I don’t need to jump start the car, inflate the tyres, use it as a torch or even as a radio … I just need a safe portable 12v power pack with say a 20Ah battery to power our cool-box when the car is parked up on days out.
You must be able to recharge it from a 12 volt supply or via a mains adapter.
Quite a number of places sell these items with all the whistles and bells at various prices, but from my own personel experiences I have found that the non-commercial types can be unreliable, especially if they are not kept at peak condition.
Most 'cool-boxes' can be plugged into the vehicle socket during the journey, and with ice packs, they should retain some degree of coolness at the journey end.
Using a standard 'sealed' battery might be a good substitute, as wee eddie as already suggested. If you have an home battery charger, then plug it in on the return home. This method could also act as a replacement for your own vehicle, if needed at a later date.
FE thanks very much I was having lots of problems with the Page not fully loading and icons not showing, even with Java Script turned off.
The Link I gave is for Deep Charge Battery's Like caravan battery, they use them on Disabled Scooters, So that they last longer on a Charge. Car Battery's only give out Big amps on Start as that's what car battery's are made for