Short answer: for DC-DC charger size, take the watt-hours a day that solar won't cover, divide by 12.8 V and by the hours you usually drive. That's the amps you need. Most van builds land on 20, 30 or 50 A. Then check your alternator, your battery's charge limits and the cable run, which for a 30 A charger is often 25 mm².
For us, the DC-DC charger is what makes UK winter trips work. In December, a roof full of solar puts back very little, so most of the charging happens while we drive between stops. Getting the size right means the battery is properly topped up by the time we park, without asking more of the van's alternator than it can give.
What a DC-DC charger does
A DC-DC charger, also called a battery-to-battery or B2B charger, sits between the van's starter battery and your leisure battery. When the engine runs, it takes power from the alternator and charges the leisure battery with the right voltage and charge profile for that battery type, rather than just connecting the two batteries together. When the engine stops, it stops, so it won't drain the starter battery.
That matters more on modern vans. Victron's manual for its Orion XS charger says a conventional alternator gives a fixed voltage (about 14 V), but a smart alternator's voltage can vary between 12.5 V and 15 V on a 12 V system. It also says Euro 6 vehicles can switch the alternator off for a while, so the charger's engine detection may need fine-tuning. The Orion XS detects the engine running from the alternator voltage, without extra wiring, though some chargers and vans need an ignition signal wire instead. Check your charger's manual and your van's.
How to work out DC-DC charger size
- Daily use. Add up the watt-hours you use in a day. The solar and battery calculator does this from your appliance list.
- Take off solar. Subtract what your panels make in the season you care about: panel watts × peak sun hours × 0.75 for losses. Plan for the worst season you'll travel in. In December, PVGIS averages give the UK about 0.52 peak sun hours a day for flat panels; in July, about 4.85.
- Divide by driving time. Amps needed = watt-hours left ÷ 12.8 V ÷ hours you usually drive a day. (12.8 V is the nominal voltage of a 12 V lithium battery; Roam uses 12 V for lead acid.)
- Round up to a size you can buy. Roam's planner picks from 20, 30 and 50 A, and never goes below 20 A.
This is how Roam's planner sizes it: the worst-season gap after solar, made up over the hours you drive, rounded up to a common size. If the answer comes out higher than 50 A, the planner treats it as a sign to drive longer, add hook-up, or fit two chargers.
Worked examples
Say you use 1,000 Wh a day and have 200 W of solar. In December, those panels make about 200 × 0.52 × 0.75 = 78 Wh, leaving 922 Wh for the DC-DC charger.
| Driving a day | Amps needed | Charger |
|---|---|---|
| 1 hour | 922 ÷ 12.8 ÷ 1 = 72 A | More than one 50 A charger: drive longer or use hook-up |
| 2 hours | 922 ÷ 12.8 ÷ 2 = 36 A | 50 A |
| 3 hours | 922 ÷ 12.8 ÷ 3 = 24 A | 30 A |
In July the same panels make about 200 × 4.85 × 0.75 = 728 Wh, leaving only 272 Wh. Even an hour's drive with a 20 A charger (about 256 Wh) nearly covers it. That's the pattern for UK vans: solar does most of the work in summer, the DC-DC charger in winter. So size the charger for the season you'll actually travel in.
Put the other way round: each hour of driving, a 20 A charger puts back about 256 Wh, a 30 A about 384 Wh and a 50 A about 640 Wh into a lithium battery, before small losses.
Alternator and smart alternator limits
Bigger isn't automatically better. The charger draws its power from the alternator, which also runs the engine, lights, heater blower and everything else. The alternator's rated output is a peak, not what it can supply all day spare. And the charger draws more going in than it puts out. Roam's planner counts the input at about 1.2 times the output current, so a 30 A charger draws roughly 36 A from the van side.
Two things help here. First, check what the van manufacturer says about adding a charging load. Second, use a charger that lets you limit its input. Victron's Orion XS lets you set an input current limit from 1 A to 50 A in 0.1 A steps, and its manual says this is particularly important to avoid overloading an alternator or other weak input sources. If you're unsure about your van, an auto-electrician can check the alternator's capacity and the factory wiring.
What your battery can take
Your leisure battery has its own maximum charge current, in its datasheet. A small battery may not accept a big charger's full output, and lead acid batteries in particular slow right down as they fill: Victron's lithium battery datasheet notes the charge process of lead acid becomes particularly inefficient once 80% state of charge is reached. That's one reason lithium pairs well with DC-DC charging on short drives. Whatever you choose, set the charger's profile to match the battery type and check the battery's charge limit before you settle on a size.
Cables and fuses
A DC-DC charger has two cables, and they're very different. The input runs from the starter battery, often under the bonnet or a front seat, to wherever the charger lives in the back. That's commonly the longest run in the van. The output is usually a short hop to the busbar.
Using the same maths as our cable size calculator, with 3% drop on 12 V and 105 °C cable:
| Charger | Input, 5 m run (1.2 × output) | Output, 1 m run |
|---|---|---|
| 30 A | 36 A: 25 mm², 50 A MIDI fuse | 30 A: 4 mm², 40 A blade fuse |
| 50 A | 60 A: 35 mm², 80 A MIDI fuse | 50 A: 10 mm², 70 A MIDI fuse |
The input fuse goes at the starter battery end, the output fuse at the busbar end. On a long input run, voltage drop matters a lot: too thin a cable and the charger sees a low voltage, may reduce its output, or may decide the engine has stopped. Your charger's manual may give its own cable and fuse sizes, and if it does, follow those. Our wiring diagram guide shows where it fits in the whole system.
Roam's Build your electrics tool works out the DC-DC size from your power budget, solar and driving, then sizes both cables and fuses and adds them to your shopping list. It's in the app now, in early access. For the full order of planning, see campervan electrics.
Questions
Is a 30 A DC-DC charger enough?
For many weekend and summer vans, yes. A 30 A charger puts back about 384 Wh an hour into lithium, so two hours' driving covers a day of about 770 Wh. If you use more, drive less, or travel in winter, work it through with your own numbers.
Do I need a DC-DC charger with a smart alternator?
It's the usual answer. Smart alternators vary their voltage (12.5 to 15 V on a 12 V system, per Victron's Orion XS manual) and may switch off for a while, which a simple split charge relay doesn't handle well. A DC-DC charger gives the leisure battery a proper charge regardless.
Can a DC-DC charger damage my alternator?
A charger that asks for more than the alternator can spare puts it under strain. Choose a sensible size, use a charger with an input current limit, and check the van manufacturer's guidance or ask an auto-electrician.
What size cable for a DC-DC charger?
It depends on the current and the run. With our calculator, a 30 A charger on a 5 m input run needs 25 mm² with a 50 A fuse, because the input draws about 36 A. Check the charger's manual for its own figures.