Liquid charging method, liquid container, and method for manufacturing the same

A method of manufacturing a liquid container (180, 220) including a container body (1, 194) and a piezoelectric device (106) having a piezoelectric element for detecting a consumption state of a liquid in the container body ( 1, 194), the piezoelectric device (106) being configured to receive an input signal to the piezoelectric element and send an output signal from the piezoelectric element via common electrodes (164, 166) for detecting a signal generated by a residual free vibration which remains in a vibration part of the piezo-electric device (106) after a drive signal is applied to the piezoelectric element to make the vibration part vibrate, the method comprising the steps of: - reducing a pressure in the container body (1, 194) to a lower pressure to atmospheric pressure; and - a liquid load inside the container body (1, 194) after the reduction step.