buy it from ebay fix it and after a while wont turn on

Hey everyone, here's whats happened. I bought an iPhone from eBay to repair it.I understood that the battery was dead and the charging circuit had a problem . So I bought a battery and a charging circuit from china . I replace them and it seems to work (but i haven't installed it permanently ).So i screwed it down , stick the double-sided tape and installed the other components .I turn up the phone and it booted (meanwhile i went to another room to take the charging cable ) when i came back i the screen was black. I tried to open it up - charging and nothing happened . I check the battery voltage and it is ok . I suppose that the new charging circuit has a problem, because i checked the amperage while i was charging it and they are 0A (after the black screen). Could you please give me some ideas about the cause behind the problem mentioned ?