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

4. It has two ports

> 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)

This alone is a killer feature that’s far less common than I expected before I started searching for it. Most power banks cut off output while they’re charging, and many require physically re-plugging the device to start powering it again.

I ended up acquiring a PiJuice for this functionality alone.

https://github.com/PiSupply/PiJuice

How many hours it lasts?

I have never measured it. At worst the main power has gone down for a 2-3 hours and it lasted at least as much. I don't have a USB power meter to measure the power taken by the Rpi so I can not give you an exact number but I would guess it to be at least around 5-10 hours if not more.

Edit: Someone linked pijuice project. I went to their website and according to them, Their 10k mah version lasts 24+ hours

> Onboard 1820 mAh off the shelf Lipo / LiIon battery for ~4 to 6 hours in constant use! (with support for larger Lipo Battery of 5000 or 10,000 mAH+ to last up to 24 hrs +)

https://uk.pi-supply.com/products/pijuice-standard

So, I would say 2i would probably last about the same?

It's not - it's a hack. Powerbanks aren't design for random surge in power output which is what Raspberry Pi requires.

Usually used for charging other batteries.

That depends on the powerbank. Many powerbanks with Quick Charge 3.0 support will supply over 4A at 5V without power delivery negotiation. It's a hack, but so is the Raspberry Pi - I wouldn't trust a powerbank for anything critical, but I wouldn't trust a Raspberry Pi either.

Can you clarify why? Surely they are just normal batteries with a charging circuit? I'd have thought it was more a problem with "pass through charging" where the battery needs to discharge (RPi) and charge at the same time.

Powerbanks are designed for continous load for a trickle charge.

An RPi can suck down a continously changing power input.

> Powerbanks aren't design for

What particularly makes them unsuitable for this?

Many things 'aren't designed' for something but work just fine in that capacity..