black screen and no power after battery and screen replacement

The connector for the Volume Button flex should have no impact on the device booting or not. The challenge here is that you made multiple changes at the same time and it becomes harder to understand what works and what doesn’t.

Let’s eliminate some variables by disconnecting the LCD and Digitizer. While you’re at it, and depending on the state of disassembly, I would also disconnect the FCAM, RCAM and Headphone Jack flexes from the top part of the logic board. Remember to always isolate the battery first, as shown here. This is a critical step that too many people overlook or think not important.

Essentially, you should have only the battery and the logic board connected (don’t forget to put the screw in place to secure the battery connection once your done). Now connect the iPad to an iTunes enabled computer to see if it is recognized. If it is, then you have a good logic board and you should add one flex/component at a time (remember the battery!) until you figure out what causes the issue.

If it isn’t recognized, then you may have a bad replacement battery. You could try you’re original battery to see if it makes a difference. If you have access to a USB Ammeter, you could use that to determine if the device is really drawing current when it says it's charging. If you don’t you should consider getting one as they are cheap and very useful.

If you have access to a multimeter, measure the battery voltage directly at the battery terminals. If you measure 3.5V or more, the battery can boot the device.

If the battery has proper voltage or appears to charge and the device still doesn’t boot, then you have a logic board issue and it will require some advanced troubleshooting by an experienced micro-solderer.