So, I have this iPhone 4s, which was broken. So i replaced the screen. While I was replacing it, I accidentally broke the black wire that connects the battery to the motherboard. So I dont know if it works, and I dont want to spend money on new battery if the screen doesn't work. I also want to test the motherboard, since I replaced the screen because the old one showed blue vertical lines. So I need to be sure both screen and motherboard is functioning. Can you test it somehow without battery?
Yes. Even with the battery disconnected you will be able to test the iPhone as long as it's plugged into the wall. I repair Apple products professionally and I am positive that it is possible, having tested an iPhone 4s this way just last week.