After lifting off the bottom, the PCB is exposed, and you can lift it out of the charger block along with the battery contacts. On the PCB we find:

A diode to protect against reverse current.

An LED and resistor that is on so long as the box gets power.

A giant 10 Ohm resistor to slightly current-limit the charge, and across which the green led and resistor are placed -- this will therefore light up as long as significant current passes to the battery.