How long a battery will run a 12V fridge

The sum is short. Battery size, times the share you can use, divided by what the fridge draws per hour. The trick is picking honest numbers.

The three numbers

1. Battery capacity in amp-hours (Ah). It's printed on the battery. An amp-hour is a measure of charge, not of how the battery will perform in your use, and the usable output can differ¹.
2. Usable share. You can't pull a battery to zero. Dometic says lead-acid batteries have a 50% usable capacity of the stated Ah capacity².
3. Fridge draw in Ah per hour (Ah/h). Use the maker's figure for conditions like yours, or better, a figure you've measured yourself. Here's how to measure it.

Then the maker's formula: divide the battery's usable capacity by the fridge's average energy consumption in Ah/h to get the hours of run time².

Worked example

These are made-up round numbers to show the method. They're not a test.

• Battery: a lead-acid battery rated at one hundred amp-hours.
• Usable: half of that, so 50 Ah².
• Fridge: the CFX3 100 spec sheet gives 1.56 Ah/h, measured at 32°C ambient and 4°C internal².
• Runtime: 50 divided by 1.56 is about 32 hours². That's a bit over a day.

Now try a gentler fridge. Using Dometic's compressor example of 3.75 A running 22 percent of the time, the draw is under 1 Ah/h². At 0.825 Ah/h, the same 50 Ah lasts about 60 hours. That's roughly two and a half days. The difference shows how much the fridge itself matters.

Why real runtime is usually shorter

• Hot days raise the draw. The maker's Ah/h is for stated temperatures only². The reasons are on the daily changes page.
• The fridge switches off at its protection level. ARB says the fridge switches off automatically once the battery drops below a set point³. The level you've picked sets where your runtime ends. More in low-voltage cut-off.
• Energy transfer is never 100% efficient. There's always some loss between energy stored and energy delivered¹.

Plan with a margin. If the sum says one day, don't plan on one day.

Other battery types

Dometic's 50 percent figure is for lead-acid, and it gives other numbers for other battery chemistries². Check your battery maker's manual for the usable amount and the limits for your battery.

Depth of discharge is how far a battery is emptied each time. For almost all rechargeable types, there is a correlation between depth of discharge and the battery's cycle life⁴. In plain words, how deeply you empty the battery is linked to how many cycles it lasts. Check your battery maker's guidance on how far to run it down.

Lithium batteries

• Follow the maker's charging and storage limits. They're in the battery's manual.
• Lithium-ion cells hold flammable electrolytes, which makes fire or explosion a hazard⁵.
• High temperatures or mechanical damage can make a cell's reaction run explosively⁵. Don't keep using or charging a damaged battery.

Daily version

You can run the sum the other way. Take the daily amp-hours from the label page, which turns Dometic's hourly figure into a day². Divide the usable amp-hours by that daily number. The answer is days of running, with no sun or charging.

Safety when you change the setup

• Adding a bigger battery means moving cables, so isolate the battery first. That goes for any battery, fuse, shunt or inverter lead.
• Keep to the manuals for the battery and the fridge.
• A licensed auto electrician should do the job if you have any doubt. It isn't routine DIY.
• In Australia, permanent mains wiring and anything at the switchboard is for licensed electricians only.