Power storage device and charging method thereof

A power storage device comprising:

• a detecting circuit configured to detect a capacity of a power storage unit;
• a memory configured to store a value;
• and
• a circuit configured to switch from first charging to second charging after the first charging is performed, the circuit comprising a first circuit, a second circuit, and a third circuit,
• wherein at least one of the first to third circuits comprises a transistor comprising an oxide semiconductor film in a channel formation region,
• wherein the first circuit is configured to compare, after the first charging, the capacity with the value stored in the memory,
• wherein the second circuit is configured to start the second charging, and
• wherein the third circuit is configured to terminate the second charging when a resistance of the power storage unit reaches a predetermined resistance.

A power storage device comprising:

• a detecting circuit configured to detect a capacity of a power storage unit;
• a memory configured to store a value;
• and
• a circuit configured to switch from first charging to second charging after the first charging is performed, the circuit comprising a first circuit, a second circuit, and a third circuit,
• wherein the first circuit is configured to compare, after the first charging, the capacity with the value stored in the memory,
• wherein the second circuit is configured to start the second charging,
• wherein the power storage device is configured to charge with a constant voltage charging at a first voltage after a constant current charging during the first charging,
• wherein the power storage device is configured to charge with a second voltage during the second charging,
• wherein the third circuit is configured to terminate the second charging when a resistance of the power storage unit reaches a predetermined resistance, and
• wherein at least one of the first to third circuits comprises a transistor comprising an oxide semiconductor film in a channel formation region.