A method for determining the charge state of a battery pack consisting of a number \u201cN\u201d of individual battery cells, includes determining the charge state of each individual battery cell SOCi for all i=1,N. The first antenna is mounted in the casing and whose position corresponds to the position of the first opening. The body includes a distal opening and a proximal opening. A surface 12A of the main-surface-side internal dielectric resin layer 12 and a surface 14A of the back-surface-side internal dielectric resin layer 14 are roughened. With respect to C, M and Y besides K, values are determined so that calorimetric values are matched.