This real-time-clock module uses two tiny Size 10 hearing-aid batteries in a custom resin-printed housing instead of a conventional larger coin-cell holder. That packaging choice is central to its compact form.

The PCF8523 keeps calendar and time information and communicates over I2C. Its battery switch-over circuit can maintain time when the main supply fails, while alarm and timer functions support applications that need a timed interrupt.

Product: https://www.questwiseventures.com/products/miniature-rtc-pcf8523-real-time-clock-breakout