I have been using Mi powerbank 2i for my Rpis for last 9 months or so. I have had zero issues with it. The advantage of using 2i are,
1. It's dirt cheap than any other solution I found($15, I use 10k mah version)
2. I can plug RPi to it and I can plugin a charger in 2i. When the main power cuts off, It'll instantly switch over to battery with 0 downtime. I think this is called passthrough charging)
3. It can provide 2.5 or 3.5 amps(not sure about exact value) output
1. It's dirt cheap than any other solution I found($15, I use 10k mah version)
2. I can plug RPi to it and I can plugin a charger in 2i. When the main power cuts off, It'll instantly switch over to battery with 0 downtime. I think this is called passthrough charging)
3. It can provide 2.5 or 3.5 amps(not sure about exact value) output
4. It has two ports