Published 12 November 2026 · OFM4x4.co.uk

A second battery is not automatically charged well by connecting it to the starting battery. Vehicle charging behaviour and cable length affect the installation.

The quick answer

Select a DC-DC charger approved for the vehicle and auxiliary battery, then size its input protection and wiring to the actual charger instructions. Check how the unit detects the engine running.


Identify the charging system

Record the vehicle year, alternator and any energy-management controls. Some chargers use an ignition or engine-run signal; others use voltage detection. Follow the chosen model's instructions rather than copying another vehicle's wiring diagram.


Match the auxiliary battery

Check chemistry, capacity, permitted charge current and temperature protection. A higher charger rating is not always better if the battery or alternator installation cannot support it.


Calculate the cable path

Both supply and return paths matter. Long or undersized cable creates voltage drop and heat. Use the manufacturer's permitted cable size, fuse location and routing guidance for the complete circuit.


Verify operation

Measure input and output behaviour under a normal operating condition and confirm the charger enters the intended mode. Recheck terminals after initial use and investigate fault codes before changing settings.


Specification checklist

Before ordering or booking work, have these details ready: Vehicle and alternator details, battery model, charger model, cable length and planned protection.


Frequently asked questions

Can I choose the charger by battery amp-hours alone?

No. Vehicle charging behaviour, battery charge limit and wiring also constrain the choice.

For an application check, contact OFM4x4.co.uk with the vehicle and component details above. Confirm the current fitting instructions for the exact product before work starts.

Technical reference: www.victronenergy.com. The relevant product manual takes priority over general guidance.