Hi @vsk
It appears as though the battery is failing to hold a charge and it needs to be replaced. Even rechargeable batteries have a lifespan.
Here’s a link to the ifixit [guide|87414] guide which may be of some help.
Here’s a link to just one [https://www.ebay.com/itm/Microsoft-Band-2-battery-replacement-/273442140643|supplier] of a replacement battery. It is shown only to give an idea of the cost of the battery. There may be other suppliers online (not too many though) that may suit you better. Just search for ''Microsoft Band 2 replacement battery'' for results.