A battery pack assembly or enclosure comprises one or more batteries having an electrochemical cell and an enclosure having at least an outer wall configured to create a sealed volume of space substantially around the batteries. An atmosphere of the volume of space comprises gas having a thermal conductivity less than 0.018 watts per meter per degree Celsius. This atmosphere of gas provides an insulative layer between the outer wall of the enclosure and the batteries. With this insulative layer, the battery pack assembly can be subjected to autoclaving without damaging the batteries. The battery pack assembly can be used to power surgical tools or other devices that are subjected to autoclaving.