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Seems the key is “121” has to be the displayed on the battery in the app
So if you are at 195km range, then switch to miles display in the car it will now show 121miles. Then tap on the 121mi battery icon in your smartphone app, and the Easter egg activates. Keeping the car displaying km and tapping on the equivalent “195km” doesn’t do it
Coincidently I was at the right range 195km/121mi while parked today so I tried it. Have not tried it at 121km
p.s. someone else posted a pic of the resulting app display, and also explanation of the various BTTF references on the screen, see post #18 here - I confirm getting the same screen for my MS
p.p.s. seems to be an Easter egg in the smartphone app only. Nothing unusual seems to happen in the car's charging screen at 121mi range