In two weeks I plan to take up my 100L bag again and walk from Tarbert to Barra, have used the references in this thread and extensively studied OS maps for finding a good route for wild camping (although I'll do in the hostels at Berneray and Howmore). I'll make a nice report as usual.
@crag scrambler: This is not strictly correct: many portable chargers have a built-in d.c. converter to change the internal battery voltage (typically 3,4V for a lithium-ion cell) to the charging voltage for the phone (typically 5V for USB chargers). During discharge, the voltage of the lithium-ion will drop, and the charger will switch off at a safe level (typically 2,2V) to prevent battery damage. The switch-off is dictated by the battery voltage, not necessarily by the remaining capacity.