Xiaomi India — issue regarding battery

Address:

Meerut, Uttar Pradesh, 250004

My phone got freezed so I make it near to mi service centre, they replace motherboard and said it's ok.
I find spot's on my display, they told me it's battery is not working properly.but before my phone got freezed there were no battery issue.as per my concern the battery is non removable from board if battery is non removable then I feel the board they have changed and battery too.Now they said battery need to be replaced.My phone is still one warranty.please clarify my doubts.