Your iPad is boot-looping; it may be a battery/charge problem but it could also be something else. If you have access to a USB Ammeter, you could use that to determine if the ipad is really drawing current when it says or is supposed to be charging.
You're replacement battery may also be sub-par...that is not uncommon for replacement batteries, especially if you bought one on eBay.
Try these things (if you haven't already) to see if anything helps:
- Try a hard reset (Home & Power button for 10 secs)
- Use a genuine Apple Lightning connector
- Use a genuine Apple charging brick and let it charge overnight
- Try to connect it to different computers (mac/pc if possible) with an updated iTunes
- Put the iPad in recovery mode (search DFU Mode). Careful, this can reset your data
- Use 3uTools for putting it in and out of Recovery Mode
- Use 3uTools to flash the firmware (you can't downgrade)
- Check the condition of all the flexes and connectors (with magnification)
- Disconnect the LCD, Digitizer, Buttons and the camera completely from the logic board and see if iTunes will recognize the iPad as a bare bones unit and allow you to see it's contents. Follow this guide but no need to remove the antennas and whatnot, just use it for the connectors.
Otherwise, you may have a dock or logic board issue. If you are uncomfortable doing these steps, consider finding a repair shop to do the troubleshooting. A shop that also does micro-soldering can go deeper if the problem is not modular in nature.