Standard test ever since alternators were fitted to motor vehicles is to check battery voltage with engine off. Voltage should be between 12.5 and 12.8. Then start engine and note voltage. In this case volts should be between 13.8 and 14.2. Increase load by switching on headlamps and or heat blowers. Voltage will drop and then should return to previous figure as regulator increases alternator output to cover load. If voltage does not reach previous figure, raise engine speed to 2000 RPM, if voltage still does not rise to previous figure alternator regulator is duff or brush gear is worn. Maybe both.