A battery temperature detecting device adapted particularly for a charger can monitor the variation of temperature of a charged battery and the detected is transmitted immediatedly to an internal circuit of the charger. The present detecting device is equipped with a shell, a heat sensitive sensor disposed is the shell and coupled to the internal circuit of the charger, and a fixing member which can firmly attach the detecting device to the surface of a charged battery whereby the heat sensitive sensor can pick up the variation of temperature of the battery and the same is delivered by wire to the internal circuit of the charger.