How Long Will a Battery Run a 12V Fridge? The Math, With Worked Examples

“Will my battery last the weekend?” is the question behind most 12V fridge setups, and the answer is simple arithmetic once you have the right two numbers. This guide shows the method Dometic publishes for its own fridges, the adjustments that matter (battery chemistry and outside temperature), and a set of worked examples. We haven’t measured any of these fridges ourselves. Every figure below comes from a maker’s published spec, an independent review, or our own estimate, and we say which. To skip the math, use the battery runtime calculator.

What’s the formula?

Dometic’s FAQ gives it directly: divide the battery’s usable amp-hours by the fridge’s average consumption in amp-hours per hour (Ah/h). The answer is hours of runtime.

runtime (hours) = usable battery Ah ÷ average draw (Ah/h)

Two numbers, and both are easy to get wrong.

Usable battery capacity. You can’t drain every battery to zero. Our calculator uses Dometic’s guidance, keeping LiFePO4 at a slightly conservative 90%:

Average draw. This is not the rated power on the listing. A fridge’s compressor cycles on and off. Dometic rates the CFX3 45 at 8.2 A maximum input but publishes an average of 1.03 Ah/h at 90°F outside and 39°F inside. Runtime math uses the average. Our power consumption explainer covers why the two numbers differ so much.

Why does outside temperature change the answer?

A fridge works harder the bigger the gap between the air outside and the set temperature inside. Our calculator scales a published draw in a straight line with that gap. At 70°F outside and 37°F inside, the gap is about two-thirds of the gap at 90°F, so the draw is about two-thirds too. Real compressors do a little worse than linear in serious heat, so treat every scaled figure as an estimate.

Here is Dometic’s published CFX3 45 figure scaled to three outdoor temperatures, with the fridge set to 37°F:

Worked example 1: a 45-quart fridge on a 100 Ah battery

Start with Dometic’s CFX3 45, a typical 45-quart single-zone fridge with a published draw:

• Published draw: 1.03 Ah/h at 90°F outside, 39°F inside.
• Set it to 37°F instead and the gap grows slightly, giving about 1.08 Ah/h.
• 100 Ah AGM: 50 usable Ah ÷ 1.08 ≈ 46 hours, just under two days.
• 100 Ah LiFePO4: 90 usable Ah ÷ 1.08 ≈ 83 hours, about three and a half days.

Now cool the weather to 70°F. The draw falls to about 0.67 Ah/h, and the same LiFePO4 battery lasts about 133 hours, or 5.6 days. At 100°F it rises to about 1.29 Ah/h, and runtime drops to about 70 hours. The battery is the same in all three cases. Only the weather changed.

That’s also why lithium is the usual upgrade for a fridge battery. A 100 Ah AGM and a 100 Ah LiFePO4 have the same label, but the lithium one gives you nearly twice the usable energy. The LiTime 12V 100Ah LiFePO4 on our accessories list is the common example, at about 1,280 Wh.

Worked example 2: a fridge with its own battery

The EcoFlow GLACIER Classic 35L is the one fridge in our data with a published fridge-mode draw: 0.57 Ah/h (165 Wh a day) at 77°F outside and 39°F inside, in Eco mode.

• 0.57 Ah/h × 12 V ≈ 6.8 W average.
• 298 Wh battery ÷ 6.8 W ≈ 44 hours.

EcoFlow claims up to 43 hours on that battery, so its own figures are consistent. That also suggests nearly all of the 298 Wh is usable. Scale the draw to 90°F outside and 37°F inside and it rises to about 0.80 Ah/h (9.6 W), so the same battery lasts about 31 hours. That’s the number we’d plan a hot weekend around.

The Anker EverFrost 2 40L is similar. Anker claims up to 52 hours on one 288 Wh battery and 4.3 days on two, measured empty, in Eco mode, at 39°F inside and 77°F outside. An empty fridge on a mild day is a best case. Treat it as a ceiling, not a plan.

Worked example 3: when real use beats the spec sheet

Lab figures assume a closed lid and steady conditions. Camping isn’t that. Gnomad Home logged the ICECO VL45 ProS’s smaller sibling, the VL35 ProS, for 123 hours of real camping. It averaged about 508 Wh a day, or roughly 1.76 A.

• Our capacity-based estimate for the VL45 ProS at 90°F/37°F: about 0.96 Ah/h. On a 100 Ah LiFePO4, that’s 90 ÷ 0.96 ≈ 94 hours.
• At Gnomad’s real-world average of 1.76 A: 90 ÷ 1.76 ≈ 51 hours.

That’s nearly a 2× gap. It isn’t a flaw in either number. Gnomad’s figure includes everything a real trip throws at a fridge, and it’s for a different (smaller) box than ours. Use the spec-based figure as the best case and plan with a margin. If your trip involves hot afternoons, a lot of lid opening or warm groceries, assume the real-world end.

Worked example 4: a power station

Power stations are rated in watt-hours. To use the formula, take the 85% usable share of the watt-hours, then divide by 12 V (the voltage the fridge’s Ah/h figure is quoted at):

• 1,000 Wh power station: 1,000 × 0.85 = 850 Wh usable ÷ 12 V ≈ 71 usable Ah.
• With the 45-quart fridge at 90°F outside (1.08 Ah/h): 71 ÷ 1.08 ≈ 66 hours.

Run the fridge from the power station’s 12 V DC outlet, not its AC inverter. The inverter has its own losses. Small DC-only units like the Anker SOLIX C300 DC (288 Wh) are a night or so for a small fridge. Our powering guide covers the options.

What about the car’s starter battery?

We don’t give runtime numbers for starter batteries. The amp-hour rating isn’t what decides when the fridge stops. Battery protection is. Its job is to switch the fridge off while the battery still has enough charge to crank the engine, and Dometic’s CFX5 manual says to switch the fridge off, or move to a house battery, if the engine will be off for a long time. Expect a few hours to a night. Our battery protection settings guide explains the cut-off levels.

How do you get more hours from the same battery?

• Pre-cool at home on AC and load cold or frozen food. Pulling warm food down is the most power-hungry thing a fridge does.
• Keep it out of the sun and give the vents room to breathe. See our hot weather tips.
• Set 37–39°F, not 32°F. Dometic recommends 39°F for refrigeration and notes that even small changes in set temperature affect consumption.
• Use Eco mode once it’s cold, if your fridge has one.
• Keep it full and open the lid less often.
• Measure. A shunt monitor like the Victron SmartShunt shows what your fridge actually draws, which beats any estimate.

Bottom line

Divide usable amp-hours by average draw, then adjust for heat and real life. A 45-quart compressor fridge on a 100 Ah LiFePO4 battery is a three-to-five-day setup in mild weather and a two-to-three-day setup in a heat wave. A built-in 300 Wh battery is about a day. If you want longer, add solar or a bigger battery. Then run your own numbers in the battery runtime calculator.