Problem: iPhone does not power on after battery replacement
Solution: First connect the back cover ribbon cable and reboot the iPhone. This involves holding down the power & home buttons at the same time for 8 seconds.
Second, try resoldering the battery connections on the system board. If this does not work resolder in the old battery, does that work? Also, you can check the battery voltage across the red and white wires, then check between the red and black wires. The voltage should be around 3.7 volts. If the voltage is not in this range you probably have faulty battery and you will want to contact customer service for an RMA number.
Problem: iPhone shows low battery and will not charge
Solution: 50% of the time performing an iPhone battery replacement corrects this issue. 50% of the time it's related to the system board which needs to be replaced to fix the issue. The system board issue can have additional symtoms such as connecting and disconnecting from iTunes, over and over again.
Also, take a look at the docking station connector pins on the iPhone. These connector pins can get out of alignment from their designated slot and prevent the iPhone from charging. You can use a very fine tool to push the pins back into place.
Problem: iPhone camera shows a black screen
Solution: This is typically the camera ribbon connector to the system board. Reset this connection and test prior to putting on the back cover. If this does not correct the issue, it's probably the camera itself that will need to be replaced.
Problem: iPhone volume controls no longer work after case is reassembled
Solution: The ribbon connector from the back cover to the system board needs to be reseated. Use you a flat tool to lift the connector off system board then click it back in place. You should feel it click into place once it's seated correctly.
Problem: How can I test the replacement battery?
Solution: The replacement iPhone battery should have around 3.7 volts between the red and white wires, and then the red and black wires.