I just found some more infos about Eco-Charge Mode here:
https://www.androidauthority.com/fairphone-3-review-1023112/
Update: I asked Fairphone about “ecocharge”, a term the team used without a full explanation regarding extending battery life. Here’s Fairphone’s response in clarifying what ecocharge actually is:
“Fairphone created a charging mechanism that charges the Fairphone 3 to 85 % in 90 mins and then completes the full charge. This mechanism will help keep the Fairphone 3 battery in service for longer – typical fast charging systems can reduce the battery capacity by up to 60-70 % after 500 cycles (approximately 18 months of usage) Fairphone 3’s charging methods will extend the life of the battery well beyond the expected capacity. A FP3 battery will still be over 90% using the same 500 cycles – 18 month timescales.”