Podmods, Maintenance, Tips and Tricks: The "black magic" of lead acid batteries

The fridge uses about 10 amps per hour. A typical deep cycle battery is rated to have about 100 amps, but should rarely be discharged below 50% (an occasional discharge to 75% is OK, but be sure to get it charged ASAP), so theoretically you have about 50 amps available.

But, there is something called the Peukert effect, which says that the faster you discharge a lead acid battery the less total capacity the battery will have. A 10 amp load is fairly large, so while you might think you have about 5 hours of run-time you'll probably only get 3-4. On the flip side, a 1 amp light bulb will have more than 50 hours of run-time, probably closer to 70. Mostly this effect is due to the mixing of the electrolyte close to the battery's lead plates, plus changes to the chemical makeup of the surface of the plates.

Another part of the "black magic" surrounding lead acid batteries is the temperature of the battery. You might get a theoretical 50 amps from a battery sitting at room temperature. Raise that to 100 F when the battery is sitting in the sun in the summer in a black battery box and you might have 75 amps available. Drop that to 25 F in the middle of winter and you might only get 25 amps out of the battery.

Back to running a fridge: with only a few hours available from a lead acid battery (that is in good shape and fully charged and isn't cold), you will do better running from propane if you don't have AC hookups available. You can run a small RV fridge for weeks from one tank of propane - if you aren't also running the furnace and cooking.