The Raspberry Pi 5's power-management integrated circuit (IC) incorporates a real-time clock as well as button cell charging circuitry, allowing the clock to be powered even when the main power is off.
A Panasonic ML-2020 Lithium Manganese Dioxide battery is available from Raspberry Pi. It has a double-sided adhesive pad to attach it to the inside of a case or another convenient place, and a two-pin plug that may be used to connect it to the battery connector on the Raspberry Pi 5 PCB.

Panasonic ML-2020 Power Source
includes a connector for the Pi 5 board.
Simple to replace and install
Adhesive pad has two sides for attaching it to the interior of a case