Phone wont detect replacement battery?

A week or so ago I bought a replacement battery and put it in, but it wasnt detecting it so I thought it was bad. But I bought another one a few days ago and that one didnt work either. My brother fixes phones and said hes never seen anything like it. It could be a possiblity I got 2 bad batteries in a row, but I think it has something to do with my phone. Charging it doesnt help, as it doesnt even show the percentage when you hold down the power button, just a black screen. Thanks for any help.