power bank efficiency?

Question

Rechargeable batteries

If you buy two identical new high-quality power banks, charge one to 100%, and then use it to charge the second one, discharged to zero, by what percentage will it be charged?
In other words, what is the efficiency of power banks?

Yes, power banks are different, but in general, their efficiency should be about the same as that of an internal combustion engine or steam engines, despite their diversity.

Answer 1

Each power bank has its own efficiency. It also depends on the wire through which you will transmit energy.

Answer 2

you will be lucky if there is 40%
It all depends on the charge technology and controllers in power banks.
In a gentle usb2 (slow) mode, you can probably get 60%, 'normal' charging speed can gobble up more energy, and 'fast' is definitely more than half, I experimented with two xiaomi a2 smartphones like this, connecting them to each other using a usb-cable type-c (wrote that fast charging is in progress), about 30% of the energy of one flowed into the strength of 12% of the other, but it’s wrong to think so, the batteries could be of different 'capacity', by eye, smartphones from a charge work for the same long time
upd: https:/ /habr.com/ru/company/madrobots/blog/377145/

On average, we can assume that the efficiency of all electronics when charging a smartphone or tablet from a power bank is about 65%.

Answer 3

From 80% to 92%: https://www.iguides.ru/qa/5448/

Answer 4

With a high degree of reliability, the efficiency of each modern converter (microcircuit and its piping, one in the source bank and one in the receiver) can be taken from 85 (slag) to 95% (top).
Batteries will have the main impact, the efficiency of which, among other things, depends on the load (more current - less). Somehow - I purposefully searched for data for 18650 (what to argue with the adherents of "electric trains"), but did not succeed.
So, it is quite possible to consider the upper limit of 60-65% ~ 0.9^4.