cilarith Sounds consistent with what the TP-Link document says. 2 Lithium-ion cells in series would add up to 7.2 V, and they could contain as much energy as a 5500 mAh LiPo cell.
Of course it is rather disingenuous to advertise both in the same figure, since when they are wired in series like that you are going to have the two (I assume) 2750 mAh cells discharged with half as much current draw. But the industry convention appears to be to just add up the charge of each cell, since that is how much phone battery it could top up if you completely ignored inefficiencies. Plus when comparing two power banks, you do not care about the voltages they use internally, you care about what you can get out of the battery at the end of the day, and marketing it as a 2750 mAh power bank would be underselling it compared to a 3.6 V 3000 mAh power bank.
Really, they should have just standardized on something more sensible, like Wh or mAh at 5 V, not mAh at 3.6 V. The simple "can top up your phone battery N times" metric does not work anyway because efficiency has not been factored into the mAh figure to begin with.
