RE: 500mah to 180mah, which resistor?
The basics as stated above are true but has left out the internal impedance of the charger itself. That resistance value you get when you divide the open circuit voltage (the output voltage of the charger with nothing connected to it) minus the voltage of the battery being charged by the desired current you have the value of the total resistance required to limit the current to the value desired. Now you have to subtract the internal impedance from that calculated value of total resistance to get the value needed for the external resistor.