Battery Isolator Switches: Where They Go and Why You Want One

A battery isolator (or master) switch breaks the main positive cable so nothing can draw from the battery. Don't confuse it with a voltage-sensitive relay or DC-DC charger, which are also sometimes called "isolators".

1. Why fit one

Storage: small loads like radios, USB sockets and monitors can flatten a battery over weeks.
Working on wiring: you can make the system dead before you touch it.
Fault finding: switching off tells you quickly whether a drain is real.

2. Where it goes

On the main positive cable, close to the battery and after the main fuse, in a spot you can reach easily. Mount it on a solid surface away from water.

3. Sizing

Choose a switch with a continuous rating above the highest current the system draws, including an inverter at full load. Also check its short-term (surge) rating if you run a big inverter.

4. What stays connected

Some things should stay on even in storage, such as a battery monitor or a solar controller feeding the battery. Check the maker's instructions: some solar controllers must not be left connected to panels with the battery disconnected.

5. Common mistakes

Undersized switch that gets hot under inverter load.
Mounted somewhere you can't reach in the dark.
Switching off the battery while the solar controller is still connected to the panels, against the controller's instructions.

6. Quick summary

A master switch cuts all loads, which is great for storage and safety.
Fit it on the main positive near the battery, rated above your biggest load.
Check your solar controller's instructions before you isolate the battery.

Work on 240V AC wiring, including inverter outputs and mains inlets, must be done by a licensed electrician. For high-current DC gear, follow the maker's installation manual and have an auto electrician check your work if you're unsure.

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