When will I know when I need to recharge it?

There are several ways:

• The App dashboard displays the battery % remaining. You can start calibration or therapy if you have at least 10% battery remaining (though at low battery levels, the session may halt early if there is not enough battery power to continue).
• When you pick up a device to perform a battery check, you do not see the white blinking light.
• If there is not enough battery charge remaining, you will not be able to calibrate or start a therapy session. Your battery must be at least 10% to start calibration or therapy.