12V Install Tools & Testers

DC-DC Charger Size Calculator

Estimate the DC-DC charger rating you need to put your daily energy back into an auxiliary battery while you drive.

This gives a preliminary estimate of the DC-DC charger size for recharging an auxiliary battery from your vehicle's alternator. It is not an installation design. Every assumption is shown and can be changed. Everything is worked out in your browser. Nothing is saved or sent anywhere.

1. Your auxiliary battery

Changing this resets the usable capacity and round-trip efficiency below to their defaults.
Defaults match our battery calculator: 80% for lithium, 50% for AGM, gel and flooded. Use your battery maker's figure.
If you enter it, the calculator warns when a charger is bigger than the battery accepts.

2. Your energy use and driving

Not sure? Work it out with the Daily Energy Use & Battery Size Calculator, or check a single load with the Battery Runtime Calculator.
Often the same as your driving time.

3. Your vehicle

Lights, fans, fridge, start battery recharge and so on, if you know them.
There's no universal figure. Alternators often can't give their full rating continuously, hot or at idle.

4. Assumptions

The defaults come from manufacturer documents listed under "Sources" below. Change them to match your own equipment.

Victron gives 92% for LiFePO4 and 80% for an average lead-acid battery.
Victron Orion-Tr Smart chargers are rated 87 to 89%. Check your charger's datasheet.
Used for charger power. Check your battery maker's absorption voltage.
Defaults to the nominal vehicle voltage. A lower input voltage means more input current.
Edit to match the chargers you're considering, separated by commas.

5. Results

How it was worked out

    Estimate the voltage drop cable size for the charger's input current →

    Before you buy or install a charger

    • This is a preliminary estimate, not a verified installation design. A charger rating on its own doesn't guarantee a full recharge. Charging profiles, how much current the battery accepts, temperature, alternator behaviour, cable losses and charger derating in heat all change real results.
    • Charging time here assumes the charger runs at its full rating the whole time. Real charging slows as the battery nears full. Lead-acid batteries (AGM, gel, flooded) need a long absorption stage, and Victron notes their charging "becomes particularly inefficient" above about 80% state of charge.
    • The alternator and vehicle wiring must suit the extra load. Don't assume an alternator can supply its rated output continuously. Ask the vehicle maker or an auto electrician.
    • Smart (variable voltage) alternators may need a charger that supports them, with its ignition or voltage-sensing wiring connected as the charger's manual shows.
    • The charger must support your battery's chemistry and the battery maker's charging specifications, including charge voltage, maximum charge current and temperature limits.
    • A 24V vehicle charging a 12V battery, or a 12V vehicle charging a 24V battery, needs a charger made for that combination.
    • Cable sizing is a separate job. It needs voltage drop, current-carrying capacity, installation conditions and protection all checked. Our Cable Size Estimator checks voltage drop only. See our 12V Cable and Fuse Size Guide.
    • Fit fuses or circuit breakers as the charger's manual specifies, with suitable isolation and installation practices. Have the installation checked by a qualified auto electrician, and check the Australian requirements that apply to your vehicle or caravan. Any 240V work must be done by a licensed electrician.

    Sources for the default assumptions

    Next steps

    Read What Size DC-DC Charger Do I Need? for the practical side, and see how driving and solar together cover your daily use with the Solar & Charging Check. Already have a charger? See how long it takes to recharge from any charge level with the Battery Charging Time Calculator. Wiring it in? See our Dual Battery Wiring Diagram.

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