards, such as 240VAC 60 Hz or 50 Hz single-phase power.

Shore Power

When a boat is equipped with shore power, it can be connected to a power source at the dock, allowing the battery to stay charged while the boat is used. Additionally, AC can be directed to receptacles throughout the boat, enabling the use of various electrical appliances.

AC Generator

The addition of an AC generator allows boaters to cruise and use all AC equipment on the boat, such as air conditioning or a washer/dryer, without relying solely on shore power. The generator also powers the DC Converter, keeping the boat's batteries charged for DC equipment.

Inverter

An inverter is a device that converts 12VDC to 120VAC single-phase at 60Hz, allowing the use of AC equipment without running a generator or being connected to shore power. This is particularly useful for anchoring and extended periods away from the dock. However, it's important to note that the inverter has a practical limit on how much current it can handle, and running too many AC appliances simultaneously can overload it.

Using a Perko Battery On-Off Switch

For boats with multiple power sources, using multiple on-off switches, sometimes in conjunction with a selector switch, can provide different benefits. For example, if you have two batteries and want to charge one of them specifically, you can add an on-off switch to the circuit to force the alternator to charge the desired battery. This setup can also be useful in emergencies when one battery is substantially discharged, as it allows you to put batteries in parallel for emergency starting.

Perko battery switches are designed for surface mounting and are easy to install. The standard-duty switches feature a bright red polycarbonate housing for safety, with a large, easy-to-turn black knob. Heavy-duty switches have a red fiber-reinforced polycarbonate housing. An optional key lock helps prevent unauthorized use of the vessel.

Models with an "Alternator Field Disconnect" feature offer protection for Non-Unitized Alternators in the event the switch is accidentally turned off while the engine is running. Perko's battery selector switches are also UL-listed, ignition-protected, and CE-approved, ensuring they can be used safely in bilges where explosive fumes may be present.

Wiring diagrams for dual batteries

One forum user has shared a detailed description of their setup, which involves using a BEP Marine 716SQ cluster with a voltage-sensitive relay. They also explain the goal of their setup, which is to ensure that there is always enough battery power to start the engine. This is achieved by having both batteries charging simultaneously but only one battery drawing power. This setup requires a Schottky diode, which can be called an Eliminator, Voltage Sensitive Relay, or Combiner, among other names. The user also mentions that BEP Marine offers a setup that combines a battery switch with a diode, which solves all charging and standby battery issues. This setup is more expensive but includes a wiring diagram.

Another user has provided a similar suggestion, recommending a setup with a starting battery and a house battery. This setup involves having common grounds for all but separate positives. They also mention that an isolation diode, or battery isolator, can be used, but there is a 1-volt drop across the poles, which can cause issues with the alternator's sense voltage.

While these forum posts do not include specific wiring diagrams, they do provide valuable insights and recommendations for dual-battery setups. It is likely that further posts in the thread contain more specific diagrams or instructions, which could be useful for those seeking to wire dual batteries on a boat.

Battery switch considerations

Battery management, including switching and charging, is essential for the safe and reliable operation of a boat. When considering how to wire a dual battery switch, it is important to keep in mind the following:

• Bank Isolation: The ability to isolate a battery bank in the event of a failure is crucial. This ensures that one failed battery does not affect the entire system.
• Cross-Connection Use: The ability to use either on-board battery bank as the sole use bank is important, especially in emergencies. This means that either battery can serve as both the starting and house load bank if needed.
• Ease of Use: A battery switching design should be easy to understand and use for the boat owner. A complex or confusing system may lead to human error and potential safety hazards.
• Battery Size and Type: Ensure that your batteries are sufficiently sized to start your engine and that you are using the appropriate type of battery for your specific application. Deep-cycle batteries, for example, can be used for cranking, but it is important to consider their capacity and the demands of your engine.
• Redundancy and Isolation: Consider the level of redundancy and isolation you require in your system. Some switches, like the 1/2/B switch, offer more redundancy and isolation than other configurations.
• Charge Management: Adding a charge management device, such as an Automatic Charging Relay (ACR), can automate the charging of both battery banks and eliminate the need for manual switching.
• Alternator Protection: To protect your alternator from load-dumps, consider installing an Alternator Protection Device (APD). This will limit voltage spikes and protect sensitive electronics connected to the system.
• Fuses and Circuit Protection: Proper fusing is essential to ensure the safety and protection of your electrical system. Consult ABYC standards and a qualified marine electrician to determine the appropriate fuse sizes and circuit protection for your setup.
• Emergency Scenarios: Plan for potential emergency situations. Ensure that your battery switching system allows you to isolate a failed bank and use the remaining bank for all functions if needed.
• Simplicity: While having a flexible and feature-rich system is important, keeping it simple will make it easier to understand, use, and maintain.

Isolating a failed battery

When a battery fails, it can be due to a bad cell, which will prevent the entire system from fully charging. In such cases, isolating a failed battery is a good idea.

One way to isolate a starter battery from the house bank is to remove the jumper on the battery switch panel. This will prevent the starter battery from draining. However, it also raises the question of how to recharge the starter battery. One solution is to run the alternator output directly to the house bank and then add an Echo Charger between the house and starter banks. This will allow the starter bank to be charged without combining the two banks.

Another option is to use a starter solenoid from an auto-parts store. This can be mounted in a protected area to keep salt off. Both battery banks can be run to it for the big wires, and the wire that flips the switch can be tied to the key for the engine. So, whenever the engine is on, the banks are connected for charging, and when the engine is turned off, the banks are separated.

A third option is to install the starter battery in a locker with the cable needed to hook it into position B on the switch unconnected. Keep the switch in the "both" position, which will result in all batteries being hooked up in parallel for all purposes (house and start). The starter battery can then be charged periodically when on shore power with a separate charger. If the starter battery is ever needed, the plan is to disconnect battery B and connect the starter battery. Then change the switch to B only, so the starter battery doesn't drain into the house batteries.

Frequently asked questions