Why is my iPhone completely dead?

My iPhone 5s was working completely good for 4 months but after some days when I upgraded to iOS 8, it started showing battery drain problems, it was 100% to 0%, in 6 hours time period. Also, I plugged in a cheap charger 20-30 times which I bought from Amazon and when it was in the device, the device said that it is not certified by Apple and unplug it, also the device was having current all over metal parts, like touch ID ring and back which, one can feel when touched. After that day I've did all the things one can do to solve the problem, like reset, some settings, new iOS upgrade etc. I've also shown the device to Apple service centre and they said that they're sure that battery is Ok and no need to replace it. Last month the device went completely off due to not plugging in charger and from that day, it is not waking up at all. There is nothing on screen when put on chargering for 12 hours too, even when pressing wake and touch ID button. Please help me as soon as possible to make it like it was, please, thank you for reading this huge description. :)