Dual Battery Wiring: Cable Sizes, Fuses and Connections
Dual battery wiring is where a lot of 4x4 and caravan setups go wrong. The auxiliary battery is fine and the charger is fine, but the cable between them is too thin, the fuses are in the wrong place, or a crimp has worked loose under the bonnet. The result is a second battery that never quite reaches full charge, or a cable that gets hot.
This guide covers the parts of a dual battery system that are about wire: cable sizes for the main charging run, where the fuses go, how to terminate heavy cable, and which joins suit Solderstick connectors. For choosing batteries and chargers, see our earlier post on dual batteries for off-roading.
Quick answer:
- Size the main run for the charger current and the distance. 8 B&S is common for short engine bay runs; 6 B&S or heavier for runs to a tray, canopy or caravan.
- Fuse both ends of any cable that connects two batteries, as close to each battery as practical.
- Terminate battery cables with properly crimped lugs sealed in adhesive-lined heat shrink. Splice connectors are for the small wires only.
- Vehicles with smart (variable voltage) alternators need a DC-DC charger rather than a simple isolator.
How a dual battery system is wired
Every dual battery setup has the same basic parts: the start battery, the auxiliary battery, something that controls charging between them, and the cables and fuses that link it all. The heavy positive cable runs from the start battery to the charger or isolator, then on to the auxiliary battery. A matching negative cable, or a solid chassis earth, completes the circuit.
There are also small wires: an ignition or trigger wire, sometimes a solar input, and the outputs from the auxiliary battery to fridges, lights and sockets. Those small wires are where most joins happen, and where splice connectors make sense.
Isolator or DC-DC charger?
This choice changes the dual battery wiring layout, so make it first.
| Feature | Battery isolator | DC-DC charger |
|---|---|---|
| How it works | Joins the two batteries when the engine is running | Converts alternator voltage into a controlled charge profile |
| Suits | Older vehicles with fixed voltage alternators | Smart (variable voltage) alternators, lithium batteries, long runs |
| Long cable runs | Voltage drop reduces charging directly | Copes better with voltage drop, though cable size still matters |
| Wiring | Heavy positive link plus earth | Heavy input and output cables, earth and a trigger wire |
Redarc, an Australian charger maker, notes that variable voltage alternators can drop too low or stop charging the auxiliary battery, so vehicles with them need a DC-DC charger. Isolators are generally suited to older vehicles, roughly pre-2015, with fixed voltage alternators. Check your vehicle and battery maker's advice.
Dual battery wiring cable sizes
The main run has two jobs: carry the current without overheating, and deliver it without losing too much voltage on the way. On a dual battery system the second job is usually the one that decides the size.
A DC-DC charger draws a little more from the start battery than it delivers, because no charger is 100 percent efficient. Size the input cable for the charger's maximum input current, which is in its manual.
Voltage drop for common runs
The table shows the drop on a 12V system, counting both the positive and negative cable (a full loop of twice the one-way length).
| Current and one-way run | 8 B&S (7.7mm2) | 6 B&S (13.5mm2) | 4 B&S (20mm2) |
|---|---|---|---|
| 25A over 1m | 0.11V (0.9%) | 0.06V (0.5%) | 0.04V (0.4%) |
| 25A over 3m | 0.33V (2.8%) | 0.19V (1.6%) | 0.13V (1.1%) |
| 25A over 5m | 0.56V (4.6%) | 0.32V (2.7%) | 0.21V (1.8%) |
| 40A over 3m | 0.54V (4.5%) | 0.31V (2.5%) | 0.21V (1.7%) |
| 40A over 5m | 0.89V (7.4%) | 0.51V (4.2%) | 0.34V (2.9%) |
A common target for charging circuits is 3 percent or less. So for a 25A charger, 8 B&S suits a short engine bay run, while a 5 metre run to a tray or canopy wants 6 B&S. At 40A over 5 metres, 4 B&S is the safe choice. These are calculations, not a replacement for the charger's install manual, which should always take priority.
Remember Australian "auto" labels: 6mm auto cable is only about 4.6mm2 and is too small for a charger's main run. Our guide to auto cable sizes in Australia explains the labels, and the 12V wire gauge guide has the formula if you want to run your own numbers.
Fuses: why both ends
A fuse protects the cable on the far side of it. In a normal circuit, power only comes from one end, so one fuse near the battery covers the whole run. In dual battery wiring both ends of the link have a battery, so a short anywhere along the cable can be fed from either side.
Fit a fuse or circuit breaker as close as practical to each battery on any cable that joins them, including the charger input and output runs. Rate each fuse to protect the cable and to suit the charger: the manual will list the size. For example, Redarc's diagrams for its 25A BCDC charger show 40A fuses.
Mount fuse holders where they are protected from water and easy to reach, and use a holder rated for the fuse size. Midi and ANL style fuses are common for these currents.
Dual battery wiring connections on heavy cable
The cables in the main run are 8 B&S or heavier, which is outside the range of any splice connector. Terminate them properly:
- Disconnect the start battery negative, and the auxiliary battery negative, before you begin.
- Cut the cable square and strip only enough insulation to fill the lug barrel.
- Use a tinned copper lug sized for the cable and the stud.
- Crimp with a tool made for that lug and cable size, then tug-test the join.
- Seal the barrel and the end of the insulation with adhesive-lined dual wall heat shrink. A 4:1 ratio tube covers the step from lug barrel to cable easily.
- Tighten terminals to the battery maker's torque and fit terminal covers on the positive side.
If you join two lengths of heavy cable, use a proper junction stud or busbar in a covered enclosure rather than a splice.
Anderson plugs for trailers and caravans
Many Australian setups run the charge cable to the rear of the vehicle and use an Anderson SB50 plug to connect a caravan or camper battery. The SB50 range takes wire from 16 AWG up to 6 AWG (13.3mm2), and the 50A housings are made for 6 AWG contacts. Crimp the contacts with the correct tool and seal the cable entry with heat shrink. Our post on off-road towing covers the towing side.
Where Solderstick connectors fit in dual battery wiring
Solderstick connectors are for the small wires around the system, not the main charge cables. Typical uses:
- Ignition trigger and signal wires for the charger (red or blue, depending on the wire).
- Extending or repairing solar input leads in the 4-6mm2 range (yellow).
- Outputs from the auxiliary fuse block to fridge sockets, lights and USB outlets (blue for 1.5-2.5mm2, yellow for 4-6mm2).
- Repairs to accessory looms in the canopy or drawer system.
Each one seals the join against dust and water to IP67, which matters in an engine bay that sees creek crossings and wash-downs. Use a heat gun to shrink them; a lighter can scorch the tube and is only a last resort. Let the join cool before you bend it into place.
Which should you use?
- If you're running a 20-25A charger with batteries side by side in the engine bay, use 8 B&S with crimped lugs and fuses at both ends.
- If you're running to a tray, canopy or caravan 4-6 metres away, use 6 B&S or heavier.
- If you're running 40A or more over a long run, use 4 B&S and check the charger manual.
- If you're wiring trigger, signal or accessory output wires, use Solderstick connectors matched to the wire colour range.
- If you're connecting a caravan battery, use an Anderson SB50 with 6 AWG contacts.
Frequently asked questions
What size cable do I need for a dual battery system?
For a 20-25A DC-DC charger, 8 B&S suits short engine bay runs and 6 B&S suits longer runs to the rear. Check voltage drop for your length and follow the charger's manual.
Do I need a fuse at both batteries?
Yes. A cable that joins two batteries can be fed from either end, so fit a fuse or breaker close to each battery to protect the whole run.
Can I use 6mm auto cable for a dual battery setup?
Not for the main charging run. 6mm auto cable is only about 4.6mm2 of copper, well short of 8 B&S or 6 B&S. It can suit accessory outputs over short runs.
Do I need a DC-DC charger with a smart alternator?
Yes, according to charger makers. Smart alternators vary their voltage and can stop charging the auxiliary battery, so a DC-DC charger is needed to control the charge.
Can I use solder seal connectors on battery cables?
No. Battery and charging cables heavier than 6mm2 need crimped or soldered lugs. Solderstick connectors suit the smaller trigger, signal and accessory wires.
Should I run a separate negative cable?
For runs to a tray, canopy or caravan, a dedicated negative back to the start battery is more reliable than a chassis earth. It also makes the voltage drop figures above a true reflection of the circuit.
What to use
For the trigger, signal and accessory side of your dual battery wiring, the 100PCS kit covers all four colours, and the 50PCS kit makes a handy spares kit for the trip. Heavy users can top up a single size with the bulk 100-pack bags.
For sealing lugs on 8 B&S, 6 B&S and 4 B&S cable, our heat shrink range includes adhesive-lined 4:1 dual wall in 12mm, 20mm and 32mm, plus 6:1 heavy wall for the biggest cables. The Ultimate Heat Shrink Bundle covers the whole range in one pack.