Not easily or cheaply, because of the voltage mismatch. Using a buck / boost converter setup tuned specifically for the UPS might work but would be pricey and probably not that reliable.

Ah gotcha, that’s the regulated 12V output with USB PD mode, and not the voltage of the battery inside.

3s would be 11.1v and 4s is 14.8v, so I’m not sure where you got 12v from.

No they’re different voltages.

Some are ok with it, on a thread I saw on reddit awhile ago about LiFePo4, people had reported a few models that worked fine with the upgrade.

Yeah you need a UPS that’s OK with the BMS charge FETs opening when the battery is full.

Different chemistry and voltage levels, power banks use Li-ion which is 3.7V nominal per cell, and the UPS needs a 12V nominal battery which you can’t get with Li-ion cells since 3x = 11.1v, and 4x = 14.8v.

However in some UPS models you can replace the batteries with LiFePo4 replacements, as that chemistry does match the voltage and charge profile (at least close enough). If it works or not depends on the UPS and if it complains about the slight differences.

It supports those, but voltages other than 5V will only happen when an appropriate handshake happens. Just a USB-A cable to bare wires has no handshake and will be 5V.

USB-A will be 5V though.

It seems to be working, but the phone is expecting a 4.2V or 4.35V max input on the battery terminals and USB is 5V. Maybe that power bank has a lot of voltage drop or a poorly regulated output so the voltage is lower.