2006 Ford Fiesta - Battery acid could boil

All dash lights including battery light come on, all dials stop working and power steering stops.Engine keeps going but have found the alternator is charging at 17.5 volts when steering fails. Therefore the battery acid could boil if this continued and could go on fire. Any suggestions? We have had a new battery and new alternator and it still does this

The computer regulates the voltage from the alternator.