Xiaomi India — mi 4a battery backup

Address:

Alappuzha, Kerala, 688522

I bought your mi4a handset before 7 month. And the handset does not have the battery backup at all. For example if the mobile showing 90 percentage charge 11 p. M with out making a call or using the net my phone charge will be 30% or below. When i reported this problem in my service centre alappuzha, kerala, india. Twise they checked and did soft ware updation. And they suggested me to change the simcard and get a new one i. I did a little the things as they suggested and but. Still i am having problem. What should i do now. Even your authorised service person can't invent the problem mi i am facing. Contact me immediately