I got this on ebay, purely to take to bits. That said, it's actually quite neat and only draws about 1.5W standby power to keep the batteries charged.
It contains a switchmode 5V supply and the circuitry required to keep the batteries on what I guess is constant trickle charge. In the event of a power cut the white LED will illuminate for at least three hours. When power is resumed the battery pack will be charged again.
The video also has my take on the cheapest and possibly tackiest Chinese style emergency light circuit possible. Like a challenge to design an emergency light with the minimum amount of components possible.
But in my last-minute addition there's a design flaw. With the LED in series between the switching circuit and the cell, the voltage available to turn on the transistor would be so low that it would probably allow the LED to glow, but not light fully. A fix would be to add another cell to make a 4.8V (6V peak) pack. But that would defeat the whole "cheapest possible" thing. (But might allow the addition of a rectifier across the LED so it glowed to show the battery pack was charging, and then lit at full brightness in the event of a power failure.)