nominal voltage of each battery cell is 2.2 volts x 6 cells = 13.6 volts = required minimum output voltage of your alternator for the battery not to discharge.
voltage required to charge each battery cell = 2.6 volts x 6 cells = 15.6 volts required to charge your battery. this is the maximum allowable charging voltage. it can be lower but any higher and your battery will get overcharged.

so that means the voltage regulator should be set at a minimum of 13.6 volts and maximum of 15.6 volts (or lower).

di ba electronic naman na ang voltage regulator ng crosswind? if so then malamang shorted na yung IC nyan kaya wala nang regulation. you'll need to change the voltage regulator IC on the alternator.