Go to settings> about phone>build number. Click on build number consecutively for 5-6 times, then you will enter the developer options.
Now go to accessibility>developer options>performance optimization>show fps and electric current > toggle on show fps and electric current.
Now check for charging rates while charging.
You can also download Ampere app from play store and check for charging rates.
If the charging rates are lower than 500 mA after disabling data and network then your battery might be unable to retain charge, then you may have to visit your nearest service center to get your battery replaced.