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I replaced the battery on my IPhone 6 a couple of weeks ago with a battery in your replacement kit. Now just a few days ago, my phone will shut off when the battery life gets to around 40%. Will not let me restart the phone unless I connect to a charger. Initially when I connect the charger, the screen will show a dead battery, but then shortly thereafter, will jump up to the % life it was before the random shutoff. I followed the battery calibration instructions upon initial install. Not sure if this is a bad battery out of the box. Please advise. Thanks.

It’s a defective battery. Contact iFixit for a replacement. https://help.ifixit.com/

Just a bad battery, man! Nobody knows how they’re treated when they come over from China - try another and odds are you’ll get a good one.