Have you ever thought about how much power gets wasted while driving down the road? Most caravanners and RVers don't. There are a few, however, who have made the effort to connect their high-current appliances to both the alternator and the leisure battery. This allows these appliances to run from the alternator output while driving, then automatically switch to the leisure battery while parked.
The best part of such a setup is that you'll never need to manually switch between the two DC power supplies. A 5-pin power relay enables your load circuit to automatically switch whenever you turn your vehicle on or off.
In today's DIY guide, we'll walk you through 12 easy steps on how to create a relay switch that automatically powers your high-current appliances from either the starter battery or the leisure battery. This ensures that fridge, water heater, air conditioner, or steam cooker serves its purpose without risking draining the starter battery.
4 Pin vs 5 Pin Relay: What's the Difference?
The most common application of a relay is controlling power to a single circuit β whether it's a 4-pin or 5-pin relay. However, with its single-pole double-throw (SPDT) configuration, a 5-pin relay can control two separate circuit paths through its NO (normally open) and NC (normally closed) terminal pins.
If you connect your appliance or accessory to the NO terminal pin, it will only turn on when you switch the relay's input power "on." When you switch the relay's input power "off," the connected appliance or accessory will also turn off.
When an appliance or accessory is connected to the NC terminal pin, it remains on whenever the relay's input power is "off." Once you switch the relay's input power "on," the connected appliance or accessory turns off.
As an electronic switch, a relay works by controlling the switching armature attached to the COM terminal. When the relay is off (with no power flowing through pin 86), the electromechanical coil remains inactive, and the switching armature rests on the NC pin. In this position, the circuit path is complete, allowing electricity to flow naturally through it and its connected load.
As soon as you switch the relay's input power "on," the coil energises and activates the electromagnet, pulling the switching armature away from the NC pin. This breaks the NC circuit path, turning both the circuit and its connected load off.
At the same time, the relay's electromagnet pulls the switching armature away from the NC pin and pushes it toward the NO pin. Once the switching armature contacts the NO pin, its circuit path is completed, allowing electricity to flow and power the connected load.
How You Set Up Your 5 Pin Relay Wiring Matters
A 5-pin relay has terminal pins 86, 85, 30, 87a, and 87. In all standard 5-pin relays, pin 30 is the common terminal that controls power flow to pins 87a and 87. Hence, the reason we call it the common (COM) pin.
How you wire a relay in your electrical setup determines how power is controlled to your load. One setup can have pin 30 powering the load through terminal 87a or 87, while another can have terminal 87a or 87 powering the load through pin 30.
Two-load switching from a shared power source
In a project where you want to use your 5-pin relay to control the switching on/off of two separate loads, pin 30 powers the loads. The switching armature controls whether power goes to load-1 (via pin 87) or load-2 (via pin 87a).
When the relay is offline, pin 30 sends electricity from the power source to pin 87a, turning on load-2. Immediately the relay comes online, pin 30 redirects power to pin 87, turning on load-1.
Once you switch off the relay, pin 30 goes back to feeding pin 87a and its connected load.
Single-load switching from dual power sources
When you want to use your 5-pin relay to switch power between two separate sources, pin 30 becomes the connection bridge to the load. The switching armature controls whether power comes from pin 87 or pin 87a.
When the relay is offline, pin 30 receives power from pin 87a (connected to power source 1), turning on the load. Immediately the relay comes online, pin 30 then receives power from pin 87 (connected to power source 2), keeping the load powered.
Once you switch off the relay, pin 30 and the load go back to receiving power from pin 87a and power source 1.
How to Create an Automatic Dual-Power Supply Switch with a 5 Pin Relay
In today's DIY hack, the idea was to guide you on how to use a 5-pin automotive relay to create a switching system for a high-current load connected to two separate power sources. It's a simple wiring setup that enables you to power your appliance or accessory from the alternator (starter battery) while driving, and then switch to the leisure battery or solar power when parked.
Therefore, our wiring setup should follow the "single-load switching from dual power sources" configuration.
What you'll need
- 5-pin automotive relay (for switching)
- 1.5mmΒ² wires (for relay's input side)
- 6.0mmΒ² cables (for short run to load and battery)
- 1.5mmΒ² butt (splice) connectors for relay's inputs
- 1.5mmΒ² ring terminal connector for relay's ground
- 30A inline fuse holders + fuse (to protect the NO wiring)
- 25A inline fuse holders + fuse (to protect the NC wiring)
- A 5A blade fuse (to connect and protect relay's input side)
- A blade piggyback tap fuse holder (for powering the relay)
- 6.0mmΒ² step-down butt splice cable joiner to connect the thin and thick cables
Step 1: Prepare Wire 1
Crimp a butt connector onto one end of a 1β3 metre, 1.5mmΒ² wire (Wire1). Then, crimp the other end to the butt connector on the tail lead of the piggyback tap fuse holder.
Step 2: Prepare Wire 2
Get a second 1β3 metre, 1.5mmΒ² wire (Wire2) and crimp a butt connector on one end and a ring terminal on the other.
Step 3: Connect the ground input of the relay
Take the Wire2 you just prepared and crimp its butt-connector end to the pre-wired lead for pin 85 of your new 5-pin relay. Then, attach the ring terminal on the other end of Wire2 to a common ground point.
Step 4: Connect the positive input of the relay
Crimp the pre-wired lead for pin 86 onto the butt connector you added to Wire1 in Step 1. Your new pin 86 connection should now have a piggyback fuse holder on the other end.
Step 5: Make the NC connection
Next, take a 1β3 metre, 6.0mmΒ² cable (Cable1) and crimp a butt splice connector onto one end. Then, add the pre-wired lead for pin 87a to this butt connector.
Step 6: Power up the NC terminal
On the other end of Cable1, crimp a 25A inline fuse holder. Then, connect the other end of the inline fuse holder wire to the leisure battery busbar using a ring terminal.
Note: Make sure the negative terminal of your leisure battery is connected to the common ground point.
Step 7: Make the NO connection
Take another 1β3 metre, 6.0mmΒ² cable (Cable2) and crimp a butt splice connector onto one end. Then, join the pre-wired lead for pin 87 to this butt connector.
Step 8: Power up the NO connection
On the other end of Cable2, crimp a 30A inline fuse holder. Then, connect the other end of the inline fuse holder wire to the starter battery busbar using a ring terminal.
Note: The negative terminal of your starter battery must also be connected to the common ground point for the setup to work.
Step 9: Make the COM connection
Get a third 0.5β2 metre, 6.0mmΒ² cable (Cable3) and crimp butt connectors onto both ends. Then, connect the pre-wired lead for pin 30 to one of the butt connectors.
Step 10: Hook up your load
Attach the positive wire of your accessory or appliance to the butt connector on the other end of Cable3. Then, connect the negative wire of your load to the common ground point.
Step 11: Identify your ignition-on fuse
Now, identify a low-current, non-critical IGN (ignition-switched) fuse slot β such as a 5A, 7.5A, or 10A fuse β that your relay input can tap into for power.
Remove the fuse currently installed in that socket and insert it into the bottom slot of the piggyback fuse holder connected to the relay.
Step 12: Power up the relay
Take the 5A blade fuse you got for the relay and insert it into the top socket of the piggyback fuse holder. Then, plug the piggyback fuse holder into the non-critical IGN fuse slot you selected in Step 11.
Congratulations, you've successfully created an automatic switching system for the dual-power supply source of your load. You now only need to switch on the main isolator switch for your leisure battery (if available), and the system will be ready for use.
How the DIY Dual-Power Supply Automatic Switch Works
Scenario 1: Powering via Pin 87a
When your vehicle's ignition is "off," the relay will also be off, with the switching armature naturally resting on the Normally Closed (NC) pin 87a. This completes the circuit loop from the leisure battery, allowing current to flow to the common (COM) pin 30 and power the connected load. The current will then flow through the load and return to the leisure battery via the common ground point on the chassis.
Scenario 2: Powering via Pin 87
When you turn your vehicle's key to the IGN (ignition) position, the relay activates, energising its electromagnet. The electromagnet then pushes the switching arm to pin 87, completing the circuit loop from the starter battery. With the circuit complete, current now flows to the (COM) pin 30, powering the load before returning to the starter battery through the common chassis ground.
When you switch the ignition "off," the relay also turns off, de-energising the magnetic core. Pin 87a then returns to its resting position, reconnecting the (COM) pin 30 to the leisure battery.
Create an Automatic Leisure Battery β Solar Charge Controller Relay Switch
If you have always wanted a simple solution for automatic power switching between a starter battery and a leisure battery, this is how you can achieve it. With this relay setup, you never have to manually switch power to your fridge, air conditioner, or any other load.
Keep in mind that this setup is not limited to a leisure battery and starter battery combination only. You can replace the starter battery connection with wires from an MPPT solar charger β such as a Renogy Rover β with automatic shutoff when the sun goes down. This allows the MPPT controller to power your load(s) during the day, helping preserve your caravan battery energy for nighttime use.
Frequently Asked Questions
What is the difference between a 4 pin and 5 pin relay?
Both control power to a circuit, but a 5-pin relay has a single-pole double-throw (SPDT) configuration, meaning it can control two separate circuit paths through its NO (normally open) and NC (normally closed) terminal pins.
What do the pins on a 5 pin relay mean?
A 5-pin relay has terminal pins 86, 85, 30, 87a, and 87. Pin 86 is the positive input, pin 85 is the ground input, pin 30 is the common (COM) terminal, pin 87a is the normally closed (NC) terminal, and pin 87 is the normally open (NO) terminal.
How does the automatic dual-power switch work?
When the ignition is off, the relay's armature rests on pin 87a, so the load draws power from the leisure battery. When you turn the key to the IGN position, the relay energises and switches the armature to pin 87, so the load runs from the starter battery (alternator) instead.
Can I use this relay setup with solar power instead of the starter battery?
Yes. You can replace the starter battery connection with wires from an MPPT solar charger β such as a Renogy Rover β with automatic shutoff when the sun goes down, letting the MPPT controller power your loads during the day and preserving battery energy for nighttime use.
Ready to build your own automatic dual-power switch? Browse our range of 12V relays for DIY campervan and caravan projects to get started.
