After the battery had 18V or more, I've ordered a new regulator and new battery. Me and my father checked again and it didnt go over 15.4V first but when the battery was recharged, it was solid around 14.7-14.9 - as it should be
it's possible the battery was part of the problem, or the wiring to the voltage regulator from the alternator contributed to the problem.
If there is resistance in the wiring such that the regulator sees voltage lower than the set point, the regulator goes provides constant maximum power to the rotor. The regulator is trying to bring the voltage up because its seeing the voltage as low.