Patent ID: 8334676

Claim:
A battery pack system comprising: a battery pack comprising a plurality of lithium ion secondary batteries electrically connected in series to each other, each of the lithium ion secondary batteries including: a positive electrode plate having a positive active material; a positive terminal electrically connected to the positive electrode plate; a negative electrode plate having a negative active material; and a negative terminal electrically connected to the negative electrode plate, wherein the lithium ion secondary battery has a property of being able to secure a flat charge-discharge capacity range defined as a capacity range that: accounts for 50% or more of an electric capacity range having, as an upper limit, a theoretical electric capacity that can be accumulated theoretically to the maximum in the positive active material; and corresponds to a range in which fluctuation of terminal voltage between the positive terminal and the negative terminal is 0.2 V or lower in both the cases where the battery is charged and discharged with electric current having a current value of 1 C at which the battery can be charged up to the theoretical electric capacity for one hour, charge-discharge control means for charging and discharging each of the lithium ion secondary batteries in a charge-discharge capacity range including at least a part of a maximum flat charge-discharge capacity range defined as a widest range of the flat charge-discharge capacity range of each lithium ion secondary battery; current detection means for detecting a value of current flowing in each of the lithium ion secondary batteries; electric quantity computation means for computing a charged electric quantity or a discharged electric quantity of each of the lithium ion secondary batteries by integrating the detected current value; and electric capacity estimation means for estimating an electric capacity stored in each lithium ion secondary battery in an electric capacity range including at least a part of the maximum flat charge-discharge capacity range based on the computed charged electric quantity or the computed discharged electric quantity.