Why my mi band showing power full but not turning on without changer

Hi,

Try replacing the battery and check if that resolves the problem.

Even though it shows that it is fully charged doesn’t mean that it can handle the load. Even rechargeable batteries have a lifespan.

Here’s a video that shows how to open a Mi Band 4 that may help. Although as can be seen in the video cosmetic damage happens to the case when trying to open it with a sharp knife so maybe it wasn’t meant to be opened.

The ifixit Démontage de la bande Xiaomi Mi guide shows the internal components better and you will need a soldering iron to disconnect/connect the battery from the circuit board