If you can hear the the voice prompt say "Battery low", you will need to charge the CK11.
Insert the micro USB (smaller) end into the charging port on the CK11 and the USB end into a power source. You can either use the included charging cord or any USB to micro USB cable to charge.
The LED will be RED while it is charging and turn off when charging is complete. You are still able to use the CK11 while it is charging.