Battery Charger output based on input Voltage
If you are talking about a cell phone charger or something like that, they are designed to maintain the output voltage for a given load and a wide range of input voltages. Most chargers don't care if they get 230V or 120V (If it is a SMPS). 
Further more the mains power supply has about 5-10% tolerance of the voltage as standard.
So the charging current will stay the same (for a well build charger.)