normal lead acid voltage output is 2.2 volts x 6 cells = 13.2 volts.

normal charging voltage of the alternator is 13.2 x 110% to 120% = 14.5 to 15.8 volts. any lower than 14.5 and the battery will be hard to charge. any higher than 15.8 and the battery plates will deteriorate.

minimum charging voltage for a lead acid battery ("trickle charge" voltage) is 13.8 volts.

so kung 13.5 lang voltage ng alternator mo then mahina na nga siguro.