You can use battery powered device without the battery plugged in. I’ve never built capacitor bank for a more modern mobile device but its probably possible with off the shelf boards. The last one I built was years ago for a dentist office. It was a mp3 player they used for the hold music. Pulled the bad battery and built a fairly small capacitor bank. The only complication was getting the right resister for the third terminal. In the case of the mp3 player it was a temperature sensor. Modern phones use this channel to communicate battery info which might complicate things a bit.