Abstract: A heat flux sensor device for measuring temperatures of a lithium-ion battery, comprising a flexible polymer substrate comprising a first side and a second side. A transducer is affixed to the first side. A first thermally conductive pad is thermally coupled to the transducer and affixed to the first side. A second TCP is thermally coupled to the transducer opposite to the first TCP and affixed to the first side. A plurality of conductive traces are electrically coupled to the transducer. A third TCP is affixed to the second side proximate to the second TCP. A fourth TCP is affixed to the second side opposite to the third TCP and proximate to the first TCP. The first TCP is thermally coupled to the fourth TCP via a first vertical interconnect access. The second TCP is thermally coupled to the third TCP via a second vertical interconnect access.