What it depends on
- Alternator rating. Find it on the alternator label, on the van's build sheet, or from a dealer using your VIN. Some vans came with optional higher-output alternators or an upfitter connection point. Check what yours actually has before you plan around it.
- What the van already uses. Headlights, the HVAC blower, heated seats, wipers, rear defrost and the engine's own electronics all come first. A winter night drive uses a lot more than a summer afternoon.
- Idle vs driving. Alternators put out less at idle. Charging while parked and idling is a different number from charging at highway speed.
- Heat. A hot alternator puts out less, and many chargers reduce their output when they get hot too.
- Smart alternators. Many newer vans use a variable-voltage alternator that drops its voltage when the starter battery is full. A charger without a mode or ignition signal for this may barely charge. Pick a charger whose manual covers your alternator type, and set it up the way the manual says.
- Input side vs output side. A charger's rating is its output. It pulls more than that from the van, because of conversion losses and because the input voltage is lower than the output voltage.
- Cable run. The run from the starter battery to the charger is often long. Too small a cable and the charger sees low voltage and cuts back.
Worked sample (planning estimates)
A 50 A DC-DC charger.
- Output: 50 A × about 14.2 V ≈ 710 W into the house battery.
- Input: at an assumed 88% efficiency, the charger pulls about 710 ÷ 0.88 ≈ 807 W.
- At about 13 V on the van side, that's roughly 807 ÷ 13 ≈ 62 A from the alternator.
Now the alternator. Say it's rated 200 A (sample number, use yours), and the van's own loads on a winter night drive are about 70 A (sample assumption).
- On paper: 200 − 70 = 130 A available.
- The charger wants about 62 A. It fits on paper, with room.
- At idle, or hot after a long climb, available output drops. That's when the margin shrinks.
A 30 A charger, same math: 30 × 14.2 ≈ 426 W out, about 484 W in, about 37 A from the alternator. Smaller draw, slower charging.
Where it gets tight is stacking: two chargers in parallel, or a big charger on a smaller alternator. If the math leaves you near the alternator's rating, size down or ask the van maker's upfitter support what the alternator is meant to carry.
All of these are planning estimates. The charger manual sets the real input current, fuse sizes and cable sizes.
What a DC-DC charger gives you
For a sense of scale: at about 710 W out, one hour of driving puts back roughly 700 Wh. In a build using 1,200 Wh a day, that's more than half a day from an hour on the road. That's why the alternator does so much of the work in winter, when solar is weak.
When to bring in a professional
The run from the starter battery to the charger carries high current near the engine bay, with fuses at both ends per the charger manual. Where you're allowed to tap into the van's electrical system depends on the van maker's guidance. Have a qualified electrical tech or upfitter design or check this run before you connect it.