Battery Isolator Wiring Diagram

Battery Isolator Wiring Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable battery isolator wiring diagram: download the diagram as SVG or open it and print to paper or PDF.

A battery isolator wiring diagram shows how to connect a dual battery system so that a secondary battery powers accessories without risking a flat starter battery.

A battery isolator is the core component of a dual battery system, a setup used extensively in 4x4 vehicles, campervans, boats, and caravans. The purpose is to keep the starter battery isolated from the auxiliary (house) battery during periods of high auxiliary load — so that running a fridge, lighting, or a winch from the auxiliary battery cannot flatten the battery you rely on to start the engine.

There are three main types of isolator. A diode-based isolator uses diodes to allow current to flow from the alternator to both batteries during charging but blocks reverse flow between the two batteries. The disadvantage is a voltage drop of approximately 0.6–0.7 V across each diode, which can undercharge batteries and shorten their lifespan. A voltage-sensitive relay (VSR), also called a battery-to-battery relay or smart relay, is a more modern and common solution: it monitors the voltage of the starter battery and closes a relay to connect the auxiliary battery only when the alternator is charging (typically above 13.2–13.8 V). When the engine is off and voltage drops, the relay opens and isolates the batteries. A DC-DC battery-to-battery charger (B2B) is the best solution for vehicles with smart alternators (those fitted with energy management systems that vary alternator output) and for charging lithium auxiliary batteries — it regulates the charge voltage and current profile to suit the auxiliary battery chemistry.

In all configurations, the wiring between the alternator or starter battery and the isolator must be rated for the maximum charging current produced by the alternator — typically 50–200 A depending on the vehicle. Undersized cable causes voltage drop, heat, and potential fire. All high-current connections must be protected by an appropriately rated fuse or circuit breaker as close to the battery positive terminal as possible — within 300 mm (approximately 12 inches) is the standard recommendation.

Always follow the isolator manufacturer's wiring instructions and consult a qualified auto-electrician for installations in vehicles, marine craft, or caravans.

How to wire battery isolator wiring diagram

Determine your system requirements Calculate the total current draw of your auxiliary loads (refrigerator, lights, inverter, water pump, and so on) and the expected daily energy usage in amp-hours (Ah). Select an auxiliary battery with adequate capacity. Determine the maximum charging current from your vehicle's alternator from the vehicle specifications — this sets the minimum cable rating for the system.
Select the correct isolator type For a standard vehicle with a conventional alternator and a lead-acid auxiliary battery, a VSR (voltage-sensitive relay) is the most practical and cost-effective choice. For vehicles with smart/variable alternators (common in modern Euro 5/6 and Euro VII vehicles) or for lithium auxiliary batteries, use a DC-DC battery-to-battery charger instead. Diode isolators are a valid choice where the VSR's relay click is undesirable, but account for the voltage drop in your charging budget.
Plan the cable routes Route the positive cable from the starter battery to the isolator and from the isolator to the auxiliary battery, keeping cable runs as short as possible to minimise voltage drop and cable cost. Avoid routing cables near exhaust systems, sharp metal edges, or areas that retain water. Plan where cable grommets will be needed when passing through bulkheads.
Disconnect the vehicle battery before beginning Disconnect the negative terminal of the starter battery. This is essential before touching any positive terminal wiring in the vehicle. Working on high-current circuits with the battery connected creates a serious risk of arc flash and fire from accidental short circuits.
Install the auxiliary battery in a secure, ventilated location Mount the auxiliary battery in an approved battery tray or box, secured against movement. Lead-acid batteries must be ventilated to prevent hydrogen gas accumulation. Lithium batteries are sealed and have more flexible mounting options but still require secure mounting to withstand vehicle vibration and impact loads.
Install cables and fuses Run the positive cable from the starter battery positive terminal, through a fuse holder installed within 300 mm of the terminal, to the IN terminal of the isolator. Run a second positive cable from the OUT terminal of the isolator, through a second fuse holder within 300 mm of the auxiliary battery positive terminal, to the auxiliary battery positive terminal. Run a combined earth cable from the auxiliary battery negative terminal to the vehicle chassis at a clean, bare-metal bolt point.
Connect the isolator control wiring and test Connect any required control wiring per the isolator manufacturer's instructions — VSRs typically require only power and earth; some also accept an ignition-switched input to force the relay closed when the ignition is on. Reconnect the vehicle battery negative terminal. Start the engine and verify the auxiliary battery is charging by measuring voltage at the auxiliary battery terminals with the engine running. With the engine off, verify the isolator has opened (auxiliary battery not connected to starter battery) by checking that a load on the auxiliary circuit does not pull down the starter battery voltage.

Specifications

VSR engage voltage (typical)	13.2–13.8 V DC
VSR disengage voltage (typical)	12.7–12.8 V DC
Diode isolator voltage drop	Approximately 0.6–0.7 V per diode
Maximum distance from battery positive to first fuse	300 mm (approximately 12 inches)
Minimum cable size (typical, runs under 2 m)	6 B&S / 13 mm² for up to approximately 100 A
Minimum cable size (longer runs / higher current)	4 B&S / 21 mm² — calculate per run length and ampacity table
Lead-acid hydrogen explosive concentration	4% by volume (minimum ventilation required in battery enclosures)
Applicable standards (AU/NZ vehicles and RVs)	AS/NZS 3001, AS 3007

Safety warnings

Always disconnect the vehicle battery negative terminal before working on any high-current wiring. Even a brief accidental short on a battery cable with no fuse in circuit can produce an arc flash sufficient to cause burns, eye injury, and fire.
Install a fuse within 300 mm of every battery positive terminal. In a short circuit event, an unfused cable from a battery can carry thousands of amps momentarily, generating extreme heat and fire within seconds. This is the single most important safety requirement of any dual battery installation.
Lead-acid batteries (flooded, AGM, and gel) produce hydrogen gas during charging. Never install a lead-acid battery in a completely sealed, unventilated space. Ensure there is a vent path to the outside of the vehicle. Hydrogen gas is explosive at concentrations as low as 4% by volume.
High-current cable terminals must be properly crimped with a hydraulic or ratchet lug crimper — never soldered as the only joint, and never connected with a bare wire folded into a terminal. An inadequately secured lug will arc and cause fire at high current.
All electrical work on motor vehicles must comply with the vehicle manufacturer's requirements and applicable national standards. In Australia: AS/NZS 3001 (caravan and RV), and relevant state transport regulations. In the UK: BS AU 145. Consult a licensed auto-electrician if in doubt.

Tools needed

Digital multimeter (for voltage and continuity testing)
Hydraulic or ratchet cable lug crimper (for ring terminal installation on heavy-gauge cable)
Cable cutters or bolt cutters rated for the cable gauge
Wire stripper for heavy-gauge cable
Drill and step drill bit (for cable grommet holes)
Torque wrench or socket set (for battery terminal and mounting bolts)
Cable ties and mounting clips for cable routing and securing

Common mistakes

Installing fuses too far from the battery positive terminal — fuses protect the cable, not just the load. Any unprotected cable between the battery and the first fuse is a potential fire hazard in a short circuit event. The 300 mm rule is not a guideline, it is a safety requirement.
Under-sizing the positive cable — using cable rated for less than the alternator's maximum output will cause the cable to run hot during charging and will result in excessive voltage drop, undercharging the auxiliary battery. Size cable for the full alternator output, not just the expected average current.
Making a poor earth connection to a painted surface — the earth cable must connect to bare, clean metal on the vehicle chassis. Painting, underseal, or corrosion between the terminal and the chassis creates resistance that prevents proper charging and causes erratic VSR behaviour.
Using a standard VSR with a smart alternator — modern vehicles with energy management systems regularly lower alternator output voltage below the VSR trigger threshold, meaning the auxiliary battery rarely receives a full charge. A DC-DC charger is the correct solution for these vehicles.
Over-rating the fuse to avoid nuisance blowing — the fuse must be rated at or below the cable's current-carrying capacity. A fuse rated higher than the cable allows the cable to overheat before the fuse blows, causing insulation failure and fire.

Troubleshooting

Auxiliary battery is not charging while engine is running: Cause: VSR is not triggering due to low alternator output voltage, poor earth connection, or a failed VSR; or fuse has blown Fix: With the engine running, measure voltage at the starter battery terminals. If above 13.2 V but the VSR is not closing, check the earth connection on the VSR and at the auxiliary battery. Check both fuses. Measure the VSR's input voltage to confirm it is receiving the alternator voltage. If the VSR control circuit has a test function, use it to verify the relay closes mechanically.
Starter battery is flat after using auxiliary loads overnight: Cause: VSR has not fully opened — relay is stuck closed — or loads are connected to the starter battery circuit rather than the auxiliary battery circuit Fix: Verify that auxiliary loads are wired to the auxiliary battery output and not directly to the starter battery. With the engine off, measure voltage at both batteries. If they are equal, the isolator is not isolating. Check the VSR for a stuck relay (a good VSR will have an audible click when the engine is switched off). Replace a faulty VSR.
Auxiliary battery charges slowly or incompletely: Cause: Excessive voltage drop in the cable run due to undersized cable, loose connection, or corroded lug Fix: With the engine running, measure voltage at the starter battery positive terminal and compare to the voltage at the auxiliary battery positive terminal. A difference of more than 0.5 V indicates excessive resistance in the circuit. Inspect and tighten all connections. Use a milliohm meter or measure voltage drop across each section of cable to identify the high-resistance point.

Frequently asked questions

What size cable should I use for a dual battery system?

Cable size depends on the maximum current and the cable run length. For most vehicle dual battery installations, 6 B&S (approximately 13 mm²) cable is the minimum for runs up to 2 m; 4 B&S (approximately 21 mm²) for longer runs or higher-current alternators. Always calculate voltage drop and confirm the cable rating exceeds the maximum continuous current. Refer to a marine or automotive cable sizing chart for your specific installation.

Where should I fuse a dual battery installation?

A fuse or circuit breaker must be installed within 300 mm of every battery positive terminal in the circuit. This protects the cable in the event of a short circuit between the battery and the first protected device. The fuse rating must be lower than the current-carrying capacity of the cable but appropriate for the expected load.

Can I use a battery isolator with a lithium (LiFePO4) auxiliary battery?

A diode isolator or standard VSR is generally not recommended for lithium batteries because the charge profile (particularly the absorption voltage) differs from lead-acid batteries and a smart alternator may not deliver the correct charge. A DC-DC battery-to-battery charger is the correct solution for lithium auxiliary batteries — it regulates voltage and current to suit the LiFePO4 chemistry.

What is a voltage-sensitive relay and how does it work?

A voltage-sensitive relay (VSR) monitors the starter battery voltage. When voltage rises above a set threshold (typically 13.2–13.8 V), indicating the alternator is charging, the relay closes and connects the auxiliary battery to the charging circuit. When the engine stops and voltage falls below a lower threshold (typically around 12.8 V), the relay opens and isolates the auxiliary battery from the starter battery.

Does the isolator need to be earthed?

Most VSR and diode isolators require a chassis earth connection to operate their control circuitry. The high-current switching path also requires a proper earth return. Ensure the earth/ground connections are made with appropriately rated cable and are bolted to clean, bare metal on the chassis. A poor earth is one of the most common causes of isolator malfunction.

Related diagrams

Free electrical calculators

Edit this diagram free in the online editor