How to Wire a DC-DC Charger in a Campervan
A DC-DC charger (also called a battery-to-battery or B2B charger) lets your alternator charge your leisure battery correctly — without the problems of a simple relay or split charge connection. Here is how to wire one.
What you need
- DC-DC charger (Victron Orion-Tr Smart, Sterling Power B2B, Renogy DC-DC, or similar)
- Cable: 6mm² or 10mm² automotive cable for input and output (check charger's required input/output current rating)
- Fuse: Inline fuse on both input (starter battery side) and output (leisure battery side), sized to the cable rating
- Ignition sense wire: thin cable (1mm² is fine) from a switched 12V live (e.g., ignition switch, fuse box)
- Ring terminals, heat shrink, crimping tool
How a DC-DC charger works
Unlike a split-charge relay that simply connects the two batteries, a DC-DC charger actively converts the input voltage to the correct charging voltage for your leisure battery. Input is typically 11–15V from the alternator; output is configured to match your battery chemistry (14.4V for LiFePO4, 14.7V for AGM).
This means:
- LiFePO4 leisure batteries are correctly charged from a lead-acid alternator
- The alternator is not affected by the leisure battery's charging needs (no risk of alternator overload)
- Charging only occurs when the engine is running (via ignition sense or voltage threshold)
Wiring overview
The DC-DC charger has four connections:
- Input positive — from starter battery positive (or a fuse box directly connected to starter battery)
- Input negative — to chassis earth or starter battery negative
- Output positive — to leisure battery positive (via fuse)
- Output negative — to leisure battery negative or chassis
- Ignition sense (remote on/off) — to a switched 12V source (optional but strongly recommended)
Cable sizing
| Charger rating | Input cable | Output cable | Fuse rating |
|---|---|---|---|
| 20A | 6mm² | 6mm² | 25A (input), 25A (output) |
| 30A | 10mm² | 10mm² | 40A (input), 40A (output) |
| 40A | 16mm² | 16mm² | 50A (input), 50A (output) |
Use the same cable size for both input and output. Keep both cable runs as short as practical.
Step-by-step installation
Step 1: Plan cable routes
Identify the route from the starter battery (engine bay) to the charger location (typically inside the van near the leisure battery). Note the cable length — the longer the run, the more important correct cable sizing.
Step 2: Mount the DC-DC charger
Mount the charger securely on a metal or wooden surface with ventilation clearance. A DC-DC charger generates heat — do not enclose it without airflow.
Step 3: Install input fuse near starter battery
At the starter battery end, add an inline fuse holder on the positive cable within 300mm of the battery terminal. Do not insert fuse yet.
Install cable from the starter battery positive, through the firewall grommet or existing cable entry point, to the charger input positive terminal.
Step 4: Install output fuse near leisure battery
At the leisure battery end, add an inline fuse holder on the positive output cable within 300mm of the leisure battery terminal.
Install cable from the charger output positive terminal to the leisure battery positive (via fuse holder).
Step 5: Connect negatives
Connect the charger input negative to the vehicle chassis (bare metal, clean connection — remove paint). Connect the charger output negative to the leisure battery negative or the leisure battery chassis connection.
Do not connect input and output negatives together at the charger — use the chassis as the common ground.
Step 6: Wire ignition sense
Connect the ignition sense wire from the charger's EN (enable) or remote terminal to a switched 12V live in the van — a wire that is live when the ignition is on and dead when the ignition is off. The fuse box often has a labelled ignition-switched output.
This tells the charger to only charge when the engine is running. Without ignition sense, some DC-DC chargers operate by voltage threshold — they start when they detect the alternator charging voltage (~13.8V+) and stop when voltage drops. This works but is less reliable than a proper ignition sense connection.
Step 7: Configure the charger
Before inserting fuses, configure the output voltage profile via the charger's app or buttons:
- LiFePO4 leisure battery: 14.4V absorption, 13.5V float (or use LiFePO4 preset)
- AGM leisure battery: 14.7V absorption, 13.8V float (or use AGM preset)
Step 8: Insert fuses and test
Insert both fuses. Start the engine. Monitor the charger via its display or app — it should show input voltage (engine running, ~13.8–14.4V) and output current flowing to the leisure battery.
Victron Orion-Tr Smart: specific notes
The Victron Orion-Tr Smart is the most popular choice for campervan DC-DC charging. Configuration via the Victron Connect app. Key settings:
- Input shutdown voltage: Set to 12.5V to protect the starter battery if the alternator output drops
- Output voltage: Configure for battery chemistry (LiFePO4: 14.4V)
- Mode: Standard (engine running) — uses the H terminal (remote on/off) for ignition sense
FAQ
Do I need to disconnect the existing split-charge relay?
Yes. If you replace a relay with a DC-DC charger, remove or disable the relay so the two batteries are no longer directly connected. Running both a relay and a DC-DC charger creates a conflict.
Can I connect the DC-DC charger directly to the van chassis instead of the leisure battery negative?
The output negative should connect to the leisure battery negative terminal (or its chassis connection), not to a remote chassis point. This ensures the full charge current goes to the battery rather than through a long chassis return path.
Will a DC-DC charger drain my starter battery?
A DC-DC charger with ignition sense will not operate when the engine is off. Without ignition sense, some chargers use a voltage-sensing mode that should also prevent discharge — but ignition sense is more reliable.