What Size DC-DC Charger Do You Need for a Van or RV?
A DC-DC charger is how most US van and RV builds recharge the house battery while driving — but the wrong size either undercharges your battery or risks straining your alternator. Here's how to get it right.
Size your charging system
Tell us your battery size and we'll recommend the right DC-DC charger — free.
Why you need one (smart alternators)
Most vehicles built since roughly the mid-2010s have a smart (variable-voltage) alternator that adjusts its output for fuel economy and emissions — sometimes dropping to 12.5V or lower, which won't properly charge a LiFePO4 house battery through a direct cable. A DC-DC charger:
- Takes whatever voltage the alternator is putting out
- Boosts and regulates it into a proper multi-stage charge profile for your battery chemistry
- Isolates the house battery from the starter battery so house loads can't flatten your starter
Check your alternator type before wiring direct
If you're unsure whether your vehicle has a smart alternator, assume it does. A DC-DC charger is the safe default for any vehicle from the last decade, and it's required for LiFePO4 house batteries regardless.
Sizing rule of thumb: C/7
A widely used starting point is C/7 — the charger's amperage should be roughly your battery's Ah capacity divided by 7:
| Battery size | C/7 charger size |
|---|---|
| 100Ah | ~15A (round up to 20-30A) |
| 200Ah | ~30A |
| 300-400Ah | ~45-50A |
This isn't a hard limit — it's a balance point. A bigger charger fills the battery faster on travel days, but draws more from the alternator.
Check your alternator's spare capacity
Add up your vehicle's typical electrical load (lights, HVAC, infotainment, ECU) and compare to your alternator's rated output. Most stock alternators (120-180A) have plenty of spare capacity for a 30-50A DC-DC charger, but if you're already running other heavy 12V accessories (winches, heated seats, large stereo systems), check the math before going to 100A+ units.
US picks
- Victron Orion-XS 12/12-30A (~$287) — compact, efficient, configurable LiFePO4 profile, doubles as a solar input on some models.
- Renogy DCC50S 50A (~$200) — higher output for larger banks, with a built-in MPPT solar input.
Wiring matters as much as sizing
The run from the engine bay to the house battery is long and carries real current — undersized cable causes voltage drop that reduces charging effectiveness even with the right charger. See the wiring & safety guide for AWG sizing on this run.