I'll try and keep this simple as i can
First check your battery charger can provide 4 or more amps (it will be on the casing / instruction book somewhere)
I can use either a smart charger ((as long as you have a spare 12v car battery / caravan battery handy)
or a normal old style charger
The old style charger is the easiest to use...
http://www.ebay.co.uk/itm/NEW-HEAVY-DUT ... 4ac35a6f76
For safetys sake your much better using a 12v battery to cig socket adaptor ( i made my own from a cig lighter extension cable and 2 battery clips like this:
and making sure the charger clips cant come into contact with each other... like they could if kicked in the pic below....
rather than doing what i did in this picture to get the reading for you...
positive to the tip and negative to the side contacts...
this is the lead i made
and how to couple up to a smart charger.......

[/img]
clip the charger to the battery terminals then attach the fridge to the same polarity terminals then switch on the charger
You can get dedicated power supplies from most camping shops if you'd rather go the proper way but i have run the camping fridges for a good while off a battery charger with no issues...
http://www.ebay.co.uk/itm/MINI-FRIDGE-C ... 2c7181f471
usual things make sure the area is well ventilated (especially when piggybacking off a battery) check the charger and any wiring isnt getting too hot
the cooler can cool 11 degrees below ambient temperature at 26^C according to the instructions...
and can fit a 4pint milk container
