Cell means a single encased electrochemical unit (one positive and one negative electrode) which
exhibits a voltage differential across its two terminals
Battery means two or more cells or batteries which are electrically connected together and fitted
with devices necessary for use, for example, case, terminals, marking and protective devices.
Cell maximum is 20 Wh. Battery maximum is 100 Wh.
Thus battery with maximum of 5 cells of 20 Wh.
As also explained further on.
For the purposes of this guidance
document and the IATA Dangerous Goods Regulations, power banks are to be classified as batteries
and must be assigned to UN 3480, lithium ion batteries, or UN 3090, lithium metal batteries, as
applicable.
Even though a powerbank might fit: "a single encased electrochemical unit one positive and one negative electrode".
You could probably throw loads of money on this debating over the meaning of the law in court. But safety will most likely win.