A dual battery system is one of the best upgrades you can make to a caravan or 4WD — but it’s also one of the easiest to get wrong. A poorly designed system will undercharge your batteries, damage your electronics, or leave you flat in the middle of nowhere.
This guide covers the key decisions you’ll face and how to get them right.
Why You Need a DC-DC Charger (Not a VSR)
The old-school approach was to use a Voltage Sensitive Relay (VSR) to connect the auxiliary battery to the alternator. The VSR detects when the main battery is charged and then connects both batteries together.
The problem: modern vehicles with smart alternators don’t maintain a steady 14.4V like the old ones. They vary the output to save fuel, which means a VSR often won’t trigger properly, and your aux battery never gets a proper charge.
The solution is a DC-DC charger (also called a battery-to-battery charger or B2B charger). It takes whatever voltage the alternator produces and converts it to the correct charging profile for your auxiliary battery — whether that’s AGM, lithium, or gel.
Choosing the Right Battery
AGM (Absorbent Glass Mat): The most common choice. Robust, affordable, handles partial state of charge reasonably well. Good for most setups. 100Ah AGM is a popular starting point.
Lithium (LiFePO4): More expensive upfront, but significantly lighter, can be discharged to 20% without damage (vs ~50% for AGM), and have a longer cycle life. If you’re doing serious off-grid camping, lithium is worth the investment.
Sizing Your System
Work out your daily power consumption first. List every appliance with its wattage and hours of use per day. A 12V fridge running continuously is typically 30-50Ah per day. Add lighting, phone charging, a CPAP machine if needed, and anything else.
Once you know your daily draw, aim for a battery bank that is 2-3x your daily consumption — this gives you buffer and avoids constant deep cycling.
Anderson Plugs and Wiring
Anderson SB50 plugs are the standard for caravan connections. Use them to connect the tow vehicle to the van so you can charge the van’s batteries while driving.
Key wiring principles: Always fuse within 300mm of the battery positive terminal. Use the correct wire gauge for your current — undersized wire is a fire risk. Run cables through proper grommets where they pass through metal.
Solar Input
Solar is excellent for extended stays. A 200W panel combined with a quality MPPT solar controller will typically keep a 100Ah AGM battery topped up during normal use.
Common Mistakes
Installing a VSR instead of a DC-DC charger in a modern vehicle. Undersizing the cable between the charger and aux battery. Not fusing both ends of a long cable run. Mixing battery chemistries without the right charger profile. Forgetting a battery monitor — you cannot manage what you cannot measure.
Getting Help
If you’re on the Central Coast and want your system designed and installed properly, we do caravan and 4WD electrical setups. We’ll spec the right components for your use case and install them correctly the first time.