I've read the same thing several times now, charging them to 15v does hold voltage better.. Assumption, feel free to correct me: If the alternator only charges the batteries when they get low, it should take longer for the battery to drop to the charging point, thus less strain on the alternator. If you have an alternator putting out 16v, it will kill a 12v battery due to the constant overcharging. If you put a 16v battery into a car with a 12v alternator, the alternator will never fully charge the battery.