I take apart anything in my house that breaks out of warranty and either fix it or see how it works.
Things I have taken apart or fixed are toys, electronics, glasses, printers, watches, lighters (trigger and pocket sized), computers, radios, game consoles/controllers, speaker systems, phones, ipods, weight scales, washing machines, and more, you name it. If it would have got thrown away, I would have taken it apart.
Another reason I take things apart is to change the appearance. For example, taking something apart to paint it or swap parts from two of the same thing that are different colors to make two unique, multicolored devices. An example of this would be taking apart a white i-phone and a black i-phone and making the back panel black and the screen white.
Could this possibly be a compatibility issue? Maybe you broke something really small such as a resistor on the motherboard which is causing this, make sure to backtrack and check for everything that got touched to see if you missed or broke something along the way, better to do it right the first time than have to do it all over again. Also i think i remember small metal frounding clips, make sure they are properly cleaned and properly seeated.
Try redoing the installation, taking time to make sure all the screws are of proper size and that everything fits neatly. Once you have the new battery in, it will probably have no charge to start, so you may need to let it stay on a charger for a while for a full charge, it will turn on by itself if you let it stay on the charger, it needs 5% battery at least to be able to load up. Does it give you a battery charging picture, battery needs charge picture, or did you try turning it on and get the apple logo?