Hi,
Have you tried replacing the battery at all?
It may be failing to hold its' charge.
Even rechargeable batteries have a life span.
Here is the link to the ifixit guide on how to [guide|52244|replace the motherboard] in the phone. It is useful as it shows (by Step.2 in the guide, you don't have to go further) how to remove the battery.
Replacement batteries are readily available online and are not expensive to buy. Just search for ''Xiaomi Redmi 2 battery'' to find suitable suppliers.