Battery light on- dash all going out.

Hello, 15.5 to 16 volts sounds incredibly high for charging voltage. 16 is definitely beyond specifications. 18 volts is way high. Shop manual procedure has you checking circuit 112 for resistance, which can cause overcharging. The alternator or engine processor could be at fault too, but doubtful that the processor is at fault. Where is your replacement alternator from? Was the voltage that high before changing it? A Ford diagnostic tool that can monitor "PIDS" is most helpful in monitoring charging system output as read by the PCM.