Is using a 50Hz rated battery charger at 60Hz safe?
Generically, it is less stressful for a transformer designed for 50Hz to operate at 60Hz. You did not include any critical details, so it is not possible to offer any definitive answer to your question. 

You did not mention the dramatic voltage difference, so we must assume that you are connecting your battery charger to a 230V circuit rather than the common 115V branch utility circuits common in North America.