There are universal smartphone battery chargers all over the internet now, which can do what you want. Depending on which Fairphone’s battery we are talking about you would have to have a look at the contact width of the charger fitting the contact slots of the battery (e.g. the slots on the Fairphone 3 battery are rather on the narrow side and need rather slim contacts on the charger to fit).
https://forum.fairphone.com/search?q=universal%20charger
Very obviously …