I have been working on a battery back up for my raspberry. The idea is classic: connect power bank to wall outlet and raspberry to power bank output. It works well with only battery power. But when the power bank is plugged to the wall at the same time it's supplying to raspberry, I get the "low power" lightning bolt signal on the right upper side of the screen, and even worse, if I unplug the power bank from the wall, my raspberry simply shuts down.... Any ideas on how to prevent this? I would like it work as a UPS where in case of a blackout, the power bank prevents the raspberry from shutting down.