Googling the part number from the photo suggests it's a charger for a Canon 7.2 V Li-Ion battery pack.
Li-Ion are normally charged at a constant current until the battery reaches a set voltage and then constant voltage until current drops to a set level. If your circuit isn't drawing sufficient current (a few 10s of milliamps?), the charger may decide the charging cycle is complete and shut off. It's not very elegant, but you might be able to fool it by putting an additional load across the output.