The TP5100 chip used in the BLF LT1 lantern for charging can input/output at the same time. In fact, the lantern will be able to work without batteries if a power supply is plugged into the USB charging port.
I think the gen2 xiaomi 20,000mAh have 5V pass through but not sure if there is a switching
when input power is disconnected. Most UPS doesn’t have switching but DIY solar controller + 12V battery charger 2in1 does. I can check it out and let you know which powerbank in my collection have non-switching pass through (always on or separated buck-boost controller).
I have tested these DIY powerbanks below if they can be UPS for RPi, Routers, etc.
1. Tomo PowerBank S4 (4x 18650; Similar to M4/M2; 5V/1A input) - Can do pass-through charging but does not seem to be able to charge the battery while using the output, this maybe due to input power limited to 1A. Unplugging the input power does not cause a surge on the output USB.
2. Soshine E3S-QC (4x 18650; QC 3.0 on USB Out 2) - Can do pass-through charging, but unplugging output causes a less than a second power surge before switching to battery. While pass through charging, the USB output port 1 (5V/1A) is turned off.
3. Epilot E4 (4x 18650; 5V 2A input) - Similar to Soshine E3S-QC, but can still provide power to both USB outputs on pass-through charging.
Battery arrangement is 3S2P, but if I were to buy one I'd make windows on the cover and install more batteries (of course all soldered or spot welded).
For reference I'd choose this fellow which at least isn't using recycled laptop batteries:
It of course is better to spot weld or solder the batteries, battery contacts can cause some meaningful voltage drop at high loads and this device can drain up to 8 A from the batteries (4 A per cell).
I've been tempted to buy it sometime, but really have no need for it right now and anyway I'd prefer for it to be professionally reviewed (HKJ).
I sometimes send stuff to be reviewed to HKJ, but I'm tight on money. If someone is interested he/she can either send the device to Henrik, or make me a donation of at least ⅔ the cost for it to be shipped to Denmark. In this case (shipped to Denmark) this other seller has it slightly cheaper at ≈€12.22-12.33 total with tracked shipping: https://www.aliexpress.com/item/32990184912.html
In order to increase the lifespan of the battery most UPSes are line interactive, so they just pass power through them without having the battery connected.
When it detects power loss it connects the battery in a few milliseconds.
You can’t use a power bank to do something like this, they have no method of disconnecting their battery.