A LM4040 5 volt reference diode used both to provide a fixed 5v for the circuit and when divided down to just 5mv, it also serves as a reference to a low power voltage comparator.

A 0.01 ohm shunt resistor is used to sample the flowing current.  When the DC current into the battery reaches about 500ma, the voltage is sufficient to toggle the comparator, which turns on the green LED using a driver transistor.  The whole circuit draws about 140ua in standby mode, so it can be left connected to the battery. If you want to turn on the LED at a lower current level, you can easily change the value of the shunt resistor.  IRC makes some small surface mounted shunt resistors of various resistances.