Finally I found the reason why I'm getting 2.61 V instead of 2.5 V, because input voltage (rising up to 15.2 V before diode D3) also impressing it's effect on voltage divider.
Also finally I got the circuit running properly. While viewing on oscilloscope I found around 120 mV difference between same ground on extreme corners of the PCB. So I simply joined these extreme away portions of ground using thick wire and Now ADC is getting proper reading and no more wrong reading. At the moment I have put board on extensive tests so that I can finalize it with few changes in the design as the suggestions showed.
Hi. Just wanted to share the end battery charger product that I made at that time after rectifying the ADC issue. It's attached herewith for the readers. Code can be improved further.
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There's a typo error in schematic that wherever +12V is mentioned, it's +15.2V actually.
Attachments
Front End Battery Charger by Azeem's High Tech Power.pdf