Wiring A Boat Battery Switch: A Step-By-Step Guide

Wiring a boat battery switch is a complex task that requires careful consideration of the boat's electrical system and battery configuration. The process involves selecting an appropriate battery switch, such as a Perko switch or a 1/2/BOTH switch, and understanding the battery layout, which could include a single engine with one starting and one house battery or twin engines with two starting and one house battery. It is crucial to ensure proper circuit protection and follow safety measures as batteries can store a large amount of energy. The wiring process involves connecting the battery positives to the switch, bypassing the main battery switch for specific loads like the bilge pump, and running the power cables to the switch panel and bus bar.

Supplies needed

• A battery switch, such as a Perko battery switch, which has complete directions for installation.
• Screws to mount the switch.
• Two positive battery cables that are long enough to reach from the switch to each battery.
• One negative battery cable that is long enough to reach from the battery negative terminal to the battery negative terminal. It is strongly recommended to use tinned marine cables that are properly crimped and heat shrink sealed.
• A location that is easily reached, not subject to immersion, and minimises battery cable lengths (the shorter the better).
• A manual-off-auto bilge switch.
• A key lock to prevent unauthorised use of the vessel (optional).
• A bus bar to run all negatives to (optional).
• A fuse block and holders.
• Inline fuses for the bilge pump.
• A Blue Sea ACR (optional).
• A Sterling Power Alternator Protection Device (optional).
• A Blue Sea Systems ON/OFF battery switch (optional).

Battery switch wiring diagrams

Single Engine, Single Battery Diagram

This setup is common for smaller boats with a single engine and one battery. The battery positive connects to the battery switch, which then leads to the engine. The battery negative is grounded to the boat's chassis. This configuration allows turning the battery on or off using the switch.

Single Engine, Two Batteries

This setup is often used for boats with a single engine but two batteries, typically a starting battery and a deep cycle or "house" battery. The positives of both batteries connect to a 1-2-BOTH type battery switch, which then leads to the engine. The negatives of both batteries are grounded to the chassis. This setup allows selecting which battery to use or paralleling them for emergency starting.

Two Engines, Two Batteries, Two Switches

This diagram illustrates a system for boats with two engines and two batteries. Each battery positive connects to its own battery switch, and the switches' outputs are joined before leading to both engines. This configuration provides independence for each engine's electrical system and the ability to start either engine from either battery.

Two Engines, Two Engine Batteries, Separate Lighting Batteries, and Two Switches

This diagram is similar to the previous one but includes a separate set of batteries for lighting. Each engine battery has its own switch, and the lighting batteries are connected in parallel to provide longer-lasting power for lighting fixtures.

Dual Battery Setup with Isolator

This diagram showcases a dual battery setup with an isolator for a boat, ensuring the main battery is isolated from the auxiliary. The isolator allows charging of both batteries while keeping them independent, preventing the depletion of both in an emergency.

Battery switch bypass loads

A battery cell bypass switch is used to protect battery assemblies in the event that one battery cell fails. When activated, the switch bypasses and isolates the failed cell from the battery assembly. This is known as a 'safe failure to mains' and occurs when there is an internal fault or failure with the UPS system.

The bypass switch is always in-circuit and is rated to carry a high continuous current for the duration of the mission. The switch has a 'Make-Before-Break' functionality, which ensures there is no voltage dropout during switching. The low switch contact resistance assures high peak current-carrying capability.

The bypass switch can be used in the following applications:

• Launch vehicle batteries
• Manned vehicle batteries
• Space platforms and instruments
• Scientific landers and rovers

Getting the source to the boat's helm

Now that you have your batteries and battery switch set up, the next step is to get the power from the house battery up to the switch panel. This will allow you to use the power to operate your boat's systems.

To do this, you will need to run two conductors from the battery switch to the switch panel. One will be a positive conductor from the battery switch, with a fuse, and the other will be a negative conductor from the ganged-together battery negatives. You should use marine-grade primary wire for this.

This wiring run can be quite long on a boat, and these two conductors will carry the current of all your electrical loads combined, so they need to be fairly thick cables. For a small boat with 3-5 loads, you will need at least 12AWG wire. For larger boats with 5-10 loads, 10AWG wire is standard. 8AWG wire is overkill for boats under 30ft.

The longer the wiring run from the battery to the switch panel, the more voltage drop you will have, so it is important to use larger cables to prevent voltage drop.

The power cables will be connected to your switch panel and negative bus bar. Most switch panels include waterproof resettable circuit breakers, which make installation easier.

The main house battery positive conductor will feed directly into the switch panel, and the main battery negative conductor will go to the negative bus bar, where all the boat's load negatives will eventually be attached.

Installing a terminal block as a breakout point

A terminal block is a great way to create a breakout point for your boat's wiring. This is where you'll connect the wires from your switch panel to the various electrical loads around your boat.

First, you'll want to get your hands on a terminal block like this one. Then, you'll need to connect the positive wires from your switch panel to the terminal block. Make sure each switch output has its own gang on the terminal block. This way, you can easily identify which positive wire goes to which load.

Next, you'll want to crimp a #8 ring terminal on the positive load wiring that runs out to the various electrical components around your boat. This will make it easy to connect the positive wires from the switch panel to the terminal block.

Once all the positive wires are connected, you can move on to the negative wires. The negative wires will connect to a negative bus bar, which is different from a terminal block because all the screws are "bussed" together. This means that the negative wires can be connected to any screw on the bus bar since they are all trying to get back to the negative post on the battery.

By using a terminal block as a breakout point, you'll have a handy, labelled location for troubleshooting or adding new electrical components in the future.

Frequently asked questions