Internal resistance estimating method, and secondary battery charging device

An internal resistance estimating method, comprising:

• an internal resistance acquiring step of acquiring an internal resistance of a first secondary battery when a first condition has a first value that is different from a predetermined first standard value and when a second condition has a second value that is different from a predetermined second standard value, wherein the first and second conditions are different conditions;
• a first internal resistance estimating step of, correcting the internal resistance by estimating an intermediate internal resistance of the first secondary battery in which the first condition has the first standard value and the second condition has the second value, the intermediate internal resistance estimated using an internal resistance calculation formula obtained by regarding the internal resistance of the first secondary battery as a function in which the first condition is a variable and performing a Taylor-expansion on the function around the first standard value;
• and
• a second internal resistance estimating step of correcting the intermediate internal resistance by estimating a new internal resistance of the first secondary battery in which the first condition has the first standard value and the second condition has the second standard value, the new internal resistance estimated based on an estimated capacity of the first secondary battery and on a capacity acquired in advance for a second secondary battery, wherein the second secondary battery has same specifications as the first secondary battery and a shorter usage period than the first secondary battery.