12 volt power and camping fridges, sized properly

A compressor fridge is the only load on a camping trip large enough to design around. A 35 to 50 litre unit typically draws 250 to 400 watt-hours per day in warm conditions, an order of magnitude more than every phone, headlamp and speaker combined. Size the battery in usable watt-hours, size the charging to match, and the rest follows.

What a fridge actually costs you per day#

Published consumption figures for portable compressor fridges are measured against a stated ambient temperature, with a closed lid and a stabilised load, in the spirit of the household appliance method in IEC 62552-3. Your trip is not that test. Four things move the number, and all are within your control:

• Ambient temperature. The compressor works against the difference between the cabinet and the air around it. A fridge in a shaded vehicle and the same fridge baking behind glass are different appliances in energy terms.
• Lid openings. Cold air falls out of a chest-format lid, and each opening replaces it with warm humid air that must be cooled and dried again. A group grazing all afternoon costs materially more than three openings a day.
• Set point. Every degree colder than you need is compressor time you pay for. Drinks do not need minus 2 degrees C.
• Insulation and airflow. A cover, shade over the unit and clear space around the condenser all reduce run time.

Plan on 250 Wh per day in mild conditions, 300 Wh as a working default, and 400 Wh in hot weather with a busy lid. Pre-chill the fridge and its contents on mains power the night before, because the pull-down from ambient to set point is the most expensive period the fridge will have.

Amp-hours are not watt-hours#

Battery capacity is sold in amp-hours which, like milliamp-hours on a power bank, is a charge unit that means nothing without a voltage. Multiply by nominal voltage to get energy:

The first two rows carry the buying decision: a 100 Ah lead acid battery and a 50 Ah lithium battery hold roughly the same usable energy. The lithium unit is far lighter, tolerates deeper discharge, holds voltage better as it empties and accepts charge faster. Lead acid and AGM are heavier and shorter lived in cyclic use, but cheaper to replace and tolerant of a simple charging setup.

Discharging lead acid past about half its capacity shortens its life sharply, so the 50 percent figure is a design rule rather than a cut-off. Lithium iron phosphate packs enforce their own limits through a battery management system, and 80 percent is the conservative planning figure.

Two worked budgets#

A three night trip, no charging. Fridge at 300 Wh per day plus lights, phones and a fan at 30 Wh per day is 330 Wh daily, so three days needs 990 Wh of usable energy.

That is a 100 Ah lithium iron phosphate battery, delivering 1,024 Wh at 80 percent depth of discharge, or a 200 Ah lead acid bank at about 1,200 Wh usable, at several times the mass. Note what happens if amp-hours are assumed interchangeable: a 100 Ah AGM battery gives about 600 Wh usable and runs out early on day two.

A seven night trip. 330 Wh per day times 7 is 2,310 Wh. At 80 percent depth of discharge that is 2,310 divided by 0.8 divided by 12.8, about 226 Ah of lithium. Nobody carries that, so a seven night trip is a charging problem, not a battery problem. Two inputs each cover roughly one fridge day:

• A 20 A DC to DC charger at about 14 V is 280 W, so one hour of driving returns about 280 Wh.
• A 100 W panel at 3 equivalent full sun hours produces 300 Wh, of which about 255 Wh reaches the battery after losses. The harvest figures behind that come from solar panels for camping.

With either input running daily, a 100 Ah lithium battery covers a week comfortably and buffers a run of cloudy stationary days. Handheld devices barely register in these totals; size them separately using power banks and batteries or the power budget calculator.

Charging: alternator, solar and mains#

Modern vehicles with smart or variable voltage alternators do not hold a steady charging voltage, and many will not push an auxiliary battery past a partial state of charge. A simple voltage sensitive relay, adequate on older vehicles, tends to leave a modern setup chronically undercharged, and it is unsuitable for lithium, which can demand more current than the alternator circuit was designed to supply.

The standard answer is a DC to DC charger: it takes whatever the vehicle offers, limits the current it draws, and produces the correct profile for your chemistry. Most units also accept a solar input and manage both sources. Size it around the driving you actually do, since at 20 A an hour of driving is roughly a fridge day.

Mains charging before departure is free capacity. Arrive full, with the fridge already at temperature, and the first day costs almost nothing.

Fridge or cooler: where the crossover falls#

The comparison is usually argued in vague terms, but a physical constant makes it concrete. Melting one kilogram of ice absorbs 333.6 kJ, and 333.6 kJ divided by 3,600 is 92.7 Wh. So:

• A fridge at 300 Wh a day performs roughly the thermal work of 300 divided by 92.7, about 3.2 kg of melting ice per day.
• Over seven days that is about 22 kg of ice equivalent. At close to 917 kg per cubic metre, 22 kg occupies about 24 litres of cooler before any food goes in.

The comparison is not exact: a cooler also loses ice to warming above 0 degrees C, and it holds a higher, less stable temperature than a fridge. But the order of magnitude tells the story. A 50 litre cooler cannot carry a week of ice and a week of food at once, which is why the crossover falls at three to four days, or at the first stop where ice is unavailable.

We do not quote prices, and this argument is usually made in currency. Make it in logistics instead: ice mass carried, cooler volume lost to it, and detours to a shop. Those are the costs you feel. The ice side is worked through in coolers and ice, and you can run your trip through the cooler ice calculator.

Cutting fridge consumption before buying more battery#

Every watt-hour you do not use is battery you do not carry, and the interventions are unglamorous:

1. Shade the fridge. Direct sun on the cabinet or condenser is the largest avoidable load.
2. Fit an insulating cover. A fitted jacket cuts heat gain through walls and lid, once, with no running cost.
3. Keep the condenser clear. A fridge wedged against a wall or buried under bags recirculates its own hot air.
4. Pack it full and pre-chilled. Cold mass holds temperature through lid openings far better than cold air.
5. Set the temperature you need. Drinks and vegetables at 4 degrees C cost less than a cabinet held near freezing.
6. Open it with intent. Decide what you want before lifting the lid.

Wiring: what to do yourself, and where to stop#

12 volt systems are low voltage, not low energy. A lithium battery can deliver hundreds of amps into a short circuit, enough to melt a spanner and set a vehicle alight. What matters is not how confident you feel, but whether a fault has a defined path to being interrupted.

Reasonable to do yourself, with care and a manual:

• Plugging a portable fridge into an existing, correctly fused vehicle outlet rated for the current.
• Using a self-contained portable power station with its own protection and standard connectors.
• Fitting a pre-made, pre-fused battery lead of the correct size, per the maker's instructions.

Have it done, or study the standards properly before starting:

• Any permanent connection to the starter battery, alternator or vehicle loom.
• Cable runs through a bulkhead or firewall, where grommets and chafe protection decide whether the job is still safe in three years.
• Sizing conductors for current and length. Voltage drop matters as much as ampacity: ABYC uses 3 percent for critical circuits and 10 percent for others, and a long thin run causes low voltage cut-outs that look like a faulty fridge.
• Mounting a lithium battery inside a living space, including ventilation and restraint against a crash load.

There is no shame in the second list. The cost of being wrong is a vehicle fire somewhere with no fire service, and it is work a competent auto electrician completes in a morning. The gear side of a vehicle setup is in the car camping checklist.

Common mistakes#

Comparing amp-hours across chemistries. A 100 Ah AGM and a 100 Ah lithium battery are not equivalent: roughly 600 Wh usable against 1,024 Wh.

Forgetting the pull-down. Cooling a warm fridge full of warm food is the most expensive thing it will do. Pre-chill on mains.

Assuming the alternator keeps up. With a modern variable voltage alternator, expect a partial charge at best without a DC to DC charger.

Running lead acid flat. Repeatedly taking lead acid or AGM below half charge shortens its life dramatically, and the damage is cumulative and invisible.

Fusing the appliance instead of the cable. The fuse protects the wire from becoming a heater. Size it to the conductor and place it at the source.

Buying battery instead of insulation. A cover, shade and lid discipline cut consumption for a few hundred grams. Battery for the same energy costs kilograms.

Frequently asked questions#

How many amp hours do I need for a 12 volt camping fridge?#

Start from watt-hours. A 35 to 50 litre fridge uses roughly 250 to 400 Wh a day, so three days without charging is 900 to 1,200 Wh. That is a 100 Ah lithium iron phosphate battery, about 1,024 Wh at 80 percent depth of discharge, or a 200 Ah lead acid bank for the same usable energy.

How much power does a camping fridge use per day?#

Typically 250 to 400 Wh per day for a 35 to 50 litre compressor unit in warm conditions, which is 20 to 33 Ah at 12 volts. Ambient temperature, lid openings, set point and shade all move the figure substantially, so treat published consumption as a best case measured against a controlled ambient with the lid closed.

Is lithium iron phosphate worth it over AGM for camping?#

For usable energy per kilogram, yes: a 50 Ah lithium battery matches a 100 Ah AGM in usable watt-hours at a fraction of the mass, and tolerates deep cycling that would destroy lead acid. AGM remains reasonable for occasional use, simple charging setups, and where replacement cost matters more than mass.

Do I need a DC to DC charger?#

In a modern vehicle, almost certainly. Variable voltage alternators do not hold a charging voltage long enough to fill an auxiliary battery through a simple relay, and lithium batteries can draw more current than the original circuit was designed for. A DC to DC charger limits the draw and applies the right profile.

Is a fridge better than a cooler for camping?#

Below three nights a cooler is simpler and lighter overall. Beyond that, ice becomes the problem: a fridge at 300 Wh a day does the thermal work of about 3.2 kg of melting ice, so a week is roughly 22 kg of ice, about 24 litres of cooler space. Once ice resupply is awkward, the fridge wins.

Can I run a camping fridge from a portable power station?#

Yes, and it is the simplest safe route. Size it in watt-hours: a station holding 500 Wh usable runs a fridge for about a day and a half at 300 Wh per day. Check it supplies a 12 volt DC output at sufficient current, because running the fridge through a mains inverter wastes energy in an unnecessary conversion.

How long will my fridge run on a 100 Ah battery?#

Lithium iron phosphate gives about 1,024 Wh usable at 80 percent depth of discharge, so at 300 Wh per day that is a little over three days. Lead acid or AGM gives about 600 Wh usable, so roughly two days. Hot weather and frequent lid openings shorten both figures.