Whether or not the charger will charge 12 cells may not depend on wattage or input voltage at all. It depends on the specific charger like Dave pointed out.

If the wattage is low then it will simply slow the charge down on some chargers. If the voltage is low then it will simply not charge on some chargers and others will not care.

It is best to see your specific chargers max number of cells, if it happens to be 12-15 then you're good to go even with a 11-12 volt input. You still have to have the same or more power on input than on the output like 360 said.