My iPad mini still won’t turn on after battery replacement

What was the original problem that led you to buy a replacement battery? Was the iPad working before you attempted the repair?

Once you give us more details, we'll be able to help just remember that this site is not staffed with iFixit personnel but rather volunteers who help out fellow DIY fixers like yourself.

Here are a few ideas:

1. If the iPad was working well before but the battery life was poor, then you either have a defective battery or something went wrong during the repair. If you have access to a multimeter, you could try to measure the voltage directly at the battery terminals. You need to have >3V for the device to boot. Leaving the battery on charge overnight may do the trick here. 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.
2. If the iPad was not working before your repair attempt and you thought a new battery would revive it, then your iPad was probably suffering from something else.

Let us know and we can help you further.