Replaced 4x AA battery with 6V 3A DC adapter, but device won't work
So I pulled the cover off one of these a couple of months ago with the same plan in mind, this is our second door, and the batteries only last 3-4 months.

The system seems to run the batteries both in parallel and in series.

The 6v of batteries in series seem to run the electronics and the 3v in parallel seem to run the rfid loop, so supplying just 6v DC gives the appearance that it all works, but failed to ever pick up tags.

I still want to hard wire the SureFlap. Plan is to make a pair of 3.3v supplied with different earths.