Why does my iPad keep on showing the dead red battery loop?

Are you using an original (or MFi certified) Apple charger and cable?

When troubleshooting battery/charging issues, I would do the following (in order). Some of these steps are much easier to do on a phone than a tablet but complete what you can and report back:

1. Always try another Lightning cable and charger, preferably Apple original or MFi certified units.
2. The Lightning cable 8-pin connector should be sitting flush with the housing and completely inserted. If it is not, there could be lint/dust/debris inside the port impeding a proper connection. You can clean it out with a toothpick, fine point tweezers or a dental pick. Just be careful not to damage the pins inside the Lightning Port.
3. Change the battery. The battery is the weakest link in the entire device and certainly for charger related issues. Use a battery utility, such as coconutBattery (for Mac) or 3uTools (for Windows) to test the battery prior to removal. Anything less than 70% of design capacity will require replacement(unfortunately, this will only work if you can boot the device).
4. If the battery is in good condition, you can use a USB Ammeter to determine if the phone is really drawing current when it says it's charging.
5. You may have a logic board issue. There is an IC (commonly referred to as Tristar) responsible for communicating with the charging device and uncertified chargers can damage it.