How to Wire a DC-DC (B2B) Alternator Charger in a Van

A DC-DC charger is the single most impactful upgrade for any campervan electrical system — it turns every kilometre of driving into free battery power. But wire it wrong and you risk blown fuses, melted cables, or a leisure battery that never reaches full charge. This guide walks you through the complete wiring process: what a B2B charger does, how to size cables and fuses, where to mount it, and the step-by-step connection sequence to get it right the first time.

1. What is a DC-DC charger and why do you need one?

A DC-DC charger (also called a B2B charger or battery-to-battery charger) sits between your vehicle's starter battery and your leisure battery. Its job is to take the variable voltage from the alternator and convert it into a proper multi-stage charge profile — bulk, absorption, float — tailored to your leisure battery chemistry.

In older vehicles (pre-2015), a simple voltage-sensing relay (VSR) was enough: the alternator put out a steady 14.4V, which was close enough to charge a lead-acid leisure battery. But modern vehicles use smart alternators that vary output between 12.4V and 14.8V based on engine load and fuel-saving algorithms. A VSR cannot compensate for this — your leisure battery gets inconsistent, incomplete charging.

A DC-DC charger solves this by:

• Boosting low alternator voltage — even when the smart alternator drops to 12.8V, the charger boosts output to the correct absorption voltage for your leisure battery.
• Limiting current — protects your alternator from overload by capping the draw at a safe amperage (typically 20A to 50A).
• Matching battery chemistry — delivers the exact voltage profile that AGM, GEL or lithium needs, including temperature compensation on some models.
• Isolating the two banks — prevents the leisure battery from draining the starter battery when the engine is off.

2. How a B2B charger works

Internally, a DC-DC charger is a switch-mode power converter. It takes the input voltage from the starter battery side (which mirrors alternator output) and steps it up or down to produce the exact output voltage your leisure battery needs at each charge stage.

The charging cycle follows a standard three-stage profile:

• Bulk — maximum current at rising voltage until the battery reaches absorption voltage (e.g. 14.4V for AGM, 14.2V for LiFePO4).
• Absorption — holds voltage constant while current tapers as the battery approaches full.
• Float — drops to a maintenance voltage (13.6V typical) to keep the battery topped up without overcharging.

The charger starts automatically when it detects engine running (input voltage above a threshold, typically 13.1V) and shuts off when the engine stops. Some models like the Victron Orion Smart allow you to configure this threshold and add a remote on/off switch or ignition signal wire for manual control.

3. Cable and fuse sizing

Getting the cable gauge right is critical. Undersized cables create excessive voltage drop, which reduces charging efficiency and generates dangerous heat. Oversized cables waste money and are harder to route.

The factor of 2 accounts for both the positive and negative cable runs. Target a maximum 3% voltage drop across the full circuit (at 12V, that means less than 0.36V total).

4. Components you need

Here is the complete parts list for a standard DC-DC charger installation:

• DC-DC charger — matched to your battery chemistry and desired charge rate (20A, 30A or 50A).
• Positive cable — from starter battery to charger input, and from charger output to leisure battery. Sized per the table above.
• Negative cable — same gauge as positive. Runs from charger negative terminals to each battery's negative post (or common chassis ground if using chassis as return path).
• Inline fuses or circuit breakers — one near the starter battery, one near the leisure battery. Use ANL or MEGA fuses for currents above 30A.
• Ring terminals — crimped and heat-shrunk onto each cable end. Match the terminal stud size to your battery posts and fuse holder.
• Cable grommet — where the cable passes through the bulkhead from engine bay to living space. Prevents chafing and water ingress.
• Ignition signal wire (optional) — thin 0.5mm2 wire from a switched ignition source (fuse box, ignition-on relay) to the charger's remote input. Required on some models for smart alternator compatibility.
• Mounting hardware — screws, vibration-dampening pads, cable clips, and ties.

5. Step-by-step wiring sequence

Follow this order to avoid accidental shorts during installation. Disconnect both batteries before you start.

Step 1: Mount the charger

Choose a ventilated location close to the leisure battery (see section 6). Secure it with screws through the mounting tabs. Leave at least 5cm clearance on all sides for airflow.

Step 2: Run the cable from the starter battery

Route the positive cable from the starter battery area to the charger location. Use a grommet where it passes through the bulkhead. Secure with clips every 30cm — avoid sharp edges, moving parts and heat sources (exhaust manifold).

Step 3: Install the fuse at the starter battery

Attach an inline fuse holder within 30cm of the starter battery positive terminal. Do not insert the fuse yet — leave it empty until the final step.

Step 4: Connect negatives first

Connect the negative cable from the charger's input negative terminal to the starter battery negative (or a solid chassis ground point near it). Then connect the charger's output negative to the leisure battery negative post. Negatives first means no live circuit exists yet.

Step 5: Connect charger output to leisure battery

Run the positive cable from the charger output terminal to the leisure battery positive post, through an inline fuse within 30cm of the battery.

Step 6: Connect charger input to starter battery cable

Terminate the positive cable (from step 2) at the charger input positive terminal.

Step 7: Connect the ignition signal (if required)

Run a thin wire from a switched 12V source (live only when ignition is on) to the charger's remote/ignition input. This tells the charger the engine is running. Some chargers detect this automatically via input voltage and do not need this wire.

Step 8: Final check and power-on

Inspect every connection. Verify no bare copper is exposed, all terminals are tight, and cables are secured away from moving or hot parts. Then insert the fuse at the starter battery end. Start the engine — the charger should begin its bulk charge within seconds. Verify with a multimeter that the leisure battery voltage is rising.

6. Mounting and ventilation

DC-DC chargers convert energy, and some of that energy becomes heat. A 30A charger running at full load dissipates roughly 40 to 60W of heat — equivalent to a small soldering iron. Without airflow, the charger will thermal-throttle (reduce output current) or shut down entirely.

Best practices:

• Mount vertically if the manufacturer recommends it — heatsink fins work best when convection can flow upward.
• Never in a sealed box — if you must enclose it, add ventilation holes or a small 12V fan with a temperature switch.
• Keep away from the battery — batteries vent hydrogen (lead-acid) or can off-gas in failure (lithium). A 15cm gap minimum is good practice.
• Accessible for maintenance — you will want to check connections, read status LEDs, and potentially update firmware via Bluetooth on smart models.

Popular mounting locations: under the passenger seat, on the bulkhead panel (living-space side), inside a ventilated electrical cabinet, or under a raised bed platform with open airflow.

7. Common mistakes to avoid

After helping thousands of van builders through the OffroadWatt community, these are the wiring errors we see most often:

8. Popular DC-DC charger models compared

Here are the most common B2B chargers used in campervan builds across Europe and North America, with key specs for comparison:

Choose based on your required charge rate, whether you want combined solar input, and your budget. A 30A charger is sufficient for most builds — it delivers roughly 20 to 25 Ah per hour of driving after efficiency losses, which means 2 hours of driving replaces 40 to 50 Ah of overnight consumption.