The three limits
A DC-DC charger moves power from the van's alternator to your house battery while you drive. Its size is the smallest of:
- What the alternator can spare. The van's own systems come first, and an alternator can run hot at idle. Use the maker's guidance where there is one.
- What the battery can take. Every lithium battery has a maximum charge current on its datasheet. A Battle Born 100Ah is 50 A; two in parallel, 100 A.
- What you need. Amps × hours of driving = amp-hours put back. 30 A for an hour is about 30 Ah, a third of a 100Ah battery.
By van
| Van | Stock alternator | A charger that fits |
|---|---|---|
| Mercedes Sprinter | 180–220 A, but Mercedes allows only about 40 A for house batteries | 30–40 A (e.g. a 50 A unit turned down to 40 A) |
| Ram ProMaster | 180 A on the 3.6L (270 A replacements exist) | 40–50 A |
| Ford Transit | 250 A on 2025 3.5L V6s (dual 250 A available) | 30–50 A; more only with a high-output unit |
How much driving puts back
| Charger | Per hour of driving | 100Ah battery from half to full |
|---|---|---|
| 20 A | about 20 Ah | about 2½ hours |
| 30 A | about 30 Ah | about 1¾ hours |
| 50 A | about 50 Ah | about 1 hour |
Most days, solar does the steady refilling and the alternator covers cloudy stretches and driving days. See what size solar panel charges a 100Ah battery.
Adjustable is worth it
A charger you can turn down lets you start conservative and raise the current once you've checked the alternator and cables stay cool. Victron's Orion XS, for example, is adjustable from 1 to 50 A and only charges while the engine is running, which matters with the smart alternators in most recent vans.
Each van page (Sprinter, ProMaster, Transit) gives the charger size with the roof layout, and the van guide sizes the rest.
Questions people ask
What size DC-DC charger for a 100Ah lithium battery?
Up to 50 A: Battle Born's datasheet gives 50 A as the maximum charge current for its 100Ah battery. A 30 A charger refills about a third of it per hour of driving and is easier on the alternator and the wiring.
What size DC-DC charger for a 200Ah battery?
The battery can take more (about 100 A for two Battle Born 100Ah in parallel), so the alternator becomes the limit. On most stock van alternators that means 40–50 A, which refills about a quarter of 200Ah per hour of driving.
Do I need a DC-DC charger with a smart alternator?
Yes. A smart alternator can drop its voltage once the starter battery is full, which leaves a house battery undercharged. A DC-DC charger boosts it to the right voltage and, like Victron's Orion XS, only charges while the engine is running.
Can I charge a lithium battery straight from the alternator?
It's risky. A lithium battery will pull as much current as the alternator gives, which can overheat it at idle. A DC-DC charger caps the current; Victron's Orion XS, for example, lets you set the input limit anywhere from 1 to 50 A.
Sources
- Orion XS 12/12-50A DC-DC battery charger manual (1–50 A adjustable, input current limit, engine running detection) Victron Energy
- Battle Born BB10012 datasheet (50 A max charge current) Battle Born Batteries
- Sprinter 220 A alternator information (PDF) Mercedes-Benz (via sprinter-rv.com)
- Ford: 2025 Transit technical specifications (PDF) Ford
Every number on this page comes from these sources. Spotted something out of date? Tell me.
Get the exact setup
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