Abstract: An estimation device is a device that estimates a charge state of an energy storage device including a negative electrode having a negative active material that contains Si. The estimation device includes a control unit that determines based on a charge-discharge history of the energy storage device whether or not to perform a first estimation method of estimating a charge state of the energy storage device by using a relationship between the charge state and a voltage of the energy storage device.

Abstract: An aspect of the present invention is a nonaqueous electrolyte energy storage device including: a negative electrode including a negative active material layer with a thickness expansion rate of 10% or more due to charge; and a separator, in which the absolute value (|dR/dP|) of an increase in resistance (dR) to a change in pressure (dP) in pressurization is 0.15 ?·cm2/MPa or less in the separator impregnated with a measurement electrolyte solution, the measurement electrolyte solution contains an ethylene carbonate and an ethyl methyl carbonate as a solvent, and a lithium hexafluorophosphate as an electrolyte salt, the volume ratio between the ethylene carbonate and the ethyl methyl carbonate is 30:70, and the concentration of the lithium hexafluorophosphate is 1.0 mol/L.