Yes, there’s an RTC battery on the bottom of the motherboard. It’s likely that it is disconnected or discharged.
If you’re comfortable disassembling the device to check or replace the battery:
- remove the top cover
- remove all M.2 cards/DIMMs - be mindful of the Wi-Fi antenna cables here and when removing the mainboard
- remove the four screws on the outer corners of the board (not those near the middle, those hold the heatsink)
- gently slide the mainboard toward the rear of the device, then lift out the front edge, then remove the mainboard
There’s also an excellent teardown video here, disassembly is around the 6 minute mark, RTC battery shown around 9:30: Librem Mini Teardown