In accordance with one embodiment, a battery system includes an electrochemical cell, a flexible sensor assembly attached to the cell, the flexible sensor assembly including an array of spin valve magnetic field sensors exhibiting an organic magneto resistance effect, and a battery management system operably connected to the flexible array, the battery management system including a memory with program instructions stored therein, and a processor operably connected to the memory and to the array, the processor configured to execute the program instructions to identify local changes in magnetic flux using input from the array of magnetic field sensors.