Campervan Battery Isolator Switch: Do You Need One and How to Wire It

A battery isolator switch cuts the connection between your leisure battery and the rest of the electrical system. In an emergency, it stops an electrical fault from continuing to draw current. In storage, it prevents parasitic drain. During installation and maintenance, it makes working on the electrical system safer.

This guide covers whether you need one, where to fit it, and how to wire it correctly. For the broader safety picture, see our campervan electrical fire safety guide.

Do You Need a Battery Isolator Switch?

Short answer: yes, for any permanent campervan electrical installation.

The reasons:

Emergency disconnection: If an electrical fault causes overheating or smoke, cutting the battery stops the current source. An isolator switch that can be reached quickly — ideally without opening any panel — gives you a fast, decisive response to an early-stage fault.

Storage: Any electrical system has parasitic loads — battery monitors, Bluetooth devices, 12V socket power feeds. Over weeks or months of storage, these drain the battery. An isolator switch lets you cut the battery completely for long periods without disconnecting terminals.

Maintenance safety: When working on the electrical system, an open isolator switch makes the system genuinely safe to work on (combined with confirming zero voltage with a multimeter — never rely solely on a switch position for safety).

Insurance: Some specialist campervan insurers require evidence of appropriate isolation in the electrical system. An accessible isolator switch on the leisure battery is standard practice.

Types of Isolator Switch

Single-Pole (Positive Only)

The most common type — a rotary or lever switch that disconnects the positive conductor only. Adequate for most campervan applications. When open, the leisure battery positive is disconnected, preventing any circuit from completing.

Standard 12V rotary isolators in the 100-350A range cost £8-20 and are widely used in marine and vehicle applications.

Double-Pole (Both Positive and Negative)

Disconnects both positive and negative simultaneously. Used in systems where complete isolation from both conductors is required — for example, where the vehicle body is used as a chassis earth and the negative bus may still be at earth potential. Provides belt-and-braces isolation.

More expensive (£15-40) and slightly more complex to wire. Not strictly necessary for most campervan builds but provides complete peace of mind.

Electronic/Relay-Based Isolators

These use a relay controlled by a key switch or digital input. The switch itself carries only the low-current relay signal, while the actual battery current flows through the relay. Good for high-current systems (500A+) where a mechanical switch would be large and expensive. Less common in campervan use. Note: relay-based isolators can fail in either the open or closed position — less predictable than a mechanical switch.

Current Rating

The isolator switch must be rated for the maximum continuous current in your system, not just typical usage.

For most mid-range campervan builds: a 250A rotary isolator is the standard. These are inexpensive (£10-15), widely available, and handle all typical loads comfortably.

Where to Fit the Isolator Switch

On the Leisure Battery Positive Cable

The most important location — wire the isolator on the positive cable between the battery positive terminal and the ANL main fuse (or immediately after the fuse if space allows):

Battery (+) → Isolator Switch → ANL Fuse → Distribution

Some builders put the fuse before the isolator and some after. Technically the fuse should be as close to the battery as possible — within 30cm. If space allows, fit: battery → very short cable → ANL fuse → isolator → main bus bar.

Accessible Location

The switch should be reachable quickly without opening panels. Common positions:

• Visible on the battery box face: Rotary knob visible and accessible when a cabinet door or hatch is open
• External switch panel: A switch panel accessible from the rear of the van without entering
• Under a seat: Accessible by lifting the seat, not dismantling the interior

The nearer the isolator is to the battery, the better — this minimises the unfused cable run between battery and isolation point.

How to Wire the Isolator Switch

The isolator switch simply interrupts the positive cable. It has two terminals: battery side (input) and load side (output).

Battery (+) ─────────────────────────────── Load Side (to ANL fuse)
                 │                  │
             [IN Terminal]    [OUT Terminal]
                     [ISOLATOR SWITCH]

Step 1: Determine Cable Size

The isolator switch cable must match your main battery cable gauge. If your main cable is 50mm², use 50mm² cable at the switch.

Step 2: Cut the Positive Cable

Route the main positive cable from the battery to the isolator switch input terminal. From the output terminal, continue to the ANL fuse and main bus bar.

Use the correct ring terminal size for the cable gauge and the switch's terminal stud size (usually M8 or M10). Crimp correctly — an isolator switch connection that fails is a dead short waiting to happen.

Step 3: Mount the Switch

The switch body must be firmly mounted — it will experience significant mechanical force from cable connection and occasional operation. Most rotary isolators mount through a panel cutout or use bolt-through mounting. Use a backing plate on thin panels to prevent deformation over time.

Step 4: Label the Switch

Label clearly: "LEISURE BATTERY ISOLATOR — OPEN = OFF". In an emergency, anyone (not just the builder) needs to be able to act immediately.

Vehicle Battery Isolation

The leisure battery isolator only disconnects your leisure system. The vehicle (starter) battery remains connected to the van's own systems at all times.

If you also want to isolate the vehicle battery (for storage or work near the starter circuit), fit a second isolator on the vehicle battery positive. This is a separate, independent switch — do not wire the leisure and vehicle isolators together.

Remote Isolator Switches

If the battery is in an inaccessible location (external underbody box, deep in a bench), a remote isolator switch mounted in a convenient location is a good alternative to a standard mechanical switch.

These use a solenoid or relay controlled by a low-current switch. The relay (carrying main battery current) is at the battery; the control switch is wherever you want it. Cost: £15-40 for the relay; £5-10 for the remote switch.

The same current rating rules apply — the relay must be rated for your maximum load.

FAQ

Can I use the main ANL fuse as the isolator?

Not practically — ANL fuses are not designed for repeated switching. The element degrades if disconnected and reconnected under voltage. Use a dedicated isolator switch and keep the ANL fuse as overcurrent protection only.

Does the isolator switch need to handle inverter peak current?

The switch must handle continuous current at your system's maximum load. For occasional high-current events (inverter starting a large motor), the switch's surge rating applies — check the spec sheet. A quality 250A rotary isolator has a surge rating of 600-1000A, adequate for all but the very largest inverter starts.

Should the isolator switch be on the positive or negative cable?

By convention, isolators go on the positive cable. This is because the negative is the return path and may also provide the chassis earth — disconnecting it does not always break all circuits (the chassis may still be connected to the vehicle battery negative). Positive isolation is more definitive.

Is a battery isolator switch the same as a battery management system?

No. A BMS (Battery Management System) is the internal protection circuit in a LiFePO4 battery that manages charge and discharge at the cell level. An isolator switch is a manual or remote-switched mechanical disconnect in the external wiring. They are different components serving different functions.