BATTERY STATE ESTIMATION DEVICE AND STORAGE BATTERY SYSTEM

After charging or discharging is stopped, a reference timing (t0) is set, and a first timing (t1) at which battery voltage (V) changes by a unit amount (N) with respect to the reference timing (t0), and a second timing (t2) at which the battery voltage changes by twice more than the unit amount with respect to the reference timing (t0) are select and set. The ratio of the time difference (t2−t1) between the first and the second timing to the time difference (t1−t0) between the reference and the first timing is found as a voltage change parameter (R) corresponding to the rate of change of the change speed of the voltage battery. An estimation device considers the change rate determined by the parameter is considered to be unchanged during an electrical current non-passage, and estimates a stable open circuit voltage at an object timing in future after the second timing.