I listen to Ak240 a lot during long flight, what I did is carry along a micro USB power bank . My tracks are all high res flacs with only three albums of DSD. The good news is with me falling asleep with the player on, they almost always last just enough for my sleeping time (13 hours or so of flight time and round 6 to 8 hours of sleeping and listening). Occasionally, it runs out of juice and I just plug the power bank in.
So on the whole I find the battery practically sufficient but still wishes it lasts longer.