USB POWER BANK // PORTABLE POWER SUPPLY

That is absolutely wrong.

The Amp-hour rating does not necessarily relate to the AMP-rating of the powerbank.

I have a power-bank where I can put my own cells in. I put in 6 good cells, so it is now about 20Ah (at 3.7V).  The converter for 5V is reasonably efficient and capable of 2A, so this one will power a pi just fine. But in this case the Ah rating is "20", and the current rating (AMP above) is only 2. The 'pi requires a current rating of about 0.7A. But because phones will charge anyway, people get away with claiming more than is technically possible. So you can buy 2A chargers that just tell the phone "I'm 2A", but then fail to deliver. The phone silently goes along.

mAh ratings are even worse when it comes to exagerating the numbers: most people are unable to verify the numbers.  I have 18650 cells that claim 5500 mAh, but are 600mAh in reality. Allow me to power my 'pi from that battery for a few minutes when I get to work. It'll work. (I'll connect the 4.2V from the battery to the 5V of the pi, and it will work like that. This saves a conversion step between the 3.3V-4.2V of the battery to the "official" 5V that the pi needs, and then back again to 3.3V, 2.5V, 1.8V that the pi actually needs internally....) (USB devices might not work: many need something closer to 5V to work. ).

Update:

I call my "raspberry pi2" "too". As the "officially 5V" line is below 4.65V, the power led is off. But...

too:~> uptime

12:40:32 up 6 min,  1 user,  load average: 0.00, 0.01, 0.01

too:~>

it works just fine. I was planning on taking an extra one of those batteries and make sure it was fully charged to make sure that this experiment went without a hitch.... I forgot. The battery was discharged yesterday to test a charger, but we didn't get around to fully charging the battery again. The battery is now at 3.7V, and the 'pi runs just fine. It has connectivity over the ethernet, my "HDMI->VGA" dongle will not work because it requires something closer to 5V. Similarly USB devices might not work, as they too require something closer to 5V. But for me, this construction is just fine. (I spent 5 minutes typing this: uptime is now 11min).

Update: After posting I tried running 4 processes at once. It ran for 22 minutes before the battery ran out. Note that although the battery claims 5300mAh, it is in fact only about 600mAh. And it was not fully charged. Also note that there was no  battery protection circuit anywhere: the pi continued to try to run when the battery got below 3.0V. This is not good for such Lithium Ion batteries. (but you can get away with it if you charge it immediately with the correct procedure....)

Hi Roger,

This is great to see. Good experiment! Will be interesting to see how long it lasts on a fresh battery. Quite long I expect if the battery is currently at 3.7V and it is still functioning.

Maybe several hours possibly : )

I expect about 40 hours on my 20Ah pack. (provided I don't use the 5V output of the power bank. Or if I only use that 5V for things that really need it like USB devices.

Update: the RPI ran 1:35 (one hour thirty five minutes) on the "about 600mAh shitty Chinese battery" now that I properly charged the battery before starting the experiment.....

P.S. Both the RPI and the battery seem to have survived the reverse polarity incident.... :-) :-(