You will get a low battery alert when the reader is running out of power. At this point you typically have about 1 days usage left so best to charge at this point which takes a couple of hours. If you run the reader and phone app concurrently then you will be able to use the app whilst the reader is charging.
It is important that if you wish to use the app and the reader together you must activate the sensor with the reader and then with your phone within 60 minutes