Phone completly dead, no signs of life.

Hi!

I have a problem with my iphone 6s plus. So the phone died by itself on table and it doesn’t show any signs of life. If I plug it to charger nothing happens, If I hold home and power button without usb or with usb nothing happens, same with volume down and power combination. I have plugged the phone in usb ampere meter and there is no any current until I press power button. Current jumps to 110-140mA for few seconds but It drops back to zero. Even if I don’t press any buttons sometimes current goes up for a second and right back down. I think It may be battery but I decided to ask professionals :) .

Thanks for all help in advanced.

If it matters it’s silver 64gig iphone 6s plus which was never opened or in service center.