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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 Oct 21, 2026

Worked schematic

DC-DC charging example with 30 A output

The worked 30 A charging point draws 38.4 A from the vehicle at 12.5 V, using the example's 90% efficiency assumption and 14.4 V output.

Interactive Crimp schematic. Pan and zoom to follow the circuit. This fixed example illustrates the calculation; your form inputs are separate.

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Red = positive · dark = return. Crossings are not junctions. Read the assumptions before adapting a circuit.

Vehicle side: 38.4 A

The positive and negative input cables are each 3 m one way. The 480 W input includes the example's conversion loss.

Battery side: 30 A

The positive and negative output cables are each 1 m one way. The charger supplies 432 W at the 14.4 V charging point.

Synthetic operating point from the worked calculator example, not measured vehicle or battery evidence. Voltages are operating values, not nominal bank ratings. Logical negative terminals are not a physical charger pinout.

Blue Sea 5191 terminal holders carry illustrative 5177 50 A input and 5176 40 A output MRBF fuses: the next available ratings above 125% of 38.4 A and 30 A. These are planning choices, not charger-manufacturer recommendations. Each holder mounts at its battery's positive terminal; expanded zero-length connections are mounting markers, not cable.

Verify actual charger limits, whole-bank fault current, maximum voltage, installation and control wiring. Recalculate after changing the operating point; your calculator inputs do not select these fuses.

Open this example in Crimp →Use your own inputs ↓

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

Vehicle input current for 30 A of DC-DC charging

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

Vehicle, battery and charger current limits

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, both the technical data and current settings documentation specify separate 1–50 A input and output limits. 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

DC-DC charger manufacturer specifications

Victron EnergyOrion XS 12/12-50A: exact technical dataRev. 11 — 06/2026, §6.1; read 11 September 2026. Voltage/current limits, maximum efficiency, temperature derating and nominal-power footnote.Victron EnergyOrion XS: engine detection and settingsInput and output limits are separately configurable from 1–50 A. Engine detection, voltage lockout and BMS controls can reduce or stop charging; checked 21 October 2026.Victron EnergyOrion XS: mounting and BMS-controlled operationActual cooling, remote charge-disable controls and manufacturer wiring requirements remain necessary.Blue Sea Systems5191 battery-terminal MRBF holderThe worked example mounts protection at both battery-positive terminals. Product photo shows a sample fuse; ratings are selected separately. Checked 21 October 2026.Blue Sea Systems5177 MRBF 50 A fuseIllustrative vehicle-side selection above 125% of 38.4 A; not a charger-manufacturer recommendation.Blue Sea Systems5176 MRBF 40 A fuseIllustrative house-side selection above 125% of 30 A. Verify actual charger requirements, maximum voltage and whole-bank fault current.
Keep goingWire size calculatorSize each conductor using its own current, length and installation conditions.Keep goingLiFePO₄ fuse size calculatorSelect and check protection separately from a wire or charging-current result.Keep goingDual battery wiring diagramsSee starter-to-auxiliary layouts and model-specific installation links.Keep goingSizing methodologyRead the implemented rules and their limits.Keep goingOrion XS 50A installationVerify the exact model, hardware and physical GND connection.

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