Quick answer
Usually because too little voltage reaches the van. The cable from the tow vehicle is long, often too thin (the charging pins on a 12-pin plug are commonly wired with about 2.5 mm² cable), and many newer vehicles have smart alternators that lower their voltage while you drive. Check fuses at both ends, the plug connections and the earth return first. The lasting fix is usually heavy twin cable through an Anderson plug to a DC-DC charger mounted near the caravan battery.
Is this a DIY job?
- Difficulty: Medium to diagnose; Pro to rewire on modern vehicles
- Time: 30 to 60 minutes to check; half a day or more to rewire
- Tools: digital multimeter, a helper, trailer circuit tester, torch
- Skills: measuring voltage with a multimeter
The checks below are safe for most people with a multimeter. Running new heavy cable, fitting a DC-DC charger or wiring around a smart alternator’s battery sensor is a job for an auto electrician.
Step by step: the checks
- Start safely. Engine off, van unplugged from mains power, and nothing loose near the batteries.
- Work out how your van is meant to charge. Look for a 12-pin plug, a separate Anderson plug, or both. Check whether there’s a DC-DC charger in the van or in the vehicle. If the van only has a 7-pin plug, it has no proper charging circuit from the vehicle.
- Make sure nothing is wired to pin 2. On Australian 7-pin and 12-pin plugs, pin 2 is reversing lights. It’s only live when the vehicle is in reverse (if at all), so it can’t charge a battery. Auxiliary power should never be wired to it. (see What’s the difference between 7-pin flat, 7-pin round and 12-pin trailer plugs?)
- Check the fuses or circuit breakers at both ends. There should be one near the vehicle’s battery and one near the van battery or charger input. A tripped breaker or blown fuse at either end stops charging. (see How do I check and replace a blown fuse?)
- Inspect the plugs. Look at the Anderson plug contacts and the 12-pin pins for corrosion, burn marks, loose crimps or bent pins. A plug that’s discoloured or melted means a high-resistance connection: stop using it and get it repaired. (see How do I wire an Anderson plug?)
- Measure voltage at the vehicle battery. Engine running, van connected. This is point A. Write it down.
- Measure voltage at the van end with the charger working. At the charger’s input (point B), or at the van battery if there’s no charger. A helper can keep the engine running. A big difference between A and B means voltage drop in the cable, connections or earth. On a DC-DC charger, the input voltage should be close to the vehicle battery voltage.
- Check the earth return. The charging current has to come back to the vehicle battery. If it returns through a thin wire, a single 12-pin earth pin or the towball and chassis, much of the voltage can be lost there. The negative should be the same size as the positive.
- Check for a smart alternator. If point A drops to around 12.5 V after a few minutes of driving, the vehicle probably has a smart alternator. A plain cable or relay won’t charge the van well from that. (see What is a smart alternator and how does it affect accessories?)
- If you have a DC-DC charger, check its signals and settings. Many need an ignition or “D+” wire to switch on with a smart alternator. Check the status lights and that it’s set for your battery type (lithium needs a lithium profile).
- Check the fridge circuit separately. The most common caravan fridge is a three-way fridge that runs on gas, 12V or 240V. On 12V it’s meant for use while towing, and it draws more current than most other loads in the van, so it needs its own heavy, fused feed from the tow vehicle and drops off quickly if the cable is thin. If the fridge isn’t cooling on the road, check it’s switched to 12V, then check its fuse and feed. When parked, switch it to gas or 240V.
- Decide on the fix. If the checks show big voltage drop, thin cable or a smart alternator, the usual fix is heavy twin cable and an Anderson plug to a DC-DC charger near the van battery, fused at both ends. (see DC-DC charger or VSR isolator: which do I need?)
Specs table
Charging-related pins on Australian trailer plugs
| Plug | Pin | Function | Colour | Use for charging? |
|---|---|---|---|---|
| 7-pin flat or round | 2 | Reversing lights | Black | No |
| 12-pin flat | 2 | Reversing lights | Black | No |
| 12-pin flat | 8 | Permanent 12V | Orange | Yes, if wired with heavy cable and fused |
| 12-pin flat | 9 | Ignition-switched 12V or auxiliary | Pink | Sometimes (often used for a fridge or charger signal) |
| 12-pin flat | 10 | Second earth | White | Return path for pins 8 and 9 |
| Anderson SB50 | – | Positive and negative, heavy contacts | – | Yes: 50 A rating with contacts to suit the cable |
A common 12-pin wiring recommendation for pins 8, 9 and 10 is 2.5 mm² cable, which is thin for a long charging run. Pins 8 and 9 are used differently by different installers: always test what your vehicle and van actually carry.
Example: voltage lost in the cable, 10 m one-way run (20 m out and back)
| Cable (copper area) | At 10 A | At 20 A |
|---|---|---|
| 2.5 mm² | 1.38 V | 2.75 V |
| 6mm auto (4.59 mm²) | 0.75 V | 1.50 V |
| 8 B&S (8.37 mm²) | 0.41 V | 0.82 V |
| 6 B&S (13.3 mm²) | 0.26 V | 0.52 V |
| 4 B&S (21.2 mm²) | 0.16 V | 0.32 V |
How this is worked out: voltage drop = current × resistance of the full loop, using copper at about 0.0172 ohm per metre for 1 mm² at 20°C. Plugs, crimps, fuses and a chassis earth add more loss, and hot cable loses a little more. The 10 m run is only an example: measure your own and use the full table in our cable size guide. (see What size cable do I need for my 12V accessory?)
To put that in context: a fully charged lead-acid battery rests at about 12.6 V, and a conventional alternator charges at roughly 13.5 to 14.8 V. Lose 1.5 V in the cable and there’s little or nothing left to push charge into the van battery, even before a smart alternator lowers its output.
What you’ll need
- DC-DC charger (with a lithium setting if your van battery is lithium)
- twin core auto cable or heavy duty single core auto cable (6 B&S is common for this run, and is the largest size a standard 50 A Anderson contact accepts)
- Anderson plug 50A with contacts to suit the cable, plus Anderson plug mounting bracket and Anderson plug cover
- circuit breaker or MIDI fuse holder (one at each end)
- cable lugs and adhesive heat shrink
- digital multimeter and trailer circuit tester
Why fuse both ends?
The charging cable connects two batteries: the vehicle’s at one end and the van’s at the other. If the cable shorts to the chassis anywhere along its length, either battery can feed the short. A fuse or circuit breaker close to each battery protects the cable from both directions. Size each one to protect the cable, not just to suit the charger.
When to call an auto electrician
- Your vehicle has a smart alternator, a battery sensor, stop-start or battery management.
- You need new cable run the length of the vehicle, through the chassis or past airbag zones.
- You’re fitting a DC-DC charger, lithium battery, solar or an inverter in the van.
- A plug, cable or terminal is getting hot, discoloured or has melted.
- You want 240V wiring changed in the van. Fixed 240V wiring is for a licensed electrician only.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Can I charge my caravan battery through a 7-pin plug? A: No. A 7-pin plug has no charging circuit, and pin 2 is for reversing lights. Use a 12-pin plug wired with heavy enough cable, or better, a separate Anderson plug.
Q: Will a 12-pin plug charge my caravan battery? A: Only if pin 8 is wired with heavy cable from the vehicle battery and fused properly. Even then, the long run and the plug’s small pins mean many vans charge poorly without a DC-DC charger.
Q: Where should the DC-DC charger go: in the car or the caravan? A: Usually close to the battery it’s charging, which for a van battery means in the van. The heavy cable from the vehicle feeds it, and the charger makes up for some of the voltage lost on the way.
Q: Why does my fridge only work while I’m driving? A: Most caravan fridges are three-way fridges that run on gas, 12V or 240V. The 12V setting is designed for towing and is usually fed from the tow vehicle while the engine runs, so it can’t flatten the vehicle battery. When you stop, switch the fridge to gas or 240V.
Q: How much solar do I need to keep my van battery charged? A: It depends on your daily use and battery size. Solar can top up what the tow vehicle can’t. (see How much solar do I need for my caravan?)
Sources
- Wikipedia, “Trailer connectors in Australia” – pin 2 is reversing lights and auxiliary power should never be wired to it; 12-pin pins 8 (permanent +12V, orange), 9 (ignition +12V, pink) and 10 (earth, white) with a recommended cross-section of 2.5 mm². https://en.wikipedia.org/wiki/Trailer_connectors_in_Australia (checked October 2026)
- ExplorOz, “Trailer wiring diagrams” – 7-pin and 12-pin pin functions and colours. https://www.exploroz.com/vehicle/electrics/trailerwiringdiagrams.aspx (checked October 2026)
- Solar4RVs, “Trouble shooting and fault finding problems on caravans” – check the Anderson connection and fusing; DC-DC input voltage should be close to the starter battery; large difference means cable too small or high resistance; ignition or D+ wire on variable-voltage alternators. https://www.solar4rvs.com.au/blogs/buyer-guides/trouble-shooting-fault-finding-problems-on-caravan (checked October 2026)
- Caravan World, “Tech: DC-DC battery management” (Collyn Rivers) – voltage drop in long thin cable; mount the DC-DC unit close to the auxiliary battery; heavy supply cable (at least 13.5 mm²) and an Anderson connector. https://caravanworld.com.au/blogs/reviews/tech-dc-dc-battery-management (checked October 2026)
- CarsGuide, “Vehicles fitted with a smart alternator in Australia” – smart alternator voltage test (about 12.5 V after 5 to 10 minutes). https://carsguide.com.au/car-advice/vehicles-fitted-with-a-smart-alternator-in-australia-97156 (checked October 2026)
- Anderson Power Products, SB50 connector datasheet – 50 A rating; accepts wire from 16 to 6 AWG (about 1.3 to 13.3 mm²). https://www.farnell.com/datasheets/273617.pdf (checked October 2026)
- Resistivity of copper (about 1.72 × 10⁻⁸ ohm-metre at 20°C), standard physical constant used to calculate the voltage-drop example. Cable areas as published in our cable size guide.
- JS Alternators, “How to test your alternator: complete guide” – resting battery and normal charging voltage ranges (as used in our alternator article). https://js-alternators.com/blogs/blog/how-to-test-alternator (checked October 2026)