Frank,
Battery manufactures are VERY liberal when quantifying the ampacities of
their products. A brand new 125 Amp hour may give you 70 amps of usage
before the voltage drops below 11.8-12.0 volts. At that point, the
battery needs recharging. That same battery, after a year of use, might
give you 50 amps of juice.
The solution, in my humble opinion, is there is no free ride when it
comes to refrigeration. The piezio electric cool boxes you describe are
very economical to purchase, however are very costly, electrically, to
run over any length of time.  Unless, you have solar power to back up
your battery charging efforts. That in itself is a costly proposition.
If you plan on using your refrig allot, my suggestion is to bite the
bullet and get a propane one. Much higher initial cost but well worth it
in the long run.
I live in Baja california in the winter time and have tried all sorts of
things in the process... Everyone winds up with propane refrigeration in
the long run, if they need reliability without being able to "plug in".
Bob Bellanca

Woodcock, Frank wrote:
>
> 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.