iPhone 7 damaging new batteries

Hello again. I'm repairing iphone 7, that had problem with charging, so i replace charger port flex, and when i turn the phone on, it works, but i discover, that the home button was broken im the process, because i also replace the lcd, and i probably rip the flex for home button. So i order a new one, the phone was turned off for 3 weeks and when i got the new home button, i replaced it, but then the phone won't turn on any more. I discover that the battery was dead, and order a new one, when i replaced it, the phone turn on normally, but when the phone stayed turned of for 1 week to wait for customer to come, the battery died. With the new battery phone work but then it damage it i think. So i dont know, what is the problem. Do you think that problem is with the new charger flex or mainboard. I already spend money for 2 batteries and charger port and lcd, dont want to spend it anymore if the problem is in motherboard. Phone was dropped several times so the lcd was broken 2 times before. Thank you.