5V UPS for Raspberry Pi
Why not to use Powerbanks
Using power banks as a cheap UPS might seems as good idea because they are widely available and more expensive ones (around 20USD) provide enough power not only for Raspberry Pi but also for tablets and other devices. Main issue with power banks (I tried Xiaomi MI 10,000 mAh) is that they contain just one DC-DC power converter so you can either charge them or discharge them. With my power bank when I disconnected power supply from power bank suddenly resetted and stopped providing power for nearly 5 seconds so my Raspberry Pi went down. Not only that. I was suspicious that when powering output power bank was not charging itself properly.
Also there is an issue with capacity. Capacity 10,000mAh means capacity of LiPol 3.7Volts battery inside power bank. After DC-DC converting you will get capacity around half - 5,000mAh for 5Volts output. If your RPi is just with all accessories is just taking out 500mA per hour this power bank will last for 10 hours maximum which is not in all scenarios (e.g. security, automation systems) enough.