Charging a 12v in an 18 volt charger will not hurt the 12v battery if it
is a modern peak charger as most good chargers are. Peak is detected by
Voltage drop of cells. RC chargers can , depending on model charge 4-
16 cells, voltage is the same it is amps that control rate of charge.
Now if you charge a 18v pack in a 12 v charger it will never charge
fully. Another way of finding peak charge is when the pack just Starts
to get warm. A warm pack means electrical energy is being converted to
heat, a bad thing to do to cells. Unfortunatly heat can be generated by
the charger itself so voltage drop method is more accurate. They will
not blow up or catch fire overcharged , they just loose alot of their
life, they cook.