If you charge one battery with another, the losses along the way are about 50% or sometimes worse. The 20,000 number is an ideal condition number and will decline over the next 300 cycles to about half. Because we can write volumes about batteries, I pause here.
I have tried two branded power banks,purported to having 20,000mah power.
With the above two devices, with full charge, I am able to charge my Samsung A8 (having 3050mah battery ) only once and that too only partially! 20% to 65%