Ultrasonic welding method using biasing members

An ultrasonic welding method comprising welding a bus bar to an external terminal of a power storage cell by applying ultrasonic vibration to the external terminal and the bus bar, wherein

• in the welding, the external terminal is sandwiched by a first holding member and a second holding member from opposite sides in a vibration direction of the ultrasonic vibration applied to the external terminal and the bus bar, the first holding member biased toward the external terminal in a first direction by a first biasing member, and the second holding member biased toward the external terminal in a second direction opposite the first direction by a second biasing member,
• in the welding, the first biasing member and the second biasing member bias the first holding member and the second holding member toward the external terminal such that the external terminal and the bus bar are vibrated at an amplitude smaller than an amplitude of the ultrasonic vibration,
• in the welding, the first biasing member and the second biasing member bias the first holding member and the second holding member toward the external terminal such that the external terminal and the bus bar are vibrated with a contact surface pressure that is 10 N/mm 2 or more applied from the first holding member and the second holding member to the external terminal and with an amplitude of 60% or less of the amplitude of the ultrasonic vibration.