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.

In my case, I am trying to power an Intel Compute Stick for hotspot router. Will try this one RPi UPS with DIY battery: 52Pi Original 18650 UPS Pro Power Supply Device Extended Two USBA Port for Raspberry Pi 4 B / 3B+ /3B