Passive cooler bags are made of insulating materials that keep the internal temperature low by preventing heat loss. For this purpose, you need to establish a low temperature inside. This is done by placing cooling packs on the bottom or in specially designed compartments. Cool packs are left overnight in the freezer before placing them in the cooler bag. When using an electric (active) cooler bag you don't need cooling packs, but in case you plan to use it without being plugged into the mains (230V) or the car battery (12V) it's good to have one charged, to keep the interior cool until you have power again.