Stepping down 12 to 6 volts can be done with a linear regulator or even resistor divider, using a 20ohm resistor which would dissipate 1.8W to get down to 6V at 300mA. However the open circuit voltage (when the charger is not on) would still be 12V and that voltage may not be handled by the charger. I would suggest you get the AC Adapter I specified. Use the pictures and it will work.