NOVELTY - The system has a plenum with an inlet in fluid communication with the fan air outlet (60), and a supply air diffuser in fluid communication with the battery pack (18). The supply air diffuser has openings to deliver a uniform air flow across the battery pack from an air flow supplied by the fan to the plenum.    USE - Battery thermal management system for managing temperature of battery pack in hybrid vehicle.    ADVANTAGE - Improves the air cooling of a multi-cell battery pack to achieve a high rate of heat rejection necessary for high power applications while maintaining a uniform cell temperature useful for maintaining and extending battery life.    DETAILED DESCRIPTION - The plenum comprises a baffle (42,73) extending from the fan air outlet and across the supply air diffuser such that the plenum is divided into multiple sections receiving equal air flux from the fan. The diffuser openings are slots of increasing size with increasing distance from the fan air outlet. The fan air outlet has a rectangular cross-section and the leading edges of the baffles are concentrated around the centre of the fan air outlet. The supply air diffuser and a discharge air diffuser are spaced from each other to form a battery compartment in which the battery pack is located. The discharge air diffuser openings are slots of increasing size with increasing distance away from the fan air outlet. The battery pack comprises a cell management board (CMB) communicative with temperature sensors, and an electronic control unit to control the operation of the fan based on temperature and current data received from the CMB. The electronic control unit has a memory having code comprising a map of a typical cell temperature distribution and instructions for comparing temperatures measured by the sensors with temperatures in the map. A warning message is generated when the measured temperatures differ by a selected threshold from the temperatures in the map. The power contactors electrically connected to the battery pack, are opened, when temperature measured by the sensors exceed a selected threshold. A heater is activated to maintain the temperature of the battery pack above the minimum battery pack temperature threshold based on the temperature signals received from the sensors. An INDEPENDENT CLAIM is included for computer readable memory storing program for battery thermal management, which comprises instructions for reading a state of charge and battery current of a battery cell in the battery pack. The battery internal resistance and heat generated or absorbed by the battery cell due to enthalpy change are determined. The heat generated by the battery cell is calculated according to a formula involving battery current, and battery internal resistance. The ambient temperature is measured and the battery heat loss at the ambient temperature is calculated. The battery heat loss is integrated at periodic intervals and the fan is set at a selected speed, when the integrated battery heat loss is greater than 0. The fan is turned OFF, when the integrated battery heat loss is less than 0.    DESCRIPTION OF DRAWING(S) - The drawing shows the bottom view of the battery thermal management system.    Battery modules (14)    Battery pack (18)    Baffles (42,73)    Fan outlet (60)    Air streams (61,63)