A power warning on a device that is plugged in is a confusing thing to see, and it is usually the battery gauge losing its calibration or a minor firmware glitch rather than a cell that is actually flat. Three steps clear it in most cases.
Before you try them: if the battery is hot to the touch, swollen or leaking, stop using the device, do not charge it, and follow Battery and power safety instead.
Restore the firmware. Go to code.circuitmess.com and click Restore firmware.
Run one full charge cycle. Leave the device on USB for at least two hours, then use it until it powers off on its own, then charge it fully again. That re-teaches the gauge where empty and full are.
Try a different USB cable. Some cables carry power only, or not enough of it. A cable you know works for data transfer is the better test.
If the warning is still there after all three, the cell may be at the end of its life. Message us here, or email [email protected], with your order number and what you have already tried. A cell that arrived faulty is replaced free of charge. One that has worn out through use or been damaged is charged, and we quote the price before we send anything.