Why DC-DC Charger Sizing Matters
Choosing the wrong size DC-DC charger is one of the most common mistakes in campervan and motorhome electrical installations. Too small, and your batteries never fully charge during a day's driving. Too large, and you risk overloading your vehicle's alternator or spending money unnecessarily.
The Victron Orion-Tr Smart range offers models from 12A to 30A, and multiple units can be wired in parallel for higher output. This guide helps you calculate exactly which model — and how many — you need.
The Sizing Formula
The basic calculation is straightforward:
Required charge current = Daily energy use (Ah) ÷ Average daily driving hours
For example, if you use 60Ah per day and drive for 3 hours, you need a charger that delivers at least 20A (60 ÷ 3 = 20A).
However, several real-world factors affect this calculation:
Factor 1: Battery Charge Acceptance
Lithium batteries accept charge current at a consistent rate until they are nearly full. A 30A charger delivers close to 30A for most of the charge cycle. Lead-acid batteries (AGM and gel) accept progressively less current as they fill, spending significant time in the absorption phase at reduced current. This means:
- Lithium: Multiply your calculated current by 1.1 (10% overhead)
- AGM/Gel: Multiply your calculated current by 1.4 (40% overhead to account for tapering)
Factor 2: Alternator Capacity
Your vehicle's alternator has a maximum output, and it must power the vehicle's own electrical systems (ECU, lights, fans, heated screens) before any surplus is available for leisure charging. As a rule of thumb:
- Small vans (Transit Custom, Caddy): 90-120A alternator — limit DC-DC charging to 30A maximum
- Large vans (Sprinter, Crafter, Ducato): 150-180A alternator — up to 60A (2x 30A) is usually safe
- Motorhomes on truck chassis: 180-220A alternator — 60A comfortably, potentially more
Exceeding these limits will not damage anything immediately, but sustained overloading in hot weather can shorten alternator life.
Factor 3: Cable Voltage Drop
The DC-DC charger's input voltage determines how much power it can deliver. Long cable runs between the starter battery and the charger cause voltage drop, which reduces the charger's available input power. A 30A charger with only 12.2V at its input terminals will deliver less than 30A output.
Keep cable runs as short as possible and size cables generously. Use our Voltage Drop Calculator to check your installation.
Sizing by Vehicle Type
Campervans (Small to Medium)
| Battery Bank | Daily Use | Drive 1-2hrs | Drive 2-3hrs | Drive 3+hrs |
|---|---|---|---|---|
| 100Ah Lithium | 30-40Ah | 30A | 18A | 18A |
| 100Ah Lithium | 50-60Ah | 2x 30A | 30A | 18A |
| 200Ah Lithium | 60-80Ah | 2x 30A | 30A | 30A |
| 100Ah AGM | 20-30Ah | 30A | 18A | 18A |
| 200Ah AGM | 40-50Ah | 2x 30A | 30A | 30A |
Motorhomes
Motorhomes typically have higher energy consumption (larger fridges, more lighting, entertainment systems) but also tend to cover longer distances between stops. Most motorhome installations work well with a single 30A Orion-Tr Smart. For lithium battery banks over 200Ah with heavy daily use, two units in parallel provide faster charging.
Narrowboats
Narrowboat engine alternators are often larger (typically 120-175A) and engines run for longer periods (cruising). A single 30A unit is usually sufficient for a 12V system. For 24V systems, use the Orion-Tr Smart 24/24-17 or consider two in parallel.
Parallel Installation
When a single charger is not enough, two Orion-Tr Smart units of the same model can be wired in parallel. The units synchronise their charge profiles via Bluetooth to ensure balanced operation.
Parallel Wiring Rules
- Both units must be the same model (both 12/12-30, for example)
- Cable lengths from the starter battery to each unit should be equal
- Cable lengths from each unit to the leisure battery should be equal
- Each unit needs its own input and output fuses
- Enable the "Networked" setting in VictronConnect on both units
Worked Examples
Example 1: Weekend Campervan
Setup: VW Crafter conversion, 100Ah lithium battery, used mainly for weekends, drives 1-2 hours to destinations.
Daily loads: LED lighting (20W × 5h = 100Wh), compressor fridge (45W × 12h = 540Wh), phone charging (15W × 3h = 45Wh), water pump (60W × 0.5h = 30Wh). Total: 715Wh = ~60Ah at 12V.
Calculation: 60Ah ÷ 1.5 hours average driving = 40A needed. With lithium overhead: 40 × 1.1 = 44A.
Recommendation: 2x Orion-Tr Smart 12/12-30 in parallel (60A capacity). However, if the van also has 200W of solar, the solar will contribute approximately 30-40Ah on a sunny day, reducing the alternator charging requirement to around 20-30Ah. In that case, a single 30A unit is sufficient.
Example 2: Full-Time Motorhome
Setup: Fiat Ducato motorhome, 300Ah lithium, full-time living, drives 2-4 hours between stops, 400W solar.
Daily loads: 1,800Wh total = 150Ah at 12V.
Solar contribution: 400W × 3.0 PSH × 0.85 efficiency = 1,020Wh = 85Ah. Remaining from alternator: 65Ah.
Calculation: 65Ah ÷ 3 hours driving = 22A needed. With lithium overhead: 22 × 1.1 = 24A.
Recommendation: Single Orion-Tr Smart 12/12-30 (30A capacity provides comfortable headroom).
Frequently Asked Questions
Can I use a bigger charger than I need?
Yes. The charger only delivers what the battery requests. A 30A charger connected to a nearly full battery might only deliver 5A. There is no harm in oversizing, though it costs more.
Will my alternator cope with two 30A chargers?
Two 30A chargers draw approximately 70A from the alternator (accounting for conversion efficiency). Most modern van alternators (150A+) handle this comfortably. Smaller alternators (90-120A) may struggle during hot weather when already powering air conditioning. If in doubt, start with one charger and add a second later if needed.
Should I combine solar and DC-DC, or just get a bigger DC-DC charger?
Solar is always recommended as a complement. Solar charges when parked (which a DC-DC charger cannot), and reduces alternator load while driving. For any system over 100Ah, a combination of solar and DC-DC charging provides the best balance of charging speed and flexibility.
What about the Orion XS?
The Victron Orion XS is the newer generation DC-DC charger with an integrated solar MPPT input. It combines a DC-DC charger and solar charge controller in one unit. If you are building a new system and want to keep things simple, the Orion XS is an excellent choice. For existing systems where you already have a separate MPPT, the Orion-Tr Smart remains a solid option.
