Dual Battery Wiring Diagram for a 4WD (Easy DIY)

How the layout works

• The alternator charges the start battery as normal.
• The start battery positive runs through a fuse near the start battery to the DC-DC charger input.
• The charger output runs through a fuse near the auxiliary battery to the auxiliary battery positive.
• The charger and both batteries have solid earths to the chassis.
• Your 12V fuse box is fed from the auxiliary battery through its own fuse, so your camping gear never drains the start battery.

Get these right

• Fuse every positive cable at the battery end, sized to the cable and charger.
• Use cable thick enough for the current and run length. Use a voltage drop calculator and aim for under 3%.
• Crimp lugs with a proper crimper and seal them with heat shrink.
• Connect the ignition wire if you have a smart alternator.
• Follow your charger's manual for fuse sizes. If you're unsure, get an auto electrician to check your work.

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