battery not charging after 88%...

hi.. this is praneeth.. Im using iphone7. i bought this device on 22/01/2017. it was about 649$.. my phones problem is-- it is not charging after 88%. after a force restart it shows 92%, again after a force restart it shows 94% and reduces to 93%, after several restarts its hows 89%.. i ve contacted with 8-10 apple advisors, they said that i should go to a service centre, so i went there, they just heard my problem and said that by updating the software the issue wont occur, and they cofirmed that it is not a hardware problem.. i ve erased my complete icloud account and my phone, started it as a new iphone, i didnt take any backup.. its been a month im roaming here and there in Apple retail centre.. I want my device to be replaced, its under warranty period and my iphone is not responding upto the mark. so kindly help me with that issue if anybody knows......im from India..