Xiaomi India — Battery damaged

Address:	Surat, Gujarat, 395007
Website:	AMAZON

Dear sir,
I have purchased Mi note 3 in may 2016, the auto stop charging after battery full is faulty and it damaged battery by overcharging, i go to service center and they says this is physical damage and out of guaranty and I have to pay INR 800/- for battery replacement,
Now i want to know that this is a non removable battery and damaged
by a faulty charging system, how can they says it that this is physical damaged even i not opens it till date, what is my fault,
It is purely charging system fault and company must replace it without any charge because it is in guaranty period.
Thanks & Regard
Mahesh Agrwal