Vehicle-to-auxiliary charging
DC-DC charger size calculator
A 50 A charger output is not a 50 A alternator load. Compare the power needed at the vehicle input with available alternator headroom, battery/BMS acceptance and the exact charger's limits.
DC-DC / B2B / leisure and auxiliary battery planning · reviewed 11 September 2026
1. Define the charging operating point
Leave unknowns blank. Evidence references travel with the circuit in its editor link and notes; use manual references, not personal information.
The charging-current setting you want to check, not battery capacity in Ah.
At the charger input, after input-cable loss, while charging at the worst vehicle operating point.
At the charger output. Use the maximum intended charge voltage; a nominal 12 V bank may charge above 14 V.
Use an evidenced lower-bound efficiency at this voltage/current/temperature. A published maximum is not guaranteed.
Exact model data, measurement conditions or an explicitly stated planning assumption.
2. Establish available vehicle current
Use sustainable alternator output at hot idle or the worst relevant speed, with manufacturer/vehicle approval. A nameplate rating alone does not establish this.
Include simultaneous vehicle loads and starter-battery recovery allowance.
Your documented margin; enter 0 explicitly if none is required by your basis.
Record source, engine speed, heat and worst load conditions. An alternator's rated maximum is insufficient.
3. Check the battery and BMS
Use whole-bank limits for the actual configuration. Other sources reserve part of the charging budget even if the house loads might consume some of their output.
For a bank without a BMS, document the applicable battery/charge-control limit. Never leave it implicitly unlimited.
Solar, shore or other sources: enter an evidenced maximum, or 0 explicitly.
Confirm the voltage/profile, current temperature and working charge-disable controls. A true BMS cutoff prohibits charging.
4. Check the exact charger
Charger suitability is unknown
A known upper bound does not fill in missing evidence or recommend a universal charger size.
- Requested output power
- Unknown
- Required vehicle input power / current
- Unknown
- Available vehicle input headroom
- Unknown
- Output ceiling from vehicle headroom
- Unknown
- Output ceiling from battery / BMS
- Unknown
- Output ceiling from charger model
- Unknown
- Partial upper bound — not a recommendation
- Unknown
Still needed
- Requested output (A)
- Minimum input voltage (V)
- Charging voltage (V)
- Efficiency (%)
- Efficiency basis
- Available alternator output (A)
- Vehicle loads (A)
- Vehicle reserve (A)
- Vehicle evidence at hot idle / worst operating point
- Battery charge limit (A)
- BMS / charge-control limit (A)
- Other simultaneous charging (A)
- Battery charge-limit evidence
- BMS / charge-control evidence
- Battery temperature, voltage/profile and charge-disable controls
- Exact charger model limits
Worked power budget
Output current and input draw are different.
Illustrative inputs: 30 A at 14.4 V output, 12.5 V minimum loaded input and assumed 90% efficiency. Vehicle output 100 A minus 40 A loads and 10 A reserve; battery/BMS limits 60/50 A minus 10 A other charging. These are synthetic values, not a charger recommendation for your vehicle.
Requested output fits the entered limits
Conditional planning result, based on your stated evidence. Operating controls can still reduce or stop output.
- Requested output power
- 432.0 W
- Required vehicle input power / current
- 480.0 W / 38.40 A
- Available vehicle input headroom
- 50.00 A
- Output ceiling from vehicle headroom
- 39.06 A
- Output ceiling from battery / BMS
- 40.00 A
- Output ceiling from charger model
- 39.06 A
- Combined output ceiling at this operating point
- 39.06 A
Determining limit: Vehicle headroom + Charger model
Method and limits
The smallest evidenced limit sets the budget.
Output power = charging voltage × output current. Input power = output power ÷ efficiency. Vehicle input current = input power ÷ minimum loaded input voltage. Headroom = sustainable alternator current − vehicle loads − reserve, floored at zero. That headroom converts to an output-current ceiling using the same voltage and efficiency assumptions.
Battery acceptance = the smaller of battery and BMS charge limits, minus all other simultaneous charging, floored at zero. Charging permission, temperature and BMS controls still apply. Amp-hours alone cannot establish charge acceptance or a charger rating. We compare this budget with vehicle headroom and the model’s input/output limits.
A partial ceiling is an upper bound from the evidence entered so far. It remains unknown as a fully checked budget until the missing vehicle, battery, efficiency and model evidence is supplied. A result within these limits is conditional on your evidence; it does not establish vehicle compatibility or authorize installation. For another model, validate the full voltage range and derated current/power behavior in its manual.
For Orion XS 12/12-50A, the calculator uses the exact technical-data current range. The online settings page still contains conflicting 1–70 A output wording. Cooling, engine shutdown detection, input lockout, state of charge and BMS requests may reduce or stop charging; the model does not simulate those controls. REDARC solar sharing and towing connections need their own model documentation.
The editor stores this checked operating point and its evidence. It does not rerun this power balance when you change voltage or charger output on the canvas. Recalculate both input and output currents here after such edits. The vehicle supply is shown at the minimum charger-input voltage, not as a verified alternator or factory harness. Battery capacity, chemistry, physical controls, fuses and fault current remain unverified.
Use this for the DC charging question in an RV, campervan leisure-bank or 4WD dual-battery system. It does not verify an AC/mains installation, towing harness, UK compliance or AS/NZS coverage.
Primary sources