DC-DC chargers with a solar input, explained

Many DC-DC chargers have a second input for a solar panel, with a built-in solar regulator. That means one box can charge your second battery from the alternator while you drive and from solar when you're parked.

What's inside: an MPPT regulator

REDARC describes its BCDC "D" models as having an inbuilt MPPT solar regulator, so the auxiliary battery is maintained from both the vehicle and solar inputs.^[1][2] An MPPT (maximum power point tracking) regulator adjusts how it loads the panel to get more power from it than a basic regulator, especially when the panel voltage is well above the battery voltage.

Which input is used when both are live?

It depends on the charger. REDARC advertises a "Green Power Priority" feature on its BCDC range, which prioritises solar over the alternator when both are available.^[1] Other brands behave differently, and some combine both inputs. Check the manual for your model.

Before you plug in a panel, check

Maximum solar input voltage (open-circuit, Voc): the panel's Voc, which rises on cold mornings, must stay below the charger's limit.
Maximum solar input current or wattage: panels over this rating waste output or may not be supported.
Panel type: some chargers are meant for panels with their own regulator removed. Don't run two regulators in series unless the manual says it's fine.
Cable run: solar blankets often sit metres from the vehicle. Use the voltage drop calculator.

We don't list panel limits for specific models here because they vary and change between versions. Always use the figures in your charger's current manual.

Is a solar-input charger worth it?

If you stay put for more than a day or two, solar is what keeps the fridge running between drives. Try the fridge runtime calculator with and without solar to see the difference.

Sources

This is general information, not a substitute for the manuals for your specific charger, battery and vehicle. Fixed wiring and anything near the starter battery should be installed by a qualified auto electrician.

Last updated: 15 October 2026