Refrigeration compressors generally have a start up surge. It usually lasts less than a second, but can last longer depending on the system. This includes refrigerators, air conditioners and heat pumps. It is the large current draw that it takes to get the compressor started. Once the compressor is started the current to run it drops significantly.

That being said...

The lower cost method might be to keep the battery or find a smaller one that can still source 100 amps during the start up surge. Put a low cost charger on it so that it is constantly being charged. The charger will source current to charge the battery as well as the low current portion of the duty cycle. The only time the battery will be drained is during the startup surge.

This will be easier if you can measure the duration of the high and low current draws by the refrigerator. This will tell allow you to size both the battery and the charger. The battery must be able to source the 100 amps for the startup and the charger must be able to source enough current to charge the battery between starts and to supply power to the fridge while it is running at low current.

 

And what, pray tell, is 100 A/h? So after six minutes it's 10 A and after 10 hr it's 1000 A?

Aside from being proportional to the voltage across an inductor, that unit has little meaning and none in this case.

It is the battery normally connected to the protable fridge, so it can work.....

 

Finally, TS provides make and model. Coldqueen B1000XT
It is a cargo fridge powered by the automotive 12V battery charging system.

http://www.coldqueen.com/#B1000

http://www.magix-website.com/mppo20/50/14C/14C27310423211DFB119C2BA529B111A.pdf

Coldqueen data sheets show <70A.
Name plate supplied by TS shows 100A.

We owe TS an apology. PSU needs to supply 100A for startup. Unit probably draws <70A while running.

Since this is automotive installation, PSU voltage will be at least 13.8V and as high as 14.5V.
A 100A battery charger could be a solution as long as the output voltage does not exceed the limits of the unit and that we do not know.

 

Finally, TS provides make and model. Coldqueen B1000XT
It is a cargo fridge powered by the automotive 12V battery charging system.

http://www.coldqueen.com/#B1000

http://www.magix-website.com/mppo20/50/14C/14C27310423211DFB119C2BA529B111A.pdf

Coldqueen data sheets show <70A.
Name plate supplied by TS shows 100A.

We owe TS an apology. PSU needs to supply 100A for startup. Unit probably draws <70A while running.

Since this is automotive installation, PSU voltage will be at least 13.8V and as high as 14.5V.
A 100A battery charger could be a solution as long as the output voltage does not exceed the limits of the unit and that we do not know.

I was looking for that datasheet, but got the "Queens fridge is turning ..... years" instead. How did you find it?

 

I was looking for that datasheet, but got the "Queens fridge is turning ..... years" instead. How did you find it?

With Google. Don't remember how.

 

in order to operate it with AC in, you will need at 2kW AC to DC converter which is not cheap.

 

Your very best & cheapest option is to leave the fridge connected to the battery and continuously charge the battery from a battery charger capable of charging at about 40A. You will need to incorporate some sort of automatic cut in/out for the battery charger when the battery is fully charged.
something like this https://www.ebay.ie/itm/12V-Storage...097754?hash=item3b02af0fda:g:RrAAAOSwahlakREf

or this https://www.ebay.ie/itm/30A-12V-Aut...583713?hash=item41b847be21:g:NYYAAOSwzJ5Xaq~q

First of all the battery will be able to supply the 100A start up current and a 30A charge rate combined with battery capacity will do the rest, without difficulty as long as the fridge is running only about 30-40% of the time. A 40 A charger will give you a 33% overhead so that it is constantly operating within it's comfort zone.
This item claims to include fully automatic control at a very reasonable price but I would check the quality first before commiting to buy.
https://www.ebay.ie/itm/40A-50A-60A...hash=item1ee2225761:m:mr8tcrndvE6Axiun25lQI1Q