Since you didn't explain why... Using a normal power bank as an UPS is a bad idea because lithium batteries have a limited amount cycles it can charge and discharge. With a power bank the power has to flow from the charger into the battery then out to the load which eats up those cycles. An UPS is a little more clever as the power goes straight to the load via a capacitor bank and a mosfet bypassing the battery. When the source power is cut the UPS runs off of the capacitors for the milliseconds it takes for the mosfet to switch the load over to the battery. This vastly extends the life of the battery and reduces the risks of fire and toxic fumes from being released.