Wiring a DC-DC charger is one of the more satisfying jobs in a van build — the install is relatively straightforward, and the payoff (arriving at camp with a fuller battery after every drive) is immediate.
This guide covers cable sizing, fuse placement, the ignition trigger, and the most common mistakes. For choosing the right charger first, see best DC-DC chargers for van & RV builds.
Our free electrical calculator shows how much each charging source contributes on your trips and sizes the battery to cover the gap.
What you need
- DC-DC charger (Victron Orion XS, Renogy DCC50S, or similar)
- Appropriately sized cable (see sizing below)
- Two ANL or MIDI fuse holders with fuses
- Ignition trigger wire (often thin, 18–22 AWG)
- Ring terminals and a crimping tool
- Heat shrink tubing
- Cable glands or loom for protection
US equipment options
Choose the charger before finalising the cable run
Start with the exact charger manual. Record the cable lengths, protection and control requirements before ordering the installation parts.
Alternator charging
Victron Orion XS 12/12-50A DC-DC Battery Charger
A 12V-to-12V charging option for a compatible vehicle and house-battery installation.
Check before choosing: Alternator headroom, cable run, battery charge acceptance and the required controls/protection.
These links open our US shop. Check the exact configuration, current price and availability there; affiliate products are purchased from the named supplier. We may earn a commission from qualifying purchases.
Step 1: Plan the cable run
The DC-DC charger sits somewhere accessible in the van — typically under a seat, in a cabinet, or near the house battery. The input cable runs from the starter battery in the engine bay to the DC-DC unit; the output cable runs from the DC-DC unit to the house battery.
Measure both runs before buying cable. Add extra for routing around obstacles — you want the cable run clean and protected from chafe, not taking a straight-line shortcut past a hot exhaust.
Step 2: Size your cables
| DC-DC charger output | Run length | Cable size |
|---|---|---|
| 30A | Up to 10 ft | 8 AWG (6 AWG preferred) |
| 30A | 10–20 ft | 6 AWG |
| 50A | Up to 10 ft | 6 AWG |
| 50A | 10–20 ft | 4 AWG |
Use the same cable size for both the input run (starter battery → charger) and output run (charger → house battery). Both sides carry the full continuous current.
Use tinned marine-grade cable
Tinned copper cable resists corrosion better than bare copper, especially in the engine bay. It costs a little more but lasts significantly longer. Avoid aluminum cable for DC runs.
For full voltage drop calculation: see the AWG wire gauge guide.
Step 3: Fuse both sides — close to the battery
This is the step most DIY builders get wrong. Each fuse goes as close to the source battery as possible — within 12 inches of the terminal. The purpose of the fuse is to protect the cable from overcurrent in the event of a short; if you put the fuse in the middle of the run, 10 feet of unfused cable can arc and start a fire before the fuse blows.
Fuse sizing: 125–150% of the DC-DC charger's continuous output:
- 30A charger → 40A fuse (ANL or MIDI)
- 50A charger → 60–70A fuse (ANL or MIDI)
Mount an ANL fuse holder directly at the starter battery positive terminal. Mount a second ANL fuse holder at the house battery positive terminal (or at your bus bar). Both must be fused — the output side is just as important as the input.
Unfused cable can cause a fire
The most common cause of van electrical fires is unfused cable. A short circuit on an unfused run will draw hundreds of amps until something melts or ignites. Always fuse within 12 inches of every battery terminal.
Step 4: Wire the ignition trigger
The ignition trigger (sometimes called the D+ or engine-running input) is a thin wire that tells the DC-DC charger the engine is actually running. Without it, the charger may run off the starter battery while parked, slowly draining it.
Find a switched 12V source — a wire that's live only when the ignition is in the ACC or ON position. Common sources:
- The fuse box (find a switched slot with a multimeter — live with key on, dead with key off)
- The D+ terminal on the alternator
- A relay triggered by the ignition
Connect this thin wire to the DC-DC charger's ignition or D+ input terminal. Consult your charger's manual for the specific terminal — most modern chargers label it clearly.
Use a relay if in doubt
If you can't find a clean switched 12V source, add a small relay coil wired to an ignition feed. The relay closes when the engine runs and opens when it doesn't. Sterling and Victron both sell remote on/off accessories that simplify this.
Step 5: Connect the grounds
Both the input ground (from the starter battery negative or vehicle chassis) and the output ground (to the house battery negative or ground bus bar) need solid connections.
For isolated DC-DC chargers (Victron Orion-Tr, Orion XS): the input and output grounds are separate. Connect the input ground to the vehicle chassis (starter battery negative or chassis ground point), and the output ground to the house battery negative or your main negative bus bar. Do NOT connect these two grounds to each other — the isolation would be defeated.
For non-isolated chargers (Renogy DCC50S): a single chassis ground is fine; consult the manual.
Step 6: Connect and configure
With cables run, fused, and grounded:
- Connect the output cable (charger → house battery positive)
- Connect the input cable (starter battery positive → charger input)
- Connect the ignition trigger wire
- Install fuses in both fuse holders last
On the Victron Orion XS and Orion-Tr Smart, open the VictronConnect app and set the battery profile to LiFePO4. Configure the charge algorithm to match your battery's spec sheet. On the Renogy DCC50S, use the mode button on the front panel to select LiFePO4 (typically mode 3 — check the manual).
Test it
Start the engine. Check the DC-DC charger display or app — it should show input voltage (typically 13.5–14.5V from the running alternator) and output amps flowing into the house battery. Drive for 10–20 minutes and confirm the house battery state of charge is rising.
Common mistakes
Fuse too far from the battery: The most dangerous mistake. Move fuses within 12 inches of the terminal.
Undersized cable: A 30A charger on 10 AWG cable will work but the cable will get warm. Use 6 AWG for a 30A run and avoid any voltage drop eating into your charge rate.
No ignition trigger: The charger runs continuously off the starter battery and can drain it flat overnight.
Wrong charge profile: Setting a lead-acid profile on a LiFePO4 battery undercharges and over-complicates the charge cycle. Set LiFePO4 mode before first use.
Forgetting the output fuse: Both sides need to be fused. The output side is just as capable of causing a fire if the cable shorts.
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Jack Kennedy is a self-builder who has built two vans. He created Roam Wired to simplify conversion planning and share free tools, supported by affiliate links.
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