Did you figure this out? Just happened to me. Booked in with the dealership, only had the car 1 month from new.

 

Try to charge the 12v on a smart charger overnight while disconnected from the car, and then monthly from then on. That usually solves the problem.

However, the dealership really should fix this. Not known it on a Megane, but, very common on the Zoe.

 

They had the car for two and a half weeks ! testing this and that was all I got (checking the app you can see last vehicle activity at the top + mileage & charge, so you can to a certain extent monitor activity on the vehicle) - most of the time it was parked in the compound . So after having a telephone tantrum things started getting done. the problem is for main agent to get paid they have to get approval from Renault for each step which seems to be wait another day for e-mail ok & start next step.
So by the end of the second week all the high voltage wiring had checked out ok . During the next step of testing (don't know what) the fault cleared itself !!! they don't know why & couldn't make it re appear , looking at the app they fully charged the battery, then it showed fully discharged and all charging history was re set. So they re set something.
I now have my car back with no issues but there's a niggling thought could it happen again ? leaving me in the S**t

 

It's happened again 6 weeks later! Car took charge to 80% overnight. Came to use car "IMPOSSIBLE TO CHARGE " light on again when I got in car. Have already booked it in for head unit not updating software and "open R link" complaints have been escalated. - getting fed up now

 

It happed again first time dealer had car 2 weeks + for testing, fault cleared itself while testing !! They don't seem to know why.
Six weeks later it's happened again ! Car chared to 80% overnight came to car "Battery Charge impossible " on
Tried 2 AC chargers at home no joy, tried disconnecting 12v vehicle battery left it half an hour Tried charging 12v battery charger immediately cut out showing full charge checked with vot meter showing 13.4 volt fully charged.
Reconnected still same fault!
Now back at dealership & they say they have sorted it . ? I'll try to find out what they did.

 

Hello Lost boy. Sounds like you had a tough time so hope it really has been sorted out now.
Same 'charging impossible' fault just happened to my 2 year old Megane e-tech techno. Took to dealer who diagnosed an insulation fault at HEVC. Repaired under warranty inside 4 days but that did include a service which happened to be due.

 

This is happening also to me. Initially we changed the compressor for cooling/heating. and it started again, they changed the compressor now again (as time wise it was linked) so they thought its connected, but no. Again same issue happening. Now new idea from French guys is to change complete motor. Really nasty failure this is...

 

Took my car into Renault garage after the 'charging impossible ' was displayed on the dash. Diagnosis- 12v battery needs replacing. I haven't picked the car up yet but I'm really nervous about it happening again. Any advise welcome to stop it from happening again?