temperatures the most insulative aerogel is silica aerogel with carbon added to it although not shown for completeness often such battery packs 100 include other devices such as electronic circuits that prevent over current over voltage under voltage control charging prevent over temperature situations during charging etc all such devices are known and present in some battery packs but have been left out for clarity reasons referring to fig3 a perspective view of a battery pack 101 with insulated battery cells 22 with thermal conductive breakers 150 will be described although the battery pack 100 with the insulating layer 130 performs well in most heat sterilization procedures some heat conducts from the contacts 12 and through the conductors 114 116 and into the battery cells 22 to reduce the conducted heat the battery pack 101 includes thermal breakers 150 that thermally disconnect the internal battery cells 22 from the terminals at a predetermined temperature the thermal breakers 150 for example include bi metallic strips 151 formed of a sandwich of two dissimilar metals that have different coefficients of expansion at room temperature e g less than 25 u00b0 c both metals are of the same length as the temperature of the bi metallic strip 151 increases for example to 40 u00b0 c one of the metals expands more than the other causing the bi metallic strip 151.