Volume mike complaint

Good day,
I am from iritty, Kannur and i bought an Asus Zenfone Max Pro M1 on 21st Feb 2019 through Flipkart... Phone was working fine untill last 10 days...Now I am facing a critical problem with its battery... Battery is fine from 100 - 50 percentage of it's use...after that it is dropping quickly to 20 or even less... Please advise ...
Awaiting for your prompt reply...