On the return, Calais side, there are a pair of chargers at the Flexi lounge - that's air-side of passports, so easier to loiter until last minute (there are also chargers at terminal if not travelling Flexi)
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There is a dedicated Flexi lane, on the right. But can check in at any lane, so (if going to the terminal / Supercharger) might be easier to use any other lane if empty. Unfortunately they are ground-side of passports, so you need to leave the charger in good time for Passports and getting to the departure area. Or just charge what you need and then "get the next train". If you must catch your train (e.g. middle of the night and only running every hour) then there is a charger, outside the Chunnel complex in Coquelles, on the French side. A little bit out of the way, but better than waiting an hour for the next train ...
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The right-hand dedicated Flexi lane then has a "bypass lane" that goes straight to passports. I am as good as certain that you can make a LEFT immediately after check-in (from dedicated right-hand Flexi lane) and still get to the roundabout - its just a large area of tarmac, but as a Flexi customer the cones are encouraging you to the "quick route"
I expect if you set SatNav to Supercharger, and pay attention!, and zoom right in on the map!!, it would be clear
Ordinary mortals are supposed to go to terminal and wait until their departure is called. This is all part of an original cockup in design (typical UK issue). It used to be that they had marshals on the roundabout to force people to go to the terminal - probably because everyone wanted to go straight to departure and that overloaded the lane queuing system there if their departure wasn't called, and because the terminal concessions complained they weren't getting any punters to fleece.
Anyway, follow signs to terminal, not "France", if you want the Supercharger (Folkestone side)