Looks like the solution is right on their website in the Questions section:still haven't bothered trying to sync it with Win 10
"Due to the programming of our Bluetooth devices, they are considered as input devices and must be added through the Control Panel though, and NOT through the normal Bluetooth settings."
Also there they have info about battery life of X1 & X3:
01/08/21
A: Battery Life:
SLIM X3 BLUETOOTH BACKLIT KEYBOARD
ST-BTSX3M (battery:1300mAh)
Level 1 - 9mA - 144h
Level 2 - 16mA - 81h
Level 3 - 27mA - 48h
Level 4 - 40mA - 32h
Level 5 - 53mA - 24h
Level 6 - 67mA - 19h
Level 7 - 82mA - 15h
Level 8 - 97mA - 13h
Level 9 - 113mA - 11h
Level 10 - 129mA - 10h
Backlit turns off if low battery - standby time is 17 days when 15% battery capacity
Without backlight - Working time is 60h.
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SLIM X1 BLUETOOTH BACKLIT KEYBOARD
ST-BTSX1M (battery:1000mAh)
Level 1 - 8mA - 125h
Level 2 - 14mA - 71h
Level 3 - 24mA - 41h
Level 4 - 35mA - 32h
Level 5 - 48mA - 24h
Level 6 - 671mA - 16h
Level 7 - 75mA - 15h
Level 8 - 90mA - 13h
Level 9 - 105mA - 9h
Level 10 - 117mA - 8h
Backlit turns off if low battery - standby time is 13 days when 15% battery capacity
Without backlight - Working time is 45h.
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Backlit decreases to L1 if no usage for 1min..Turn off if no usage for 3mins.
Enter into sleep mode if no usage of 30mins
How can it be that the batteries last longer on level 1&2 backlight than without any backlight? Looks like a mistake