12V camper trailer basics: battery, load and charging terms explained

A camper trailer’s 12V system only does three things: it stores energy, releases it at a rate your gear can use, and gets refilled before the next night. Battery size, charger type, panel wattage and cable runs are all just decisions about one of those three. Learn to write the numbers down in the right units and most of the confusion disappears.

This guide covers the 12V camper trailer basics an Australian owner needs before spending money — the terms on every spec sheet, a load budget you can work out on paper, what each charging source actually contributes, and the standards and safety rules that changed for new installations in November 2023.

Start with the units, not the products

Five units carry almost all the meaning. Everything else on a product page is packaging.

Unit	Symbol	What it measures	Where you meet it
Volt	V	Electrical pressure	“12 V” system label, charger output
Amp	A	Rate of flow	Appliance draw, fuse rating, charger rating
Watt	W	Power (volts × amps)	Appliance and panel ratings, inverters
Amp hour	Ah	Charge delivered over time	Battery capacity
Watt hour	Wh	Energy (amp hours × volts)	Comparing batteries and daily consumption

Watt hours are the unit worth thinking in. Amp hours only mean something once you also state the voltage, which is why a 100 Ah figure on its own tells you very little and a figure in watt hours can be compared with anything.

“12 V” is a label, not a reading

A 12 V system rarely sits at 12.0 V. Terminal voltage moves with state of charge, temperature, and whether the battery is being charged or discharged at that moment. Lead-acid voltage falls fairly steadily as the battery empties; lithium iron phosphate holds a much flatter voltage until it is nearly empty, which is exactly why a voltmeter is a poor fuel gauge on a lithium setup and a shunt-based monitor is not.

12V camper trailer basics: capacity versus usable capacity

The number printed on the battery is the rated capacity. The number you can actually plan around is smaller, because manufacturers publish a recommended maximum discharge for their own products.

Century Batteries states that its flooded and AGM deep cycle batteries “are typically recommended to be discharged to a maximum of 50% of their capacity, whilst Lithium Pro can be discharged to up to 80%”.

Battery type	Maker’s recommended maximum discharge	Usable from a rated 100 Ah	Rough energy at 12 V nominal
Flooded or AGM deep cycle	50%	50 Ah	about 600 Wh
LiFePO4 (Century Lithium Pro)	80%	80 Ah	about 960 Wh

The percentages above are Century’s, published for Century’s own products. The arithmetic is ours. Another brand will publish different figures for its batteries, and yours is the one that governs your setup — take it from the manual that came with the battery, not from a table on any website.

Two other published figures change the buying decision. Century rates Lithium Pro at 3000+ cycles at 80% depth of discharge, and says its lithium batteries weigh up to 50% less than an equivalent AGM. That weight difference is a payload decision as much as an electrical one, so it belongs in the same conversation as packing without wasting payload and keeping heavy gear low.

Build the load budget before you buy the battery

Sizing a battery by feel is how people end up with too much weight and still run flat. Work in watt hours per day.

1. List every 12V load. Fridge, lights, water pump, fans, device charging, heater, anything else.
2. Take the draw from the data plate or manual, not from memory or a forum post.
3. Estimate honest run time. A compressor fridge does not run continuously; it cycles. Get the duty cycle from the fridge maker, or measure it once with a monitor.
4. Multiply, sum, then add margin for the hot night, the flat day and the guest who charges three devices.

The table below is illustrative arithmetic. Every figure in it is a placeholder for the number on your own appliance.

Load	Rated draw	Hours per day	Wh per day
12 V compressor fridge	45 W while running	8 (cycling)	360
LED lighting	12 W	4	48
Water pump	60 W	0.3	18
Device charging	20 W	3	60
Daily total			486 Wh

Read that against the usable capacity table. About 486 Wh a day against roughly 600 Wh usable from a rated 100 Ah AGM means you reach the maker’s discharge limit partway through the second night with nothing coming in. The same load against roughly 960 Wh usable from a rated 100 Ah lithium buys most of a second night. Neither answer is “buy a bigger battery” until you have also worked out the recharge side.

Where the charge actually comes from

You need to return roughly what you took out, every day, from some combination of three sources.

Mains charging

The simplest and most predictable source, and the only one that reliably completes a full charge cycle overnight. It is also the source you do not have when you are away from powered sites, so it should not be the basis of your plan for a remote trip.

Charging from the tow vehicle

Many recent vehicles use a variable-voltage alternator whose output is managed for the starting battery, not for a second battery at the far end of a long cable run. A DC-DC charger exists to take whatever the vehicle offers and deliver a controlled multi-stage charge to the trailer battery instead.

Whether your specific vehicle needs one, and which model suits it, is a question for your vehicle manufacturer and the charger manufacturer. Do not settle it from a forum thread about a different model year. The same applies to cable size, fusing and voltage drop over the run from the vehicle to the trailer — those are installation calculations, not opinions, and getting them wrong shows up as heat rather than as an error message.

Solar

Two terms decide what a solar setup is worth. Victron Energy describes a PWM controller as “in essence a switch that connects a solar array to a battery, with the result that the voltage of the array will be pulled down to near that of the battery”, meaning the panel is not used at its maximum power point in most cases. An MPPT controller instead adjusts its input voltage to harvest maximum power from the array and converts it to what the battery needs, decoupling the two voltages.

Victron states that an ultra-fast MPPT controller “will improve energy harvest by up to 30% compared to PWM charge controllers”, and that MPPT outperforms PWM in cold to temperate climates while both perform roughly the same in subtropical to tropical conditions. That second point matters in Australia: the same controller choice is worth more in a Tasmanian or alpine winter than it is in a Darwin dry season.

A panel’s watt rating is a test-condition rating, not a promise of daily harvest. Shade, angle, dust, cloud and heat all reduce it, so plan on watt hours you have actually measured rather than nameplate watts multiplied by hours of daylight.

Generators

If you carry a generator, the safety rule outranks the electrical one. Victorian health guidance is explicit: “Do not use a portable generator indoors. This includes inside a garage, carport, basement, crawlspace, or other enclosed or partially enclosed area, even with ventilation,” and “Operate the generator outdoors, away from windows.” The same guidance says never to use a charcoal grill, lantern or portable camping stove inside a home, tent or caravan, and warns that opening doors and windows will not prevent carbon monoxide building up. If anyone feels sick, dizzy or weak, get to fresh air immediately and seek medical advice.

Match the charger to the chemistry

Century’s instruction is short and worth following literally: “Always use a charger compatible with your battery’s technology. Different Deep Cycle types require specific charging profiles and voltage settings.”

For lithium there is a temperature condition as well. Century specifies that its lithium batteries be charged in temperatures between 0 °C and 45 °C using a compatible lithium charger or charging system, and that the battery cannot be installed on its end — side mounting is acceptable, but not in high-vibration situations. Century also notes the built-in battery management system protects against overcharging, over-discharge, short circuits and extreme temperatures.

The temperature limit has a practical Australian consequence. A frosty morning in the High Country or a Tasmanian winter camp can put a trailer battery below the charging window while the sun is already on the panels. A system that handles this correctly does so because the equipment was specified for it, not because the owner remembered.

What changed for new installations in November 2023

REDARC’s summary of the current standard reports that AS/NZS 3001.2:2022 superseded AS/NZS 3001:2008 on 18 November 2023 after a twelve-month phase-in, and sets out several requirements that directly affect where a battery may live in a camper trailer:

• it applies to new electrical installations carried out after the effective date rather than to existing installations, but alterations such as a battery replacement must comply with the current standard;
• batteries must comply with AS IEC 62619;
• a lithium battery cannot be installed in a habitable living area unless it is in a sealed enclosure, or the location is sealed off from the habitable area and vented to the exterior;
• a lead-acid enclosure requires two vents, one at the top and one at the bottom, because the gases involved are lighter than air;
• batteries mounted on the chassis are permitted provided they are operated within their specifications and protected from physical damage;
• restricted areas apply to extra-low voltage equipment as well as to mains equipment.

REDARC recommends the work be performed by a qualified auto electrician or technician. The standard itself is published by Standards Australia and is not free to read, so treat the list above as the set of questions to put to your installer rather than as the text of the standard. If you are swapping an AGM battery for lithium in an older trailer, that swap is an alteration, and the enclosure and ventilation questions arrive with it.

The mains side is not yours to touch

The 12V and 230V systems in a trailer are separate, with different rules. Building and Energy in Western Australia states that caravans are required to have a residual current device installed under AS/NZS 3001, and that only a licensed electrician can install RCDs. The same guidance says the supply extension lead must be one continuous length rated at 15 amperes, should not exceed 10 metres, and should be fully uncoiled before use. Energy Safe Victoria’s outdoor guidance similarly says to ensure the power supply cord to your caravan is in good condition and rated at 15 amps, and to watch for overhead powerlines when erecting antennas.

Licensing and inspection requirements differ between states and territories. Confirm yours with the electrical regulator in your jurisdiction before assuming a rule you read for another state applies to you.

Lithium handling and charging safety

ACCC Product Safety publishes consumer guidance for lithium-ion batteries that translates directly to a camper trailer parked in the sun:

• store batteries in cool, dry places out of direct sunlight, and never in hot locations such as parked vehicles;
• allow a battery to cool after use before recharging it;
• never use a battery showing denting, crushing or other damage, overheating, swelling, leaking or venting gas;
• charge on non-flammable surfaces such as concrete, ceramic or steel, and never on beds, sofas or carpet;
• monitor charging and disconnect when it is complete;
• avoid damaged chargers or cables, and get replacement batteries from the original or a reputable supplier.

For disposal, ACCC directs consumers away from household rubbish and recycling bins. B-cycle, Australia’s official battery stewardship scheme run by the Battery Stewardship Council and authorised by the ACCC, asks that terminals be taped with clear sticky tape and the battery taken to an accredited drop-off point, noting that binning batteries is a fire risk.

Measure instead of guessing

A shunt-based battery monitor counts amp hours in and out rather than inferring them from voltage, which is the only way to know what your setup actually does. Over a few trips, record:

• watt hours consumed per night, noted by season and by ambient temperature;
• the lowest state of charge you reached, and on which night;
• how much each source returned — mains, vehicle, solar — on a good day and a bad one;
• anything that behaved differently after a long corrugated road.

Three trips of that data will tell you more than any calculator, and it turns the next purchase into a decision instead of a guess. A shakedown trip close to home is the cheapest place to collect it.

A 12V routine before departure

1. Confirm the battery is fully charged, from the monitor rather than from the voltage.
2. Check terminals and connections for tightness, corrosion and damage.
3. Confirm the battery is secured in its mounting and in the orientation its maker permits.
4. Check fuses and breakers are the specified rating and that spares are aboard.
5. Inspect visible cabling for chafe, heat marks or damaged insulation.
6. Clean the solar panels and confirm the controller is reporting sensibly.
7. Confirm the DC-DC charger operates once the vehicle is running.
8. Test the loads that matter — fridge, pump, lights — before you leave the driveway.
9. Inspect the mains lead and RCD arrangement; stop and get an electrician if anything is damaged.
10. Pack the manuals for the battery, charger and fridge, so the specification travels with the trailer.

Fold these into the wider pre-trip checklist rather than running them as a separate ritual. The fridge in particular sits across both lists, since its power budget and its packing both decide whether it holds temperature — a point covered in the kitchen packing checklist.

Scope and limitations

This guide is desk research for Australian conditions. The depth-of-discharge, cycle-life, weight, charging-temperature and mounting figures are Century Batteries’ published figures for Century products; the PWM and MPPT comparison is Victron Energy’s for Victron equipment. Both are used here as examples of how manufacturers state their own requirements, not as universal values. Your battery, charger and controller have their own numbers, and those are the ones that apply.

The standards summary reflects REDARC’s published explanation of AS/NZS 3001.2:2022, not the standard itself. Electrical licensing, inspection and compliance requirements differ between states and territories, and any work on the mains side belongs with a licensed electrician. The load budget table is illustrative arithmetic built from placeholder figures so the method is visible; it is not a measurement of any particular appliance.

The 12V camper trailer basics worth carrying away are the smallest ones: write consumption in watt hours per day, know the usable capacity your battery maker actually recommends, and confirm that your daily recharge matches your daily draw. Everything else is a product decision, and product decisions get much easier once those three numbers are on paper.