How long can a 12 V fridge run from a battery?
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The short answer
Divide the battery's usable energy in watt-hours by the fridge's average draw in watts, and you get the run time in hours. Use the average your fridge's maker publishes for stated test conditions, not the peak figure on the label. Usable energy is less than the battery's full rating, so check the depth of discharge for your battery type.
The formula
Hours = usable battery energy (Wh) ÷ average fridge draw (W)
Usable battery energy is the battery's voltage multiplied by its amp-hour rating, then by its usable share (its depth of discharge). Victron's worked sum for a lithium pack uses the same steps, with 0.80 as the usable share. Victron Energy blog on lithium and AGM batteries
Many fridges list draw in amp-hours per hour (Ah/h) at 12 V. To turn that into watts, multiply by the voltage: 1 Ah/h at 12 V is about 12 W. Dometic publishes its CFX3 figures in Ah/h, so do this conversion before you use the sum. Dometic CFX3 55 product page
Why average draw is lower than rated power
A compressor fridge does not run flat out. It switches on and off to hold its set temperature, so its average draw is the running current multiplied by the share of time it runs. Dometic's support page describes this method for compressor coolers, with an example of 3.75 A running for 22% of the time. Your fridge's running share will differ. Dometic support: most energy efficient cooler
The label figure is a ceiling, not an average. Dometic lists a rated DC input current of 7.5 A for the CFX3 35 and describes it as a maximum rating, not a consumption figure. Dometic CFX3 35 product page
Ambient temperature changes the figure, and maker figures state the conditions they were taken at. The CFX3 55 is listed at 1.1 Ah/h with 32°C outside and 4°C inside. Dometic CFX3 55 product page Only compare figures whose printed ambient and inside temperatures match your own conditions, and use the figure for the hottest conditions you expect.
The maker sources used for this guide do not say how a fuller fridge or frequent lid opening changes the average. Check your fridge's manual for a figure before you rely on one. For the wider picture of running kit from the vehicle, see power on the road.
Usable capacity versus nominal capacity
The amp-hour number on a battery is its nominal capacity, and you cannot use all of it. Victron's blog uses 80% depth of discharge for lithium iron phosphate (LiFePO4) batteries and 50% for AGM batteries. Victron Energy blog That post is from 2015, so check the current depth-of-discharge guidance from your own battery's maker before you use its percentage.
Your fridge's battery protection setting
Dometic's CFX3 series has a three-stage battery protection system that cuts the fridge off when the battery voltage drops too far. Dometic says to use High or Medium when the fridge runs from a starter battery, and Low when it runs from an accessory or dual battery. If the fridge will not start at a setting, drop one step. Dometic CFX3 35 product page
The setting changes how far the battery can be drained. Dometic says the protection either prevents excessive discharge from a starter battery or makes more use of a supply battery. Dometic CFX3 series operating manual Protection reads the voltage under compressor load, so a battery can show a healthy voltage at rest and still trigger the cut-off. Dometic CFX3 35 product page
After a cut-off, the battery is only partly charged. Recharge it before restarting, and avoid restarting the fridge over and over. Dometic CFX3 series operating manual
Worked example
Example figures, made up to show the method. Use your own fridge's and battery's published figures.
- Battery: 100 Ah nominal, 12.8 V (made-up figures)
- Usable share: 80%, the lithium figure in Victron's blog
- Usable energy: 12.8 × 100 × 0.80 = 1,024 Wh
- Fridge average draw: 12 W (made-up figure, the same as 1 Ah/h at 12 V)
- Run time: 1,024 ÷ 12 = about 85 hours, or roughly 3.5 days
The real figure will be shorter. Dometic advises against splitters, pigtails and extensions because they add line loss and lower the voltage at the socket. Dometic CFX3 35 product page Add your fridge to your overlanding kit list, and see the camp kitchen setup guide for how a fridge fits into cooking and storage.
FAQs
Why is the label current higher than the average?
Dometic describes the 7.5 A DC input figure on its CFX3 35 page as a maximum rating, not a consumption figure. Use the average figure from the maker's test conditions to work out run time.
Is amp-hours the same as watt-hours?
No. Multiply a battery's amp-hours by its voltage to get watt-hours, as in Victron's worked sum. A fridge's Ah/h figure at 12 V converts to watts by multiplying by roughly 12.
Which battery protection setting should I use?
Dometic says High or Medium for a starter battery and Low for an accessory or dual battery. If the fridge will not start, lower the setting by one step.
Next: plan your recharging on the move
Once you know how many hours your battery gives, work out how much charge you need while you drive, using charging while driving.
More in Power
Sources
- Dometic CFX3 35 product page: energy consumption of 0.98 Ah/h at 90°F ambient and 39°F inside; rated DC input current of 7.5 A is a maximum rating; battery protection FAQ on High or Medium for a starter battery, Low for an accessory or dual battery, dropping one step if it won't start, and protection reading voltage under compressor load (checked )
- Dometic CFX3 55 product page: energy consumption of 1.1 Ah/h at 32°C ambient and 4°C inside; 3-stage battery protection (checked )
- Dometic support: consumption estimated as current draw multiplied by average running time, with an example of 3.75 A at 22% running time; energy-label classification at 25°C ambient and 5°C interior (checked )
- Dometic CFX3 series operating manual, "Using the battery protection": Medium or High for a starter battery, Low ideal for a dedicated supply battery; after a cut-off the battery is only partly charged, so recharge it and avoid repeated restarts (checked )
- Victron Energy blog, March 2015: usable energy calculated as voltage x amp-hours x depth of discharge, 80% for LiFePO4 and 50% for AGM; the 2015 date means check your battery maker's current guide (checked )