My iPhone 6 Reboot restart itself

Hello,

Firstly sorry for everyone who look my question because my english is bad.

My iphone keep reboot every 2 minutes and i have checked the battery to unauthorized store,They check use a box have a voltage I don't know what the box name,but it's function is can make phone battery on screen 100% condition,after they test my phone is keep rebooting.

I have been reset my phone,keep like that.

If i got to apple store,how much the cost?

And my phone isn't in my country create or assembly,Is it can service in my country apple store?

Thanks.