Understanding Dual Battery Systems
A dual battery system separates your vehicle's starter battery from your leisure battery bank, allowing you to power lights, fridges, and other appliances without risking a flat starter battery. In a Victron-based system, a DC-DC charger manages the connection between the two batteries while charge controllers and inverters serve the leisure side.
This guide provides wiring diagrams for the most common campervan and motorhome configurations, from simple single-battery setups to advanced multi-source systems.
Diagram 1: Basic 12V Campervan System
This is the most common starting point for campervan conversions. A single leisure battery is charged by both solar and the vehicle alternator via a DC-DC charger.
Components
- 1x Victron Orion-Tr Smart 12/12-30 (DC-DC charger)
- 1x Victron SmartSolar MPPT 75/15 (solar charge controller)
- 1x 100-200W solar panel
- 1x 100Ah lithium leisure battery (with built-in BMS)
- Fuse box, bus bars, and appropriate cabling
Wiring Overview
Alternator charging path:
- Starter battery positive terminal → 40A fuse (within 300mm of battery) → 16mm² cable → Orion-Tr Smart INPUT positive
- Orion-Tr Smart OUTPUT positive → 40A fuse → 10mm² cable → leisure battery positive bus bar
- Orion-Tr Smart INPUT negative → starter battery negative (or chassis ground)
- Orion-Tr Smart OUTPUT negative → leisure battery negative bus bar
Solar charging path:
- Solar panel positive → 15A fuse → SmartSolar MPPT PV input positive
- Solar panel negative → SmartSolar MPPT PV input negative
- SmartSolar MPPT battery positive → leisure battery positive bus bar
- SmartSolar MPPT battery negative → leisure battery negative bus bar
Critical notes:
- Always connect the battery to the MPPT before connecting the solar panels
- All negative connections should share a common bus bar or grounding point
- The Orion-Tr Smart and SmartSolar can charge simultaneously without conflict
Diagram 2: Campervan with Inverter
Adding an inverter allows you to run 230V appliances from your leisure battery. This diagram builds on the basic system by adding a Victron Phoenix inverter.
Additional Components
- 1x Victron Phoenix Inverter 12/500 or 12/800 (pure sine wave)
- 1x 200A class T fuse (between battery and inverter)
- 35mm² or 50mm² cable (battery to inverter, keep as short as possible)
Inverter Wiring
- Leisure battery positive bus bar → 200A class T fuse → 50mm² cable → Phoenix inverter DC positive
- Leisure battery negative bus bar → 50mm² cable → Phoenix inverter DC negative
- Phoenix inverter AC output → RCD/MCB consumer unit → 230V sockets
Important: Keep the cable between the battery and inverter as short as physically possible — ideally under 1.5 metres. Inverters draw very high currents at 12V (a 500W load draws over 40A). Long or thin cables cause significant voltage drop and power loss.
Diagram 3: Motorhome with Shore Power
Motorhomes often need the ability to charge from campsite hookup as well as solar and alternator. This configuration uses a Victron MultiPlus inverter/charger which handles both inverting and mains charging in one unit.
Components
- 1x Victron MultiPlus 12/1600/70 (inverter/charger)
- 1x Victron Orion-Tr Smart 12/12-30 (DC-DC charger)
- 1x Victron SmartSolar MPPT 100/20 (solar charge controller)
- 200-400W solar array
- 200Ah lithium battery bank
- 1x Victron SmartShunt 500A (battery monitor)
Shore Power Path
- Campsite hookup → 16A plug → RCD in vehicle inlet → MultiPlus AC input
- MultiPlus AC output → consumer unit → 230V sockets and appliances
- MultiPlus DC connections → leisure battery bus bars (same as inverter wiring above, using 50mm² cable with 200A fuse)
When plugged into shore power, the MultiPlus simultaneously charges the batteries and passes mains power through to your 230V circuits. When you unplug, it automatically switches to inverter mode, powering your 230V circuits from the batteries. The changeover takes less than 20 milliseconds — fast enough that most appliances do not notice.
Battery Monitoring
The SmartShunt connects in the negative cable path between the battery negative terminal and the negative bus bar. All negative connections (from the MultiPlus, MPPT, Orion-Tr, and loads) must go to the bus bar side, not directly to the battery terminal. This ensures the SmartShunt sees all current flowing in and out of the battery and can accurately calculate state of charge.
Diagram 4: Advanced System with GX Monitoring
For full system visibility and remote monitoring, add a Victron Cerbo GX. This connects to all Victron components and provides a dashboard via the VRM online portal.
Additional Components
- 1x Victron Cerbo GX
- 1x GX Touch 50 or 70 screen (optional)
- VE.Direct cables (for MPPT and SmartShunt)
- Ethernet cable or Wi-Fi for internet connectivity
GX Connections
- Cerbo GX VE.Bus port → MultiPlus (using RJ45 cable)
- Cerbo GX VE.Direct port 1 → SmartSolar MPPT
- Cerbo GX VE.Direct port 2 → SmartShunt
- Cerbo GX USB port → Orion-Tr Smart (via VE.Direct USB adapter, if monitoring is needed)
- Cerbo GX Ethernet/Wi-Fi → Internet for VRM portal access
The Cerbo GX enables DVCC (Distributed Voltage and Current Control), which coordinates all connected chargers to deliver the optimal charge to your battery. It also enables the ESS (Energy Storage System) assistant in the MultiPlus if you want to use the system for domestic grid-tied storage.
Cable Sizing Reference
Correct cable sizing is critical for safety and performance. Here are the recommended minimum cable sizes for common connections in a 12V system:
| Connection | Typical Current | Max 2m | Max 5m |
|---|---|---|---|
| Battery → Inverter (500W) | 45A | 16mm² | 35mm² |
| Battery → Inverter (1600W) | 150A | 50mm² | 70mm² |
| Starter → DC-DC (30A) | 30A | 10mm² | 16mm² |
| Solar Panel → MPPT | 10-15A | 4mm² | 6mm² |
| MPPT → Battery | 15-20A | 6mm² | 10mm² |
| SmartShunt → Bus bar | Full system | 50mm² | 50mm² |
Use our Voltage Drop Calculator to verify cable sizes for your specific run lengths.
Earthing and Grounding
Proper earthing is essential for safety and correct operation of monitoring equipment:
- Vehicle chassis ground: The starter battery negative is typically connected to the vehicle chassis. The leisure system should have its own dedicated negative bus bar
- Single ground point: Connect the leisure negative bus bar to the vehicle chassis at one point only, to avoid ground loops that can interfere with battery monitoring
- SmartShunt placement: Must be the only path between the battery negative terminal and all other negative connections
Common Wiring Mistakes
- Bypassing the SmartShunt: Any negative cable connected directly to the battery terminal (instead of the bus bar) creates an unmeasured path and ruins state-of-charge accuracy
- Undersized inverter cables: Thin cables between battery and inverter cause voltage drop, reduce available power, and generate dangerous heat
- Missing fuses: Every cable connected to a battery must be fused within 300mm of the battery terminal
- Solar panels connected before battery: Always connect the MPPT to the battery first, then the solar panels. Connecting panels first can damage the charge controller
- Mixed battery types: Never mix lithium and lead-acid batteries in the same bank, or even batteries of different ages and capacities
