[Rich_]

My Dad has one of these Dave.

His is Ok but it is not a fridge, the stuff needs to be cold when you put it in, & it struggles a bit in hot weather. It won't keep stuff frozen any longer than cool blocks would , but it keeps sandwiches & drinks reasonably cool.

I wouldn't run anything off the car battery for too long while the engine is not running. The battery does not need to loose much charge before it will refuse to start your engine, even though it would run the appliance for a while. I would, if you can justify the expense, & weight get separate leisure battery from a caravan dealers & put it in the boot when you need it, especially if you intend running it all day. You can get a 85ah one for about £35. You can get cigarette lighter fittings that you could use to connect the box to this.

I think the cool boxes also come with a mains adapter too
.