This gadget charger is located around food court area at Terminal 2, not sure if another ones also available at Terminal 1 and 3 as well. There are some different gadget brand available, so you just pick the one you need and connect it to your gadget, place it inside that tiny locker, lock it and keep the key with you till you want to remove it and board your next flight!
I would say Changi is my favorite airport and one of the best in the world.