I have recently bought a 12 volt battery operated cool box (about the
size of a 15" PC monitor) for my campervan . The idea was to keep wine/
beer cool. A lead connects from the chiller in the box lid to the cigar
lighter socket.

While parked up, I plugged the cool box into the accessory socket of a
powerpack that is used for the mobile phone and other low drain
equipment. The power pack comes from Argos Superstore (UK)

I was surprised to find that the power pack was flat after only hours
and checked the capacity to find that it was only 15 AmpHour.

I later checked the cool box with an meter and found that it drew 4.2
Amps which explained why the power pack lasted such a short time.

I will be trying it with an electric hook-up and a battery charger to
see if the power pack + cool box will work.

Meanwhile is there anyone with knowledge of a small thermostat on a lead
that can be placed inside the cool box to switch on and off the 12 volt
supply and therefore reduce the power drain?

I am considering getting a large leisure battery with a capacity of 125
AmpHours. This would last about 30 hours, so a recharge would still be
required quite often without a thermostat device.