iPhone 6 shuts down after battery replacement

Hi all,

the battery in my iPhone 6 has worn off and I had it replaced. The only problem I had was that it was draining too fast, but had no issues with any sudden shutdowns or charging problems.

I got the phone back with the new battery, but it started to shut down unexpectedly at 30-40%. It goes completely black and reboots only after I plug it in. It then immediately goes back to 30-40%. I charge it for some time, use it without any problems, get down to 30-40% and bam, it goes off (so I can't even drain it completely).

same problem reported by some one else...

However, mine is replaced in authorized Apple service station...I too kback to same service station they are not give proper answer..

They says "After batery replace some cases it happens" :(

do let me know the root cause possible solution.