A battery system comprising: a chargeable and dischargeable lithium ion battery unit (1, 100); and a controller configured to estimate a polarization state of the lithium ion battery unit (1,100), wherein the controller is configured to: calculate a concentration distribution in an active material (141b, 142b) of the lithium ion battery unit (1, 100) by using a diffusion equation, and calculate a first polarization elimination time taken for the concentration distribution in the active material (141b, 142b) to fall within an allowable range when the charge and the discharge of the lithium ion battery unit (1,100) is not performed, calculate a concentration distribution in an electrolyte of the lithium ion battery unit (1,100) by using a diffusion equation, and calculate a second polarization elimination time taken for the concentration distribution in the electrolyte to fall within an allowable range when the charge and the discharge of the lithium ion battery unit (1,100) is not performed, and determine that polarization of the lithium ion battery unit (1,100) is eliminated when a time (T off) for which the charge and the discharge of the lithium ion battery unit (1, 100) is not performed is longer than a longer polarization elimination time (T pr) of the first polarization elimination time and the second polarization elimination time. a chargeable and dischargeable lithium ion battery unit (1, 100); and a controller configured to estimate a polarization state of the lithium ion battery unit (1,100), wherein the controller is configured to: calculate a concentration distribution in an active material (141b, 142b) of the lithium ion battery unit (1, 100) by using a diffusion equation, and calculate a first polarization elimination time taken for the concentration distribution in the active material (141b, 142b) to fall within an allowable range when the charge and the discharge of the lithium ion battery unit (1,100) is not performed, calculate a concentration distribution in an electrolyte of the lithium ion battery unit (1,100) by using a diffusion equation, and calculate a second polarization elimination time taken for the concentration distribution in the electrolyte to fall within an allowable range when the charge and the discharge of the lithium ion battery unit (1,100) is not performed, and determine that polarization of the lithium ion battery unit (1,100) is eliminated when a time (T off) for which the charge and the discharge of the lithium ion battery unit (1, 100) is not performed is longer than a longer polarization elimination time (T pr) of the first polarization elimination time and the second polarization elimination time. calculate a concentration distribution in an active material (141b, 142b) of the lithium ion battery unit (1, 100) by using a diffusion equation, and calculate a first polarization elimination time taken for the concentration distribution in the active material (141b, 142b) to fall within an allowable range when the charge and the discharge of the lithium ion battery unit (1,100) is not performed, calculate a concentration distribution in an electrolyte of the lithium ion battery unit (1,100) by using a diffusion equation, and calculate a second polarization elimination time taken for the concentration distribution in the electrolyte to fall within an allowable range when the charge and the discharge of the lithium ion battery unit (1,100) is not performed, and determine that polarization of the lithium ion battery unit (1,100) is eliminated when a time (T off) for which the charge and the discharge of the lithium ion battery unit (1, 100) is not performed is longer than a longer polarization elimination time (T pr) of the first polarization elimination time and the second polarization elimination time.