A controller for a rechargeable battery and a temperature estimation method and a deterioration determination method for a rechargeable battery enabling an accurate battery temperature to be obtained through calculation. The controller calculates a heat generation amount of Joule heat generated in the rechargeable battery and a heat generation amount of chemical reaction heat generated in the rechargeable battery to calculate the battery temperature of the rechargeable battery based on the calculated Joule heat generation amount and the calculated chemical reaction heat generation amount. When a cooling device is connected to the rechargeable battery, the controller preferably calculates the battery temperature of the rechargeable battery using the Joule heat generation amount, the chemical reaction heat generation amount, and a value indicating the cooling capability of the cooling device.