How long will my battery last

I think that you have to divide the Amps that your fridge uses into the capacity of the Battery. In other words...if your fridge uses 2.5 Amps per hour running flat out then it should run on the 55AH battery for 22 hours before the battery is totally drained. That is my understanding.

Personally the fridge and I sleep in the same compartment so the latter gets turned off after lights out. Everythiong stays pretty cool inside the fridge through the night.Cheers, Willem
Never a dull moment

David
Don't forget the fridge does not run continuously and it's cycling time is governed by the ambient temperature, the temp you have set the fridge to and the fridges ability to insulate itself.
So the way you "operate" the fridge can have a big affect on the car battery life !
Rod

A waeco fridge should drag about 4Amps, flat out in 35 degree heat in the tropics, but in cooler climates or on lower settings there is such thing as dutycycle, where the thermostat cuts the fridge on and off for the required temperature.
At christie engineering we get 20 calls per day about the same problems.
An optima battery will be the best battery you can get, but there are larger types than the one you bought, which can run your fridge longer.
We reccomend running the fridge for 24 hours only as your voltage will be getting close to critical level. Anything below 11.6 volts, you will be starting to corrode the spiral wound plates. This is why we built our own battery chargers as you can recharge this battery in around 1 hour, then you have another 24 hours for your fridge, so you can camp in one spot for more than 1 day without stuffing your batteries.

David,
I worked it out for my Engle and it works diffrently every trip it's very much down to all the "other" conditions, amount of beer in the fridge, amount of times opened climate etc etc, the longest I have my fridge running on the battery alone without charging would be around 14 to 16 hours, or there abouts. As long as you air on the conservative side and you don't plan on not running the car for extended times you should be right. The theoretical answer will give you a ball park figure but there are so many other varients that dictate how long the battery will last before the auto cut out cuts in (mouthful) it would be very difficult to calculate accurately, this is only my opinion. When I go on a trip the performance of my battery improves with use, I run a small mechaincal clock in the back of the fourby to have a small constant discharge on the battery at all times, I've been told that leaving the auxillary battery doing nothing is very harmful, something to bare in mind. Keep the shiny side up

My rule of thumb is pull up at a camp site around 4 in the afternoon stay the next day and move on the second morning and the fridge has run the whole time. Also a 12v light for about an hour each night.

Voltage on the aux battery still showing a tad over 12 volts.