If your Buzz is very low on battery, it will flash one red light three times and vibrate in a sweep around your wrist to warn you.
When your Buzz dies and if it has some power left, pressing the power button will cause a red light to flash 3 times.
You can see the battery level when you open the Neosensory app. If you prefer not to use the app, plug in your Buzz to charge using the cable included in your box, or any USB-C cable you have. The battery level will flash briefly when you plug it in and again when you unplug your Buzz. One red light means it has very low power and needs to be charged soon; one green light means it has some power left; two lights mean it has enough power, and three lights mean it is fully charged.
It will take about 30-40 minutes for your band to charge fully. Please make sure the cable is inserted all the way in.
Depending on the settings and your environment, your band can last over 24 hours on one full charge.