How to Size a DC-DC Charger for Your Van or RV

A DC-DC charger is the right way to charge an auxiliary (house) battery from your vehicle’s alternator — whether you’re in a van, RV, truck camper, or boat. It does something a simple battery-to-battery wire or relay can’t: it isolates the two battery banks, charges the aux battery with the correct profile for its chemistry, and works safely with modern smart alternators that don’t hold the voltage a relay needs to pass significant current.

Why You Need a DC-DC Charger (Not a Relay or Isolator)

The traditional “battery isolator” or solenoid relay approach connects the starter and aux batteries in parallel when the engine runs — voltage from the alternator flows directly into both banks at once.

This has two problems:

Wrong chemistry profile. Alternators regulate to a fixed voltage — typically 13.8–14.2V for a 12V system. That might be fine for an AGM starter battery, but a LiFePO4 aux battery needs a specific charge profile with controlled absorption and no float. A direct connection gives the lithium battery whatever voltage the alternator happens to produce, which is not the same as a proper lithium charge curve.

Smart alternator incompatibility. Most vehicles built after roughly 2012 use variable-voltage “smart” alternators that drop output when the battery is detected as full. When a relay suddenly connects the aux battery and pulls the voltage down, the smart alternator sees a load spike, misreads the situation, and reduces its output — sometimes to near-zero. The result is the aux battery barely charges despite the engine running.

A DC-DC charger solves both problems. It sits between the starter and aux banks, draws what it needs from the alternator at whatever voltage the alternator is producing, and outputs a properly controlled charge profile tailored to your aux battery chemistry.

How DC-DC Chargers Are Sized

DC-DC chargers are rated by their output current — the amperage they deliver to the aux battery. Common sizes in the Victron Orion and Orion XS lineup are 12A, 17A, 18A, 30A, and 50A.

The right size depends on three things:

1. Aux Battery Capacity

The general rule: charge at a rate of C/5 to C/2 — meaning 1/5 to 1/2 of your battery’s amp-hour capacity. A 100Ah LiFePO4 battery can comfortably accept 20–50A of charge. A 200Ah bank could take 40–100A.

You don’t need to charge at the maximum rate the battery can accept — but you want enough current to meaningfully charge the bank during a typical drive. A 12A charger going into a 200Ah depleted bank is going to take a very long time to make a dent.

2. Alternator Headroom

The DC-DC charger draws from the starter battery and alternator. The alternator has to serve everything: engine systems, vehicle electronics, the starter battery, and now your DC-DC charger’s input. If your alternator is producing 80A total and 50A is consumed by the vehicle, you have 30A of realistic headroom.

A 30A DC-DC charger pulling 30A input continuously is going to keep the alternator near its limit. A 17A or 18A charger gives more room. Sustained high alternator load also generates heat — relevant for long highway drives in hot weather.

Rule of thumb: Don’t put a DC-DC charger larger than 30% to 40% of your alternator’s rated output without understanding how much of that output is already spoken for.

3. Wiring Constraints

Larger chargers require heavier cable runs. A 30A charger needs 10 AWG minimum for short runs; a 50A charger needs 8 AWG or 6 AWG depending on the run length. Both the input (from starter battery) and output (to aux battery) sides need to be properly fused.

In a van build where the battery is in the back and the alternator is in the engine bay, a 10–15 foot cable run is common. Every additional foot of run increases voltage drop — which reduces how much current the charger can pull effectively. A proper wire sizing calculation (taking into account run length and acceptable voltage drop) should precede your charger size decision.

Common Sizing Scenarios

Isolated vs Non-Isolated

DC-DC chargers come in two types: isolated and non-isolated.

Isolated means there is no direct electrical connection between the input and output sides of the charger — the two battery banks share no common ground path through the device. This is the right choice for most van and RV builds, and it is required when the two batteries are in separate chassis (like a tow vehicle charging a trailer battery).

Non-isolated is lower cost and slightly more efficient because it skips the transformer — but it does share a ground path. This is fine in a single-chassis build where the batteries are in the same vehicle and share a common chassis ground anyway.

For most van builds: isolated. For boats, where galvanic corrosion from shared grounds is a real concern: always isolated.

Don’t Forget the Temperature Cutoff

If your aux battery is LiFePO4, the DC-DC charger needs to stop charging if battery temperature drops below 0°C — charging cold lithium causes permanent damage. The Victron Orion XS supports temperature-based charge cutoff via a temperature sensor connected to the charger or via communication with the battery’s BMS. Configure it, don’t skip it.

Browse the Blue Marine DC-DC charger collection — we carry the full Victron Orion and Orion XS lineup in isolated and non-isolated configurations. For the full background on why alternator charging with lithium requires specific attention, see our alternator charging guide. If you’re still in the planning phase for your battery bank, see our RV battery bank sizing guide first — the right charger size follows directly from the right bank size. Questions about your specific build? Schedule a free consultation with our team.

Related reading:
Alternator Charging with Lithium Batteries: What You Need to Know
How to Size a Battery Bank for Your RV: A Practical Guide
Victron Orion DC-DC Chargers and Converters: Full Guide