NOVELTY - The sensor has a substrate (110) layered with a battery cell element that is selected from a group consisting of an anode (220), a cathode (230), and a porous separator between the anode and the cathode used in a battery cell (100). A thin film resistive temperature device (RTD) (120) is provided on the substrate, and changes resistance with change in temperature. A battery cell housing (310) encloses the thin film RTD. The thin film RTD comprises a negative sensor terminal to couple a contact of a thin film thermistor (130) to a negative battery terminal.    USE - Integrated temperature sensor for use in a battery (claimed) i.e. electrical battery, used in a mobile device and an electric vehicle.    ADVANTAGE - The thin film RTD on the substrate coupled to terminals on the battery cell housing allows an external control unit to determine thermal gradient across three-dimensional (3D) volume and tailor a thermal management system to keep cells at desirable thermal condition, thus providing ability to maintain a battery pack at constant thermal condition to increase safety, life of the battery pack and to allow for the cells to be repurposed or reused during battery cell recharging. The sensor built on the flexible substrate can be well aligned during roll-to-roll type manufacturing process such that additional process operations of layering the substrate with battery cell elements can be made easier in an inexpensive manner, so that a number of sensors in a battery unit can be increased, thus improving safety of the battery unit. The sensor monitors the cells with battery cell sensors, thus reducing risk of thermal condition degrading or damaging the battery unit or device powered by the battery unit.    DETAILED DESCRIPTION - The substrate is made of stainless steel, polyimide (PI), polyethylene naphthalate (PEN) and polyethylene terephthalate (PET). An INDEPENDENT CLAIM is also included for a method for fabricating an integrated battery temperature sensor.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a battery temperature sensor with a thin film thermistor and a TFT on a flexible substrate.    Battery cell (100)    Substrate (110)    Thin film RTD (120)    Thin film thermistor (130)    TFT (140)    Anode (220)    Cathode (230)    Battery cell housing (310)