Hi Craig,
I think I know what you’re after, a cheap and effective UPS made by modifying off-the-shelf USB battery banks has been a bit of a search for the holy grail on this forum recently.
There was an extensive discussion on all the challenges involved in this thread very recently, which is likely why @AndrewBG @Robert93820 and @Jeff105671 are being so direct in their advice. The short version is that all the things that make battery banks cheap is also what makes them terrible UPS systems and filling that weakness is where all the expense and complexity of a UPS comes from.
