Use DC voltmeter in range up to 20V. In good car voltage at battery point should not be higher 13,8V (good regler). Alternator often have 14,4V at alternator point, not on battery terminals, on battery terminals should be max 13,8V (small voltage fall happen because resistance in cables, and other things).
Water from car Lead Acid batteries start to evaporate at 14,2V. In summer on high temp high over voltage and high temp kills battery!
Voltage of alternator depends from type of alternator (type and model of car). If old car old alternator type (clasic) Alternator give 14,4V at alternator terminals and max 13,8V on battery terminals. If you turn on light and fan that voltage goes down. Smart alternator adjust voltage (new generation of car / regler) under load like that, you always have 13,8V at battery terminals.
You have problem with regler, change it !!!!