Apple - Why would my iPhone 6 be stuck in a dead battery boot loop?

In such cases I, myself, do a boot into DFU mode which actually uses way less energy mainly because the display stays off. Then I wait for about a half an hour and then I reboot the iDevice in order to have it properly booted.

P.S. Follow this procedure in order to enter an iDevice into DFU mode:

1. Make sure your iDevice is plugged into a power source via it's USB cable;
2. Hold down both Home and Power buttons on the iDevice for about 10 seconds until the screen goes off;
3. Keep holding the Home button and release the Power button;
4. Open iTunes to check if the iDevice is connected. A message saying that an iDevice in a Recovery mode is connected should show up.

Its a software bug it should not boot until the battery is strong enough. Instead it boots to soon and the screen stays on keeping the battery from charging.

Fix one if it does come up to the unlock screen quickly unlock it and then off the screen it will then stop rebooting and charge up.

Fix 2 if you cant get it unlocked hold down the home screen while rebooting and leave it on the screen that says to connect to itunes. Let it charge on this screen with a original charger and cord.

Remember its the screen being on thats stopping it from charging.

This is a defect in the current OS its not reading you battery voltage and rebooting to soon. Ours only started doing this after the latest OS update.

I was able to get out of the loop by following the above steps..

1. Plug in iPhone into USB on PC
2. Hold the home and power button together until screen is black. Release the power button and only hold the home button until iTune symbol appears.
3. Now, unplug the iPhone and plug it into the wall charger, NOT the USB charger on PC. This action allowed the phone to keep charging without entering the boot loop
4. If the Apple logo reappears, then try step 2 again.
5. The phone will clear itself and power again with the login screen.
6. It worked for me in less than an hour.