Your regular wall charger for the phone may well run right off your ebike battery.
Hook up the AC prongs to the battery + and -, and see if it works.
More than half the little SMPS (lightweight) type AC adapters for things I've tried have worked on DC above around 50V.
I used 15vDC output 100-130VAC input wallwarts on both CrazyBike2 and an early version of SB Cruiser to run all the LED lighting on them, for example.
The heavy (transformer) types can''t, but there's not a lot of those made these days.