Anatomy · 12 V power

Two batteries

What a dual-battery system does, why the charger matters on a modern vehicle, and what happens overnight with one battery, a relay or a DC-DC charger. An illustrative model, not test data.

Batteries, charger, relay and fridgeChart: the battery the fridge runs on, one line per setupEngine off overnight, on the chart and the clockCurrent flowingHalf charge: conventional guidance for lead-acid and AGM is not to go below about half routinelyWon't-start lampA dual-battery system keeps the battery that starts your engine apart from the one that runs your fridge, and charges that second battery while you drive. The chart follows the battery the fridge runs on through a day's driving, the night and the next morning, for three setups. With one battery, the fridge runs on the battery that starts the engine and can flatten it. A voltage-sensitive relay keeps the two apart, but it only charges while the alternator's voltage is high, and a smart alternator drops its voltage once the starting battery is topped up, so the second battery can be left short of full. A DC-DC charger takes whatever the vehicle gives it and charges the second battery in stages, properly and completely. The batteries are treated as lead-acid or AGM, so the chart marks half charge, and the charger and display on the board are generic. Opposite Lock offers the interVOLT DCC Pro: a 12 V, 25 A in-vehicle DC-DC charger that charges your auxiliary battery using the main battery as the source. It is solar ready. The kit gives you the charger and the display; your nearest Opposite Lock fitment centre partner will spec the auxiliary battery and tray to suit your vehicle, and can confirm what your vehicle's alternator does. Fuses, cable sizes, earth leads and battery mounting are fitment work, so the drawing leaves them out or shows them only roughly. It is an illustrative model, not test data: time is sped up, and the chart's levels and the morning results come from the model. No battery capacities, charge or draw currents, or runtimes are claimed.

Check your own

Will your setup last the trip?

It depends on your fridge, your battery and how you travel, so work it out for your own setup. Power Plan adds up the loads you will run, checks them against your battery and gives you a verdict.

Open Power Plan (opens in a new tab)

The auxiliary battery and tray are not in the kit: your nearest Opposite Lock fitment centre partner (opens in a new tab) will spec them to suit your vehicle.