I have a scale to weigh myself, I've had it for at least 30 years. The display is liquid crystal display (LCD).  This scale has no user-accessible battery, yet it reliably displays my weight every time I step on it.  I also have a MFA authentication card, recently acquired from a software vendor which requires MFA for login (TOTP), it is the exact size of every credit card I've ever had, so there is no apparent battery.  Yet the LCD display on the card displays every time I press the "button". How is it that these LCD displays can work with no apparent source of electric power?