Andrew Gabriel presented the following explanation :
However, some phones detect the difference between about 5V
and 5.5V in order to detect if they're plugged into a real
USB port (5V) in which case they limit their current draw
to low levels, or mains adaptor (5.5V) in which case they'll
do things like fast charging at much higher current draw.

Certain (maybe all) Nokia phones can be used with a Nokia sourced
charger which come in two versions - the standard one and the fast
charger. There is no marked difference in the output voltage on the
chargers, so I did wonder how the phone might detect which type of
charger it was plugged into. I guess the voltages must be different
despite the markings.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk