Thanks for all the replies folks.
Why did I ask? Technical curiosity mostly, noting my phone can be run through the mini/micro USB slot, so wondered if the Etrex could. Didn't want to try it in case of blue smoke.
AA batts widely available - yeah, except in rural France on Mondays when absolutely everything is shut. I was carrying a steadily diminishing supply of duracells on tour, and was musing about whether a phone back-up pack would do in an emergency.
I was already carrying a phone charger and camera charger, so would not chose to carry an AA charger as well.
USB chargers are becoming ubiquitous - just saw an add from Ebuyer or Savastore for a twin 13A socket face plate with 2 USB charging sockets built in. You know it makes sense
Bryn