Hey Scoot. 13.5 is ok. The alternator is equiped with a regulator internally. Will vary slightly depending on how much electrical equipment is drawing off the battery. If it the charge rate is too high or to low the ecu will detect a fault. What is the actual problem you are having. More info needed to be accurate. I have been told by auto electrical that its not charging correctly 13.5 volts idling, and only 13.6 @ 3000 rpm, they say it should be 14.4 to 14.8 volts. everything seems to be working fine. being a taxi I have a lot more crap running (2way radio, scanner, tv, etc, so batteries only last 8 to 12 months anyway.