Category: 4WD, caravan & 12V

Quick answer

A VSR (voltage sensitive relay, or isolator) simply connects your auxiliary battery to the vehicle’s charging system when the engine is running. A DC-DC charger converts the vehicle’s power into a proper multi-stage charge for the auxiliary battery. Choose a DC-DC charger if your vehicle has a smart (variable-voltage) alternator, your auxiliary battery is lithium, the cable run is long (canopy, tray or caravan), or you want solar charging built in. A VSR can still suit an older vehicle with matching lead-acid batteries close together under the bonnet.

Is this a DIY job?

• Difficulty: Medium for a VSR under the bonnet; Pro for a DC-DC charger with long cable runs, lithium or a smart alternator
• Time: 2 to 6 hours
• Tools: heavy cable crimper, cable cutter, multimeter, drill and grommets
• Skills: high-current wiring and fusing

Step by step: choosing

1. Find out what alternator you have. Many newer vehicles (roughly 2015 on, and some older ones) have smart or variable-voltage alternators, often alongside stop-start or a battery sensor on the negative terminal. Check the owner’s manual or ask the vehicle maker. A smart alternator may drop its output once the starting battery is full, so a VSR may not see enough voltage to charge the auxiliary battery properly.
2. Check your auxiliary battery type. Lithium (LiFePO4) batteries need a charge profile to suit them. A DC-DC charger with a lithium setting is the usual choice. A VSR generally suits matching lead-acid batteries.
3. Measure the cable run. Long cables lose voltage. A DC-DC charger mounted near the auxiliary battery makes up for some of that loss. A VSR can’t, so the auxiliary battery may only partly charge. (see What size cable do I need for my 12V accessory?)
4. Decide on solar. Many DC-DC chargers have a built-in solar input (often MPPT). With a VSR, you need a separate solar regulator.
5. Consider cost and simplicity. A VSR is simpler and cheaper. A DC-DC charger costs more but usually charges more completely.
6. Check the charger’s input rules. Some DC-DC chargers need an ignition or “D+” signal to work with smart alternators. Follow the maker’s instructions.
7. Fuse both ends. Fit a fuse or circuit breaker near the starting battery and near the auxiliary battery (or at the charger output), sized for the cable.

Specs table

Situation	Usually suits
Older vehicle, conventional alternator, two lead-acid batteries side by side	VSR
Smart or variable-voltage alternator	DC-DC charger
Lithium auxiliary battery	DC-DC charger with lithium profile
Auxiliary battery in a canopy, tray or caravan (long run)	DC-DC charger near the auxiliary battery
Want solar charging through the same unit	DC-DC charger with solar input
Lowest cost, simplest install	VSR (if the situation above suits)

What you’ll need

• DC-DC charger or voltage sensitive relay
• heavy duty single core auto cable or twin core auto cable
• circuit breaker / MIDI fuse holder (one each end)
• cable lugs and adhesive heat shrink
• Anderson plug 50A (for caravan or trailer connections)

When to call an auto electrician

• Your vehicle has a smart alternator, a battery sensor, stop-start or a battery management system.
• You’re installing a lithium battery or running cable to a canopy, tray or caravan.
• The auxiliary battery isn’t fully charging after a day’s driving.
• You want a combined system with solar, an inverter and a battery monitor.

Related questions

Q: Can I use a VSR with a lithium auxiliary battery? A: Generally not recommended. Lithium batteries need a suitable charge profile, and they can draw very high current from the alternator through a VSR. A DC-DC charger with a lithium setting is the usual approach.

Q: How do I know if I have a smart alternator? A: Signs include stop-start, a sensor on the battery negative terminal, or charging voltage that varies a lot while driving. Check the owner’s manual or ask the vehicle maker. (see What is a smart alternator and how does it affect accessories?)

Q: Does a DC-DC charger need solar? A: No. Solar input is an option on many units. It’s useful if you camp for several days without driving.

Q: Where should the DC-DC charger go? A: Usually close to the auxiliary battery, with heavy cable from the vehicle side. Follow the maker’s limits on heat and mounting.

Sources

1. REDARC, “Choosing the right charger for a dual battery system” – isolators connect on sensed voltage and rely on alternator output; DC-DC chargers set the charge to suit the battery and handle voltage drop to a canopy better; isolators generally suit older fixed-voltage vehicles, DC-DC chargers newer variable-voltage ones; solar input. https://www.redarcelectronics.com/au/discover/choosing-the-right-charger-for-a-dual-battery-system/ (checked October 2026)
2. REDARC, “BCDC v DC-DC dual battery chargers” – variable-voltage (smart) alternators need a DC-DC charger for the auxiliary battery; chemistry-specific charge profiles including lithium; solar charging. https://www.redarcelectronics.com/au/discover/bcdc-v-dcdc-dual-battery-chargers/ (checked October 2026)

Quick answer

Work out how much power you use in a day, then size your solar to replace it, with a margin for cloudy days and less-than-ideal panel angles. Panels rarely produce their full rated output: shading, heat, dust, panel angle, time of year and where you are in Australia all reduce it. A common planning approach is to estimate daily solar harvest as panel watts × an assumed number of peak sun hours × a system efficiency factor, then compare it with your daily use. Your battery size decides how long you can last when the sun doesn’t cooperate.

Is this a DIY job?

• Difficulty: Easy to calculate; Medium to install portable panels; Pro for roof-mounted panels, regulators and wiring into an existing system
• Time: 30 minutes to work out
• Tools: appliance labels or manuals, a battery monitor (if you have one), calculator
• Skills: simple maths

Step by step

1. List your 12V appliances. Fridge, lights, water pump, fans, USB charging, diesel heater, TV, and anything run through an inverter.
2. Find each one’s power draw. Look for amps (A) or watts (W) on the label or in the manual. For 12V, amps ≈ watts ÷ 12.
3. Estimate hours of use per day. A fridge cycles on and off, so use the maker’s daily energy figure if it gives one, or measure it with a battery monitor.
4. Work out daily use in amp-hours (Ah). For each appliance: amps × hours per day. Add them up.
5. Add inverter losses. Anything run through an inverter uses more than its rated power from the battery. Allow extra, or check the inverter maker’s efficiency figure. (see How do I choose and install an inverter?)
6. Estimate daily solar harvest. For planning: panel watts ÷ 12 × assumed peak sun hours × efficiency factor = Ah per day. The peak sun hours you can expect depend on location and season; use a conservative figure for winter or southern trips. An efficiency factor (for heat, dust, angle and cable and regulator losses) is often assumed to be around 0.7, but treat it as a rough guide.
7. Compare and add a margin. Your solar harvest on a typical day should comfortably exceed your daily use if you want to stay put. Cloudy days can produce much less.
8. Size the battery for the gap. Battery capacity covers nights and cloudy days. Lithium lets you use more of its rated capacity than AGM.
9. Choose a suitable regulator. MPPT regulators generally get more from the panels than PWM regulators, especially in low light and on larger systems. PWM can suit a small system on a budget. Many DC-DC chargers include one. (see DC-DC charger or VSR isolator: which do I need?)
10. Check with real numbers. Once set up, a battery monitor shows what you actually use and harvest. Adjust from there.

Specs table

Worked example (illustration only, not a recommendation)

Item	Draw	Hours/day	Ah/day
12V compressor fridge (average)	2.5 A	24 (average, including cycling)	60
LED lights	2 A	4	8
Water pump	5 A	0.5	2.5
USB devices	2 A	3	6
Total			about 77 Ah

Solar estimate using the planning formula, assuming 5 peak sun hours and an efficiency factor of 0.7:

Panel size	Estimated Ah/day
200 W	200 ÷ 12 × 5 × 0.7 ≈ 58 Ah
300 W	≈ 88 Ah
400 W	≈ 117 Ah

In this example, 200 W wouldn’t keep up on a good day; 300 W would cover the daily use with a small margin; 400 W gives more room for cloud or extra loads. Fewer sun hours (for example in winter) reduce every figure. Your own appliance draws and conditions will differ.

What you’ll need

• solar panel 12V or portable folding solar panel
• MPPT solar regulator (or a DC-DC charger with solar input)
• Anderson plug 50A and solar extension lead
• battery monitor
• inline fuse holder / circuit breaker

When to call an auto electrician

• You’re installing roof panels and running cable into the van.
• You’re adding solar to a system with a DC-DC charger, lithium battery or inverter.
• The battery isn’t charging as expected from solar.
• Any 240V work. Fixed 240V wiring in a caravan must be done by a licensed electrician.

Related questions

Q: Portable or roof-mounted solar? A: Roof panels are always connected and need no setting up. Portable panels can be aimed at the sun and placed in sun while the van is in shade. Many people use both.

Q: Why am I getting less solar than my panels are rated for? A: Panel ratings are measured in ideal lab conditions. Heat, angle, dust, shading, time of year and cable losses all reduce real output.

Q: Do I need an MPPT regulator? A: It’s not essential, but MPPT generally harvests more energy than PWM. Many DC-DC chargers have MPPT built in.

Q: How long will my battery last without sun? A: Divide the battery’s usable capacity by your daily use, after allowing for any partial solar. Lithium lets you use more of its rated capacity than AGM.

Sources

1. Signature Camper Trailers, “How much solar do you need?” – planning formula (panel watts ÷ 12 × 5 peak sun hours × 0.7 efficiency); daily use in Ah; cloudy-day reduction (manufacturer guide; assumptions not independently sourced). https://signaturecampertrailers.com.au/how-much-solar-do-you-need/ (checked October 2026)
2. REDARC, “Regulating the sun” – MPPT regulators harvest more from the panels, especially in low light and on larger systems; PWM suits small, budget systems. https://www.redarcelectronics.com/au/discover/regulating-the-sun/ (checked October 2026)
3. WorkSafe ACT, “Electrical incident” – 240V installations in caravans and transportable structures should be done by a licensed electrician and certified to AS/NZS 3001. https://www.worksafe.act.gov.au/health-and-safety-portal/safety-alerts/electrical-incident (checked October 2026)
4. Consumer, Building and Occupational Services Tasmania, “Using electricity safely in caravans and tents” – don’t do DIY electrical work on a caravan; only a licensed electrician can. https://cbos.tas.gov.au/topics/technical-regulation/electrical-standard-safety/using-electricity-safely-in-caravans-and-tents (checked October 2026)

Quick answer

Cable size depends on two things: the current the accessory draws and the total length of the cable run (there and back). At 12V, even a small voltage loss matters, so long runs need much thicker cable than you’d expect. Always compare cable by its copper area in mm², not its “mm” label: a cable sold as “6mm auto cable” has roughly 4.6 mm² of copper, not 6 mm². Then fuse the cable close to the battery so the fuse protects the cable.

Is this a DIY job?

• Difficulty: Medium
• Time: 15 minutes to work out
• Tools: accessory’s current rating (from its label or manual), tape measure, the table below
• Skills: simple maths

Step by step

1. Find the current. Check the accessory’s label or manual for amps (A). If it only gives watts, divide by 12 for a rough figure (for example, 60 W ÷ 12 V = 5 A).
2. Measure the run. Measure the one-way distance from the battery (or fuse block) to the accessory, along the route the cable will actually take.
3. Decide the acceptable voltage drop. Many installers aim to lose no more than about 3% (around 0.36 V on a 12V system). Charging circuits such as DC-DC chargers and fridges often perform better with less: check the maker’s instructions, which usually specify a cable size.
4. Look up the table below. Find your current and pick a cable whose maximum length is longer than your one-way run.
5. Check the cable’s current rating. The table only covers voltage drop. The cable must also be rated for the current: use the ratings table below or the maker’s figure, and reduce it for heat and bundling.
6. Choose the fuse. Fit a fuse or circuit breaker close to the battery, rated to protect the cable (at or below the cable’s rating) and suitable for the accessory. (see Where should the fuse go when wiring an accessory?)
7. Use twin-core or a proper return. The negative needs the same size as the positive if it runs back to the battery. A thin earth wire undoes a thick positive cable.

Specs table

Auto cable labels, copper area and current rating

The “mm” in an auto cable’s name is roughly its outside size, not its copper. What carries the current is the conductor cross-section in mm². Tycab, a major Australian cable maker, publishes these figures for its single-core auto cable:

Sold as	Stranding	Copper area	Tycab current rating at 30°C
3mm auto cable	14/0.32	1.13 mm²	20 A
4mm auto cable	23/0.32	1.84 mm²	28 A
5mm auto cable	36/0.32	2.90 mm²	37 A
6mm auto cable	57/0.32	4.59 mm²	48 A

Heavier cable (B&S)

B&S is the Australian name for the American wire gauge (AWG) sizes, but Australian battery cable is often made a little lighter than the true AWG area. Compare mm², not the gauge number.

Size	True AWG area	Tycab battery cable area	Tycab current rating at 30°C
8 B&S	8.37 mm²	7.71 mm²	74 A
6 B&S	13.3 mm²	13.5 mm²	103 A
4 B&S	21.2 mm²	20.26 mm²	135 A
2 B&S	33.6 mm²	32.07 mm²	188 A
0 B&S	53.5 mm²	49.0 mm²	246 A

Ratings drop with heat and bundling. Tycab’s ratings are for a single cable at 30°C. In hotter places, multiply by Tycab’s derating factor: 0.9 at 40°C, 0.76 at 50°C, 0.60 at 60°C and 0.45 at 70°C. A 6mm cable rated at 48 A carries only about 29 A in a 60°C engine bay. Cables bundled together in a loom lose more again. Twin-core cable is rated lower than single-core of the same size for the same reason; check the rating for the exact cable you buy.

Maximum one-way run for about 3% voltage drop at 12V (copper, 20°C)

Cable	5 A	10 A	20 A	30 A	50 A	100 A
3mm auto (1.13 mm²)	2.4 m	1.2 m	too small	too small	too small	too small
4mm auto (1.84 mm²)	3.9 m	1.9 m	1.0 m	too small	too small	too small
5mm auto (2.90 mm²)	6.1 m	3.0 m	1.5 m	too small	too small	too small
6mm auto (4.59 mm²)	9.6 m	4.8 m	2.4 m	1.6 m	too small	too small
8 B&S (8.37 mm²)	17.5 m	8.8 m	4.4 m	2.9 m	1.8 m	too small
6 B&S (13.3 mm²)	27.8 m	13.9 m	7.0 m	4.6 m	2.8 m	too small
4 B&S (21.2 mm²)	–	22.2 m	11.1 m	7.4 m	4.4 m	2.2 m
2 B&S (33.6 mm²)	–	–	17.6 m	11.7 m	7.0 m	3.5 m
0 B&S (53.5 mm²)	–	–	28.0 m	18.7 m	11.2 m	5.6 m

How the table is worked out: voltage drop = current × resistance of the full loop (out and back). Copper resistance is about 0.0172 ohm per metre for 1 mm² at 20°C. Real cable gets warmer and loses a little more. B&S rows use the true AWG area; Tycab’s slightly lighter battery cable gives runs up to about 8% shorter at 8 B&S and 0 B&S. “Too small” means the current is at or above the cable’s rating, or leaves no margin for heat (for example 3mm cable rated at 20 A carrying 20 A, 5mm rated at 37 A carrying 30 A in a warm engine bay, or 6 B&S rated at 103 A carrying 100 A). “–” means the cable is far bigger than needed.

What you’ll need

• twin core auto cable (size from the table)
• heavy duty single core auto cable (for high-current runs)
• cable lugs and crimp terminals
• inline fuse holder / circuit breaker / MIDI fuse holder
• split loom tube and firewall grommet

When to call an auto electrician

• The run is long and the current is high (DC-DC chargers, inverters, winches, caravan charging).
• You’re not sure what the accessory draws, or it has high start-up current (compressors, fridges, inverters).
• Cable or terminals are getting hot, or accessories cut out under load.

Related questions

Q: Is “6mm” cable 6 mm²? A: No. In Australian auto cable, “6mm” usually describes the outside diameter including insulation. The copper area is about 4.6 mm². Always compare mm².

Q: Why does my fridge cut out when I’m driving or camping? A: Often because the cable is too thin for the length, so the voltage at the fridge drops below its cut-out setting. Thicker cable or a shorter run usually fixes it.

Q: Can I use thicker cable than I need? A: Yes. Thicker cable has less voltage drop and runs cooler. It costs more and is harder to route, but there’s no electrical downside.

Q: Does the negative cable need to be as thick as the positive? A: Yes, if it carries the same current back to the battery. The voltage drop happens on both sides of the circuit.

Sources

1. Solar4RVs, “Understanding cable size types: mm, mm², B&S, AWG” – auto cable “mm” is outside diameter; 3mm/4mm/6mm copper areas; B&S to mm² conversions; heat derating note (AS/NZS 3008). https://www.solar4rvs.com.au/buying/buyer-guides/understanding-cable-size-types-mm-mm2-b_s-awg/ (checked October 2026)
2. Tycab, automotive cable catalogue – single-core auto cable (ASC) and battery cable (ABC) stranding, mm², current ratings at 30°C and temperature derating factors. https://www.tyree.nz/wp-content/uploads/2019/09/Automotive-Comp.pdf (checked October 2026)
3. HS Auto Parts, “Tycab cable – how to select the right gauge” – Tycab battery cable B&S sizes, mm² and rating guide. https://hsautoparts.com.au/tycab-cable-selection-guide/ (checked October 2026)
4. Technical review, Ozautoelectrics (October 2026) – compare cable by conductor cross-section in mm²; “3mm” auto cable is 1.13 mm².
5. Resistivity of copper (about 1.72 × 10⁻⁸ ohm-metre at 20°C), standard physical constant used to calculate the voltage-drop table.

Quick answer

Put the fuse in the positive cable as close to the battery (or other power source) as practical, so almost none of the cable is left unprotected. Rate the fuse at or below what the cable can safely carry, and above what the accessory normally draws. The fuse’s job is to protect the cable from overheating if it shorts to the body, which is why it belongs at the source end, not at the accessory.

Is this a DIY job?

• Difficulty: Easy
• Time: 15 to 45 minutes, depending on access to the battery
• Tools: spanners for the battery terminals, crimp tool, wire stripper, multimeter, cable ties
• Skills: basic crimping and reading a current rating

Fitting a fuse holder is straightforward. Get help if you’re feeding high-current gear (inverters, winches, DC-DC chargers), working on a vehicle with a battery monitoring sensor on the negative terminal, or wiring anything in a hybrid or electric vehicle.

Step by step

1. Work out the accessory’s current. Check its label or manual for amps. If it only lists watts, divide by 12 for a rough figure (for example, 120 W ÷ 12 V = 10 A).
2. Size the cable first. Choose cable by copper area in mm² for the current and run length. The fuse is chosen to suit the cable, so this comes first. (see What size cable do I need for my 12V accessory?)
3. Pick the fuse rating. It must be above the accessory’s normal draw (including any start-up surge the maker mentions) and at or below the cable’s current rating. If the accessory maker specifies a fuse, use that, as long as it’s no higher than the cable can carry.
4. Pick the fuse type. Getting the protection right matters more than anything else in this job, because it’s what stops cable damage and fires. Blade fuses suit most small accessories. Higher currents need MIDI or ANL fuses, or a circuit breaker (see the table below). Under the bonnet, under the vehicle or anywhere it can get wet, use a waterproof (sealed) fuse holder or circuit breaker. A circuit breaker can be reset instead of replaced, which suits accessory circuits that may trip occasionally.
5. Disconnect the battery. Negative terminal off first, then you can work near the positive safely.
6. Mount the fuse holder close to the battery. Fix it firmly, away from sharp edges, exhaust heat and moving parts, and where you can reach it to check or replace the fuse. The short cable between the battery and the fuse is the only unprotected part, so keep it as short as you practically can and protect it with loom tube.
7. Connect the battery side. Use a properly crimped lug on the battery positive terminal or an approved power stud. Don’t stack lots of lugs on one terminal.
8. Run the cable to the accessory. Use loom tube, grommets through any metal panel, and secure it every so often so it can’t rub through.
9. Run a proper earth. Use a negative cable of the same size back to the battery or a clean, solid chassis earth point.
10. Fit the fuse last. With everything connected and checked, fit the fuse, then reconnect the battery: negative on last.
11. Test. Switch the accessory on and confirm it works. Check that nothing, including the fuse holder, gets warm after a few minutes of running.

Two power sources, two fuses. If a cable can be fed from both ends, such as the link cable between a main and an auxiliary battery, or the feed to a DC-DC charger, fit a fuse or circuit breaker near each battery. A short anywhere along the cable is then cut off from both sides. (see How does a dual battery system work and do I need one?)

Specs table

Common fuse and breaker types for 12V accessories

Type	Typical ratings	Notes
Standard or mini blade fuse	Up to 40 A	Colour-coded by rating; plugs into an inline holder or fuse block
MIDI (bolt-down) fuse	30 to 200 A (32 V DC)	Time-delay type that tolerates short surges; the maker states 150 to 200 A sizes are for short-circuit protection only
ANL fuse	35 to 750 A	Bolt-on, for heavy cables such as inverter and winch feeds
Circuit breaker, Type I (auto reset)	Various	Resets itself and keeps cycling until the fault clears
Circuit breaker, Type II (modified reset)	Various	Stays open while power is applied; resets when power is removed
Circuit breaker, Type III (manual reset)	Various	Stays off until you press the reset button or lever

For accessory feeds, a manual-reset (Type III) breaker is usually the safer choice, because it stays off until someone looks at why it tripped.

Blade fuse colours

Rating	Colour
5 A	Tan
7.5 A	Brown
10 A	Red
15 A	Blue
20 A	Yellow
25 A	Clear (natural)
30 A	Green
40 A	Orange

Maxi fuses use a different colour code. Always go by the number printed on the fuse.

Matching fuse to cable (examples)

Cable ratings from one Australian cable maker, for a single cable at 30°C. Ratings drop with heat and bundling, so in an engine bay choose a fuse well under the figure.

Cable sold as	Copper area	Maker’s rating at 30°C	Fuse must be no higher than
3mm auto cable	1.13 mm²	20 A	20 A, lower in hot spots
4mm auto cable	1.84 mm²	28 A	28 A, lower in hot spots
6mm auto cable	4.59 mm²	48 A	48 A, lower in hot spots

Fuses come in set ratings, so choose the nearest standard size at or below the limit. Twin-core and bundled cables have lower ratings than single-core; check the figure for the exact cable you buy.

Other brands rate the same sizes much lower. For example, some 3mm auto cable sold in Australia is rated at 10 A, and some 4mm twin-core at 15 A. Go by the rating printed on the cable or packet you actually buy, and if in doubt use the lower figure.

What you’ll need

• inline blade fuse holder (waterproof type under the bonnet)
• blade fuses
• MIDI fuse holder or ANL fuse holder (high-current feeds)
• manual reset circuit breaker
• fuse block (if you’re adding several accessories)
• cable lugs and crimp terminals
• split loom tube and rubber grommets
• auto cable (sized for the job)

When to call an auto electrician

• The accessory draws high current (inverter, winch, air compressor, DC-DC charger) or needs cable heavier than 6mm auto cable.
• The vehicle has a battery sensor on the negative terminal, a smart alternator, or a stop-start system. Adding loads in the wrong place can upset charging. (see What is a smart alternator and how does it affect accessories?)
• You’re adding several accessories and need a fused distribution block or auxiliary battery.
• Fuses keep blowing. That usually points to a short or a fault, not a fuse that’s too small.
• The vehicle is a hybrid or EV (high-voltage systems are professional-only), or the wiring runs near airbags or SRS looms.

Related questions

Q: Should the fuse go near the battery or near the accessory? A: Near the battery. A short can happen anywhere along the cable, and only the part after the fuse is protected.

Q: Can I use a bigger fuse so it stops blowing? A: No. A fuse rated higher than the cable lets the cable overheat before the fuse blows. Find out why it’s blowing instead.

Q: Does the negative cable need a fuse? A: In a normal negative-earth vehicle, the fuse goes in the positive cable only. The negative is at earth potential, so a short to the body doesn’t draw current through it.

Q: Can I tap into the vehicle’s fuse box instead? A: For small loads, a fuse tap on a suitable circuit can work. Stay within that circuit’s rating and avoid safety circuits such as airbags.

Q: Fuse or circuit breaker: which is better? A: Both protect the cable. A breaker can be reset after a trip; a fuse is cheaper and simpler. For high currents, choose a type rated for the job.

Sources

1. Tycab, automotive cable catalogue – low tension single-core cable by stranding, with copper areas and current ratings at 30°C (14/0.32 1.13 mm² 20 A; 23/0.32 1.84 mm² 28 A; 57/0.32 4.59 mm² 48 A) and temperature derating. https://www.tyree.nz/wp-content/uploads/2019/09/Automotive-Comp.pdf (checked October 2026)
2. Littelfuse, ATO and MAXI blade fuse datasheet (via Farnell) – blade fuse colour codes by rating; maxi colour code differs. https://www.farnell.com/datasheets/82316.pdf (checked October 2026)
3. Littelfuse, MIDI 498 series datasheet – 32 V DC, 30 to 200 A, time-delay; 150 to 200 A for short-circuit protection only. https://www.enika.eu/data/files/Littelfuse_MIDI_0498_auto32V.pdf (checked October 2026)
4. Eaton Bussmann, ANL low-voltage limiter fuse datasheet – 35 to 750 A, 32 V AC / 80 V DC. https://static.chipdip.ru/lib/651/DOC044651693.pdf (checked October 2026)
5. Littelfuse, aftermarket circuit breakers catalogue – Type I automatic reset, Type II modified reset, Type III manual reset definitions. https://www.littelfuse.com/~/media/automotive/datasheets/circuit-breakers/littelfuse_aftermarket_circuit_breakers.pdf (checked October 2026)
6. Narva, 3mm single core cable – sold as 3mm and rated 10 A. https://narva.com.au/products/narva-10a-3mm-white-single-core-cable-100m (checked October 2026)
7. Wiltronics, 4mm twin core auto cable – 23/0.32 stranding, 1.85 mm², rated 15 A. https://www.wiltronics.com.au/product/52830/4mm-twin-core-auto-cable (checked October 2026)

Quick answer

Choose the plug and contact size to suit your cable (for most 12V accessory and charging jobs, a 50 A Anderson-style connector with 6 B&S or 8 B&S contacts). Crimp the contacts on with a proper crimp tool, push them into the housing until they click, and wire every plug the same way round so positive always meets positive. Fuse the positive cable close to the battery, and fit a cover on any plug that’s live when unplugged.

Is this a DIY job?

• Difficulty: Medium
• Time: 15 to 30 minutes per plug
• Tools: cable cutter, wire stripper, Anderson-style crimp tool (hex or ratchet type sized for the contact), heat gun, multimeter
• Skills: crimping heavy cable properly

The plug itself is simple. The quality of the crimp is what matters: a poor crimp gets hot under load.

Step by step

1. Pick the right connector and colour. The SB50 suits most fridges, solar panels and DC-DC charger inputs. The SB120 and SB175 are for high-current jobs. The real rating depends on the contact and cable, so match the contact to your cable size. Use the same housing colour as the plugs already on your vehicle and trailer, or they won’t join.
2. Choose your cable. Use twin-core cable sized for the current and the length of the run. (see What size cable do I need for my 12V accessory?)
3. Disconnect power. If the cable is connected to a battery, remove the fuse or disconnect the battery first.
4. Strip the cable. Strip only enough insulation for the copper to sit fully in the contact barrel, with no bare copper showing past the barrel.
5. Crimp the contacts. Put each contact in the correct die of the crimp tool and crimp firmly. Tug hard on each one: it must not move. Don’t use pliers or a hammer crimper unless it’s designed for that contact.
6. Add heat shrink over the back of the contact and cable if the contact design allows it.
7. Decide your polarity convention and stick to it. Anderson-style connectors are genderless: two identical plugs join by turning one over. Hold each plug with the face towards you and the same edge up, and put positive on the same side every time. Then any two plugs you wire will join positive to positive. If you’re adding plugs to an existing setup, check how the existing plugs are wired and match them.
8. Insert the contacts. Push each contact into the housing from the back until it clicks behind the spring. Pull gently to check it’s locked.
9. Check polarity with a meter. Before connecting anything, measure at the plug face and confirm positive is where you expect it. Then plug two plugs together and confirm positive reaches positive.
10. Fuse it. The positive cable must have a fuse or circuit breaker close to the battery or power source, sized to protect the cable. (see Where should the fuse go when wiring an accessory?)
11. Mount and cover it. Use a mounting bracket or dust cover, especially for plugs on bumpers or drawbars that stay live.

Specs table

Common Anderson-style sizes

Anderson Power Products makes the genuine SB series. The number in the name (SB50, SB120, SB175) is a size, not a promise: the current a connector can actually carry depends on the contact fitted and the cable crimped into it. A small contact in a big housing is only as good as the small contact.

Connector	Maker’s UL rating	Contact cable range	Typical use
SB50	50 A	16 to 6 AWG (1.3 to 13.3 mm²)	Fridges, solar panels, DC-DC charger inputs, most 12V accessories
SB120	Up to 190 A with the largest contact and cable (rated at 75°C)	6 to 1 AWG (13.3 to 42.4 mm²)	High-current charging, battery links
SB175	175 A with the largest contact and cable (rated at 65°C)	6 AWG to 1/0 (13.3 to 53.5 mm²)	Winches, jump leads, very high current

All three are rated to 600 V by the maker. In a 12V vehicle the limit is always current and heat, never voltage.

Contact sizes

Connector	Contact	Cable size
SB50	6 AWG / 6 B&S	13.3 mm²
SB50	8 AWG / 8 B&S	8.4 mm²
SB50	10–12 AWG	3.3–5.3 mm²
SB120	6 AWG	13.3 mm²
SB120	4 AWG	21.1 mm²
SB120	2 AWG	33.6 mm²
SB120	1 AWG	42.4 mm²
SB175	6 AWG	13.3 mm²
SB175	4 AWG	21.1 mm²
SB175	2 AWG	33.6 mm²
SB175	1 AWG	42.4 mm²
SB175	1/0 AWG	53.5 mm²

Choose the contact to match the cable, and the cable to match the current and run length. (see What size cable do I need for my 12V accessory?)

Housing colours and voltage

The housing colour is a voltage key. Genuine housings of different colours have different moulded keys, so in general they won’t plug into each other. That’s meant to stop a 12V plug being joined to a 24V or 48V system. Anderson Power Products’ recommended colour key is:

Colour	Recommended voltage
Yellow	12V
Orange	18V
Red	24V
Grey	36V
Blue	48V
Green	72V
Black	80V
Brown	96V
Purple	120V
White	144V

In Australian 12V fit-outs the maker’s key often isn’t followed. Grey housings are widely used for 12V, and red is common on 12V battery boxes and power leads. The key only protects you if every plug in your system uses the same colour. Check what’s already on your vehicle, trailer and accessories before you buy, because a plug of a different colour won’t join them.

What you’ll need

• Anderson plug 50A (with contacts to suit your cable)
• Anderson plug crimp tool
• twin core auto cable (size for your load and length)
• adhesive heat shrink
• Anderson plug cover or Anderson plug mounting bracket
• inline fuse holder or circuit breaker

When to call an auto electrician

• You’re wiring a charging circuit for a caravan or trailer battery, especially with a DC-DC charger or a smart alternator.
• The cable runs the length of the vehicle, through the firewall or near airbag zones.
• An Anderson plug or its cable gets hot in use. That points to a bad crimp, a loose contact or undersized cable.
• You need SB120 or SB175 connectors with heavy cable.

Related questions

Q: Which side is positive on an Anderson plug? A: There’s no single rule printed on the plug. What matters is that every plug you use is wired the same way when viewed the same way up, so positive always meets positive. Check with a multimeter.

Q: Can I solder Anderson contacts instead of crimping? A: A correct crimp with the right tool is the common method for these contacts. Whatever method you use, the joint must be mechanically strong and not get hot under load.

Q: Why did my Anderson plug melt? A: Usually a poor crimp, a contact not fully seated, corrosion, or more current than the cable or connector can carry.

Q: Do I need a fuse if I have an Anderson plug? A: Yes. The plug isn’t a fuse. Every positive cable needs a fuse or circuit breaker close to its power source.

Sources

1. Anderson Power Products, SB50 connector datasheet (via Farnell) – 50 A rating, 16–6 AWG contact range, genderless design, colour-keyed housings. https://www.farnell.com/datasheets/273617.pdf (checked October 2026)
2. Anderson Power Products, SB50 datasheet (via Farnell) – silver-plated contacts for 6 AWG (13.3 mm²), 8 AWG (8.4 mm²) and 12–10 AWG (3.3–5.3 mm²); “genderless design mates with itself”. https://www.farnell.com/datasheets/3046067.pdf (checked October 2026)
3. Anderson Power Products, SB50 connector datasheet – recommended voltage key colour code (yellow 12V, orange 18V, red 24V, grey 36V, blue 48V and higher); 600 V rating. https://www.e-kart.fr/images/stories/technique/anderson/sb50-connector.pdf (checked October 2026)
4. Anderson Power Products, SB120 datasheet (via Mouser) – 190 A UL rating at 75°C with largest cable; #6 to #1 AWG contacts. https://www.mouser.com/catalog/specsheets/DS-SB120(7)[1].pdf (checked October 2026)
5. Anderson Power Products, SB175 datasheet – 175 A UL rating at 65°C with largest cable; #4 to 1/0 AWG range plus a #6 contact; colours used to identify voltages. https://static.chipdip.ru/lib/221/DOC011221459.pdf (checked October 2026)
6. Home of 12 Volt (Australian 12V supplier), red to grey 50 A Anderson adaptor – genuine Anderson colours don’t mate with other colours; grey used for 12V, while many newer battery boxes and 12V leads use red. https://www.homeof12voltonline.com.au/products/red-50-amp-anderson-to-grey-50-amp-anderson-genuine-andersons (checked October 2026)
7. Narva, heavy-duty 50 amp connector housing (red) – red 50 A housing sold for chargers, inverters and fridges; keyed to prevent accidental connection. https://www.narva.com.au/products/57200R/heavy-duty-50-amp-connector-housing-red (checked October 2026)

Quick answer

Choose a pure sine wave inverter with a continuous rating above the total wattage of everything you’ll run at once, and a surge rating that covers start-up loads such as fridges, compressors and power tools. Mount it close to the battery with short, heavy cables and a fuse or circuit breaker near the battery, because inverters draw very high current at 12V. Plug appliances straight into the inverter’s own outlets. Connecting an inverter into a caravan’s or vehicle’s fixed 240V wiring is licensed electrical work.

Is this a DIY job?

• Difficulty: Medium for a small inverter with its own outlets; Pro for large inverters; licensed electrician only for any fixed 240V wiring
• Time: 1 to 3 hours for a stand-alone install
• Tools: heavy cable crimper, cable cutter, socket set, multimeter, drill
• Skills: high-current DC wiring

Step by step

1. List what you’ll run. Find the wattage on each appliance’s label. Add up everything you’ll use at the same time.
2. Choose the continuous rating. Pick an inverter rated comfortably above that total. Running an inverter at its limit for long periods makes it hot and less efficient.
3. Check the surge rating. Motors and compressors (fridges, blenders, power tools) can draw several times their running power for a moment at start-up. The inverter’s surge rating must cover that.
4. Choose pure sine wave. Pure sine inverters produce power like a household outlet, so laptops, chargers, motors and sensitive electronics run properly. Modified sine inverters are cheaper, but some appliances run hot, noisy or not at all.
5. Check your battery can supply it. At 12V, the DC current is roughly the AC watts ÷ 12, plus inverter losses. A 1,000 W load draws more than 80 A from the battery. Make sure the battery, and its charging, can support that. (see How much solar do I need for my caravan?)
6. Choose the location. Close enough to the battery to keep the DC cables short, but not inside the battery box or right beside the battery: some makers specify a minimum gap, so check the manual. It should be dry, well ventilated, protected from damage and away from battery gases.
7. Disconnect the battery negative first before you fit any fuse or cable. Reconnect it last, once everything else is done.
8. Fit the DC fuse and isolator. A fuse or circuit breaker rated for the inverter (from the maker’s instructions) goes as close to the battery positive as practical. An isolator switch makes it easy to turn off.
9. Run short, heavy DC cables. Use the cable size the inverter maker specifies, and keep the run as short as possible. (see What size cable do I need for my 12V accessory?)
10. Connect the chassis earth if the inverter maker requires it, using the specified cable.
11. Use the inverter’s own outlets. Plug appliances directly into the inverter, or use an appliance lead rated for the load.
12. Leave fixed 240V wiring to a licensed electrician. Connecting an inverter to a caravan’s power points, changeover switch or shore power needs correct earthing, RCD protection (an RCBO, which also gives overcurrent protection) and compliance with AS/NZS 3001.
13. Test and check heat. Run your normal loads and check cables, lugs and the inverter don’t get hot.

Specs table

Approximate DC current at 12V (allowing for about 10% inverter loss)

AC load	Approx. battery current
100 W	about 9 A
300 W	about 28 A
500 W	about 46 A
1,000 W	about 93 A
2,000 W	about 185 A

Real current varies with the inverter’s efficiency and the battery voltage. Check the maker’s figures.

Pure sine vs modified sine

	Pure sine wave	Modified sine wave
Output	Smooth, like household power	Stepped approximation
Laptops, chargers, electronics	Run normally	May run hot, buzz or fail
Motors and compressors	Run normally	May run hot or noisy
Cost	Higher	Lower

What you’ll need

• pure sine wave inverter
• ANL fuse holder or circuit breaker (rated per the inverter maker)
• battery isolator switch
• heavy duty single core auto cable (size per the inverter maker)
• cable lugs and adhesive heat shrink
• battery monitor (to watch consumption)

When to call an auto electrician

• Any inverter above a few hundred watts, or any install with long DC cable runs.
• You want the inverter connected to a caravan’s or vehicle’s 240V power points, changeover switch or shore power. That’s licensed electrical work and must comply with AS/NZS 3001.
• The inverter shuts down under load, alarms for low voltage, or cables get hot.
• You also need a DC-DC charger, solar and a larger battery to support the inverter.

Related questions

Q: What size inverter do I need for a coffee machine or microwave? A: These are high-wattage appliances. Check the label: you’ll need an inverter rated above that figure, a battery bank that can supply the very high DC current, and heavy cabling. For many setups, it’s a big step up.

Q: Can I plug my inverter into my caravan’s power inlet? A: Don’t do this yourself. Back-feeding power into a caravan’s 240V wiring can be dangerous. A licensed electrician can install a proper changeover arrangement with RCD protection.

Q: Why does my inverter beep and shut off? A: Usually low battery voltage under load, cables too thin or long, or the load is beyond the inverter’s rating. Check battery charge and cable size first.

Q: Is it safe to leave an inverter on? A: Inverters draw some power even with nothing plugged in. Switch it off when not in use to save your battery.

Sources

1. REDARC, “Guide to AC wiring when installing a pure sine wave inverter in a vehicle” – fixed AC wiring by a licensed electrician; RCBO and changeover switch requirements; AS/NZS 3001. https://www.redarcelectronics.com/eu/resources/redvision-wiring-guides/guide-to-ac-wiring-when-installing-a-redarc-pure-sine-wave-inverter-in-a-vehicle (checked October 2026)
2. WorkSafe ACT, “Electrical incident” – fatal incident involving non-compliant 240V wiring in a trailer; 240V installations in caravans and transportable structures to be done by a licensed electrician and certified to AS/NZS 3001. https://www.worksafe.act.gov.au/health-and-safety-portal/safety-alerts/electrical-incident (checked October 2026)
3. Solar4RVs, “Understanding cable size types: mm, mm², B&S, AWG” – typical cable sizes for inverters. https://www.solar4rvs.com.au/buying/buyer-guides/understanding-cable-size-types-mm-mm2-b_s-awg/ (checked October 2026)
4. REDARC, “RS Series 12V & 24V Pure Sine Wave Inverters instruction manual” – DC fuse on the positive as close as practical to the battery; short DC cables sized to the maker’s table; ventilation clearance; don’t mount in a battery compartment or close to a battery; chassis earth; no-load current draw; permanent AC installs by a licensed electrician. https://cdn.intelligencebank.com/au/share/yE9N/zJpl/rDD2O/original/RS+Series+Pure+Sine+Inverters+Instruction+Manual (checked October 2026)

Quick answer

A dual battery system adds a second (auxiliary) battery that runs your fridge, lights and other accessories, while the starting battery is kept separate so you can always start the engine. When the engine is running, the auxiliary battery is charged through a voltage sensitive relay (VSR) or a DC-DC charger, and often from solar as well. You probably need one if you camp for more than a night without power and run a 12V fridge, or if you use accessories for long periods with the engine off.

Is this a DIY job?

• Difficulty: Medium for a simple under-bonnet VSR system; Pro for DC-DC, lithium, canopy or long cable runs
• Time: half a day to a full day
• Tools: heavy cable crimper, cable cutter, socket set, multimeter, drill, grommets
• Skills: high-current wiring, fusing, battery mounting

Before you wire anything, disconnect the negative terminal of each battery first, and reconnect the negatives last.

How it works

1. Starting battery stays connected to the starter and vehicle electrics as normal.
2. Charging device sits between the two batteries: – a VSR (isolator) links the batteries when it senses the alternator is charging, and separates them when the engine stops; or – a DC-DC charger takes power from the vehicle side and charges the auxiliary battery with a proper multi-stage charge profile. Many also accept solar input. (see DC-DC charger or VSR isolator: which do I need?)
3. Auxiliary battery (usually deep-cycle AGM or lithium) powers your accessories.
4. Accessories (fridge, lights, USB, inverter) are wired to the auxiliary battery, each with its own fuse.
5. Solar (optional) charges the auxiliary battery through a regulator, or through a DC-DC charger with a built-in solar input.

Step by step: planning a system

1. Work out your daily power use. Add up what your fridge and other accessories draw over a day, in amp-hours (Ah). The fridge maker’s data is a starting point. (see How much solar do I need for my caravan?)
2. Choose the battery type and size. Deep-cycle AGM is heavier and you can only use part of its rated capacity without shortening its life. Lithium (LiFePO4) is lighter and lets you use more of its capacity, but costs more and needs a compatible charger and a cooler location.
3. Choose the location. Under the bonnet (short cables, but hot), in a canopy, tray or cargo area (cooler, but longer cables), or in a caravan. Lithium batteries generally don’t suit hot engine bays: heat shortens their life, and some makers say not to fit them there at all.
4. Choose the charging device. A VSR suits matching lead-acid batteries close together on vehicles with a conventional alternator. A DC-DC charger suits smart alternators, lithium batteries, long cable runs and solar.
5. Size the cables and fuses. The link cable between the batteries carries high current and needs to be heavy, with a fuse or circuit breaker near each battery. (see What size cable do I need for my 12V accessory?)
6. Mount the battery securely. Use a proper tray or box with a hold-down that won’t let the battery move in a crash or on corrugations.
7. Wire accessories to the auxiliary battery with individual fuses, ideally through a fuse block, and a low-voltage cut-off where needed.
8. Test the system. With the engine running, check the auxiliary battery is charging. With the engine off, check accessories run from the auxiliary battery and the starting battery stays isolated.

Specs table

VSR vs DC-DC at a glance

	VSR (isolator)	DC-DC charger
How it charges	Connects auxiliary battery straight to the vehicle system when charging	Converts vehicle power into a multi-stage charge for the auxiliary battery
Smart (variable-voltage) alternators	Often won’t fully charge the auxiliary battery	Designed to cope
Lithium auxiliary battery	Generally not suitable	Common choice (with a lithium charge profile)
Long cable runs (canopy, caravan)	Voltage drop reduces charging	Compensates better
Solar input	Needs a separate regulator	Many have one built in
Cost	Lower	Higher

What you’ll need

• deep cycle battery AGM or lithium battery 12V
• voltage sensitive relay or DC-DC charger
• battery tray or battery box
• heavy duty single core auto cable / twin core auto cable
• circuit breaker or MIDI fuse holder (one near each battery)
• cable lugs and adhesive heat shrink
• fuse block
• battery monitor (optional)

When to call an auto electrician

• Your vehicle has a smart alternator, stop-start or a battery management system.
• You’re fitting a lithium battery or a battery in a canopy, tray or caravan.
• You need to run heavy cable through the firewall or along the chassis.
• You’re adding an inverter or 240V outlets. Fixed 240V wiring needs a licensed electrician. (see How do I choose and install an inverter?)
• The auxiliary battery isn’t charging, or the starting battery goes flat when camping.

Related questions

Q: Can I just run my fridge off the starting battery? A: You can, but you risk a flat battery and a car that won’t start. A dual battery system or a portable power station keeps the starting battery for starting.

Q: Do I need a DC-DC charger or will a VSR do? A: If you have a smart alternator, a lithium auxiliary battery, a long cable run or want solar input, a DC-DC charger is usually the better choice. A VSR can suit older vehicles with matching lead-acid batteries close together. (see DC-DC charger or VSR isolator: which do I need?)

Q: Can I put the second battery in the cabin? A: Only a battery designed for it, in a sealed or vented box, securely restrained. Flooded batteries vent gas and shouldn’t be in the cabin. Many lithium and AGM batteries are sealed, but follow the maker’s instructions.

Q: What size auxiliary battery do I need? A: Work out your daily use, then allow for how many days without charging you want and how much of the battery’s capacity you can use. Lithium usually lets you use more of its rating than AGM.

Sources

1. REDARC, “The truth about alternators” – smart alternators vary their output; a simple isolator won’t charge an auxiliary battery properly from one; DC-DC charger recommended; solar and vehicle charging together. https://redarcelectronics.com/blogs/blog/the-truth-about-alternators (checked October 2026)
2. REDARC, “BCDC v DCDC dual battery chargers” – vehicles with variable-voltage alternators need a DC-DC charger; lithium charge profiles; solar input; isolator suits traditional alternators. https://www.redarcelectronics.com/au/discover/bcdc-v-dcdc-dual-battery-chargers/ (checked October 2026)
3. REDARC, “The REDARC call log: lithium battery FAQs” – lithium batteries aren’t designed for engine bays; near-full usable capacity compared with AGM. https://www.redarcelectronics.com/au/discover/the-redarc-call-log-lithium-battery-faqs/ (checked October 2026)
4. Solar4RVs, “Understanding cable size types: mm, mm², B&S, AWG” – cable sizing for DC-DC chargers and battery links. https://www.solar4rvs.com.au/buying/buyer-guides/understanding-cable-size-types-mm-mm2-b_s-awg/ (checked October 2026)

Quick answer

A smart alternator (also called a variable-voltage or computer-controlled alternator) is controlled by the engine computer, which raises and lowers its output to save fuel and cut emissions. It often charges at a low voltage while you cruise and charges harder when you lift off the accelerator or brake. That’s fine for the starting battery, but it means an auxiliary or caravan battery joined by a simple relay (VSR) often won’t charge fully, so most installers use a DC-DC charger on vehicles with a smart alternator.

Is this a DIY job?

• Difficulty: Easy to identify; Pro to wire accessories correctly
• Time: 15 to 20 minutes to check
• Tools: digital multimeter, torch, owner’s manual
• Skills: basic use of a multimeter

Anyone can check for the signs below. Wiring a dual battery or caravan charging system to a smart alternator vehicle is best left to an auto electrician, because the battery sensor and the vehicle’s computer have to be wired around correctly.

How a smart alternator works

A conventional alternator has its own regulator that holds the charging voltage fairly steady whenever the engine runs. A smart alternator takes its orders from the vehicle’s computer instead. The computer watches the battery (usually through a sensor clipped to the battery negative terminal) and decides how hard the alternator should work.

When the battery is well charged, the computer turns the alternator down, so the engine has less load and uses less fuel. When you lift off the accelerator or brake, it turns the alternator up and uses the car’s momentum to recharge the battery. Smart alternators started appearing around 2010 and were common in new vehicles by about 2015.

Step by step: how to tell if you have one

1. Check the battery negative terminal. Look for a small electronic module with a plug and wires, clamped to the negative post. That’s the battery sensor. It’s a strong sign of a smart charging system. Don’t remove it.
2. Look for other clues. Stop-start, or a note in the owner’s manual about battery “registration” or a battery management system, also point to smart charging.
3. Start from cold. Ideally the car has sat overnight. Switch off the fan, radio, lights and other loads.
4. Measure at startup. Set the multimeter to DC volts and measure across the battery terminals with the engine running. Expect about 14 V or a little more at first.
5. Measure again after 5 to 10 minutes of idling or driving. If it has dropped to about 12.5 V, it’s very likely a smart alternator. A conventional alternator usually stays between about 12.8 and 14.4 V.
6. Confirm with the vehicle maker or a workshop scan tool if the readings are unclear. There’s no complete published list of vehicles with smart alternators. (see How do I know if my alternator needs replacing?)

How it affects accessories

• VSRs (voltage sensitive relays): A VSR connects the second battery when it sees charging voltage. If the alternator is sitting low, the VSR may not switch on, or the auxiliary battery only gets a partial charge.
• Caravan and camper batteries: The voltage is already low at the alternator, and then more is lost along the long cable to the van. The result is a van battery or fridge that barely charges while you tow. (see Why isn’t my caravan battery or fridge charging while towing?)
• DC-DC chargers: A DC-DC charger boosts and controls the charge for the auxiliary battery, so it copes with a low or changing input. Many need an ignition or “D+” signal wire on smart alternator vehicles so they know the engine is running. (see DC-DC charger or VSR isolator: which do I need?)
• The battery sensor: Many smart alternator vehicles have a sensor on the battery’s negative terminal that measures current flowing in and out of the starting battery. Whatever you fit, a good, solid earth is essential. Check the charger and vehicle instructions for where accessory earths should connect: many say to use a body or chassis earth point rather than stacking cables straight onto the negative post, so the sensor still sees the real current. Getting this wrong can confuse the charging system, and at least one DC-DC charger maker warns it can blow the charger’s input fuses.
• Replacing the battery: RACV notes that some vehicles with a battery management system need the new battery registered with the system. This is done with a scan tool. (see What’s the difference between flooded, EFB, AGM and lithium car batteries?)

Specs table

Typical battery voltage with the engine running

Reading	Conventional alternator	Smart alternator
Just after a cold start	About 14 V	About 14 V or a little more
After 5 to 10 minutes, loads off	Stays about 12.8 to 14.4 V	Can drop to about 12.5 V
Full range you might see while driving	Fairly steady	About 12.3 V to 15 V

Readings vary between vehicles and conditions. A low reading on a smart alternator vehicle doesn’t by itself mean the alternator is faulty.

Effect on common accessory setups

Setup	With a smart alternator
VSR joining two batteries under the bonnet	Often doesn’t charge the second battery fully
Caravan battery charged through pin 8 of a 12-pin plug	Usually poor charging: low voltage plus long thin cable
DC-DC charger near the auxiliary battery	Designed to cope; may need an ignition or D+ wire
Lithium auxiliary battery	DC-DC charger with a lithium profile

What you’ll need

• digital multimeter (to check)
• DC-DC charger (if you’re charging an auxiliary or caravan battery)
• heavy duty single core auto cable or twin core auto cable
• circuit breaker / MIDI fuse holder (one at each end of the charging cable)

When to call an auto electrician

• You’re adding a dual battery, DC-DC charger or caravan charging to a vehicle with a smart alternator.
• Your vehicle has a battery sensor, stop-start or battery management, and you’re not sure where accessory wiring and earths should connect.
• The auxiliary battery isn’t charging after a day’s driving, or a warning light appeared after accessories were fitted.
• You’ve replaced the starting battery and the vehicle now shows warnings or stop-start has stopped working.

Related questions

Q: Can I turn off or disable my smart alternator? A: On some vehicles a dealer can switch the charging system to a fixed voltage, but it isn’t possible on every make, and it can increase fuel use and may affect warranty, so ask the dealer first. For charging an auxiliary or caravan battery, a DC-DC charger is the usual answer because it works whatever the alternator is doing. (see DC-DC charger or VSR isolator: which do I need?)

Q: Is a smart alternator bad for my car? A: No. It’s designed to save fuel and keep the starting battery charged. The issues come up when you add a second battery or charge a caravan from the vehicle.

Q: Will a VSR work with a smart alternator? A: Often not well. The alternator’s voltage can sit too low for the VSR to connect, or for the second battery to charge fully. A DC-DC charger is the usual fix.

Q: Why is my alternator only putting out 12.5 volts? A: On a smart alternator vehicle that can be normal once the battery is charged. On a conventional system, a low reading points to a charging fault. (see What does the battery light on my dashboard mean?)

Q: Do I need an ignition wire for my DC-DC charger? A: On many smart alternator vehicles, yes. The charger can’t always tell the engine is running from the battery voltage alone. Follow the charger maker’s instructions.

Q: How does a dual battery system work? A: A second battery runs accessories, and a VSR or DC-DC charger keeps it charged from the vehicle while protecting the starting battery. (see How does a dual battery system work and do I need one?)

Sources

1. CarsGuide, “Vehicles fitted with a smart alternator in Australia” – how smart alternators work; first appeared around 2010 and common by 2015; voltage test method (about 14 V at start; about 12.5 V after 5 to 10 minutes indicates smart; 12.8 to 14.4 V indicates fixed voltage); effect on second batteries; no definitive list of vehicles. https://carsguide.com.au/car-advice/vehicles-fitted-with-a-smart-alternator-in-australia-97156 (checked October 2026)
2. Hema Maps, “Smart alternators” – smart alternator output can range between 12.3 V and 15 V; reduces voltage to cut emissions; VSRs and secondary batteries. https://www.hemamaps.com/blogs/hema-maps-gear/alternator (checked October 2026)
3. Redarc, “How do I know if I have a variable voltage alternator?” – battery sensor usually on the battery negative terminal; also called smart or computer-controlled alternators. https://redarcelectronics.com/us/resources/chargers-isolators-faqs/how-to-identify-smart-alternator/ (checked October 2026)
4. Enerdrive, EN3DC30 installation guide addendum V2 – on smart alternator vehicles the negative cable must connect to the vehicle side of the battery sense module; otherwise the charger’s input fuses can blow. https://www.mygenerator.com.au/assets/images/resource/pdf/EN3DC30-Install-guide-Addendum-V2.pdf (checked October 2026)
5. Solar4RVs, “Trouble shooting and fault finding problems on caravans” – DC-DC chargers on variable-voltage alternators may need the ignition or D+ wire connected. https://www.solar4rvs.com.au/blogs/buyer-guides/trouble-shooting-fault-finding-problems-on-caravan (checked October 2026)
6. RACV, “Car battery replacement” – some vehicles with battery management systems require new batteries to be registered with the system. https://racv.com.au/on-the-road/roadside-assistance/car-battery-replacement.html (checked October 2026)

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

1. Start safely. Engine off, van unplugged from mains power, and nothing loose near the batteries.
2. 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.
3. 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?)
4. 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?)
5. 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?)
6. Measure voltage at the vehicle battery. Engine running, van connected. This is point A. Write it down.
7. 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.
8. 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.
9. 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?)
10. 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).
11. 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.
12. 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.

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

1. 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)
2. ExplorOz, “Trailer wiring diagrams” – 7-pin and 12-pin pin functions and colours. https://www.exploroz.com/vehicle/electrics/trailerwiringdiagrams.aspx (checked October 2026)
3. 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)
4. 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)
5. 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)
6. 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)
7. 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.
8. 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)