A battery unit includes: a module including m cell groups that are connected in series and each include n cells connected in parallel; m voltage detectors; (m×(n−1)) heat flow detectors; a storage; and a state estimator. The storage stores a table map of OCV vs. SOC characteristics (A1) and a table map of HF vs. SOC characteristics (A2). For each of the m cell groups, the state estimator is configured to: determine, based on the (A2), a true SOC of (n−1) cells that corresponds to a heat flow detected; determine, based on the (A1), an OCV of the (n−1) cells that corresponds to the determined true SOC; calculates an OCV of a remaining cell from the determined OCV and a detected composite voltage; and determine, based on the (A1), a true SOC of the remaining cell that corresponds to the calculated OCV.