How long a camping fridge runs on a battery depends on the electrical draw of the fridge and the size of the battery. A typical car battery has about 50 Ah of energy, which can run some of the best 12-volt camping fridges for about 30-40 hours since they usually have around a 1 Ah per hour average draw. If you find the average energy consumption rate of the portable fridge (in Ah or Wh) and the total capacity of the battery (in the same Ah or Wh), you can calculate how long it will run. But the ambient temperature, fridge temperature setting, and other factors will also impact how long the fridge will run on the battery.