This image is a typical spec sheet for a Yanmar/Hitachi alt:


The regulation voltage is specified at +/- 0.3V which, in and of itself is pretty pathetic, but this is what you get from a $6.25 regulator. If the design voltage is for 14.4V then this means you could possibly get one at 14.1V or one at 14.7V. Kind of a luck of the draw kind of specification. D'oh..


Of course the regulation voltage limit only tells part of the story. You then need to subtract a voltage gradient caused by the thermistor, of -0.01V for every degree rise in Celsius above a 20C baseline, which is 68F (see red arrow & red box in image). If the alternators set point was 14.4v by the time the alternator gets to 200F the limit voltage has been reduced to just 13.67V and this does not even consider any voltage drop in the system wiring. It is not at all uncommon to see an alternator charging large deep cycle banks at 225F - 230F so 200F is being kind.



Proper Absorption Voltages:


The absorption stage and absorption voltage is critically important to maintiaining healthy batteries. Attaining the proper voltage and then keeping it at desired set point for the proper duration means faster & healthier overall charging.. Thermistor type alternators rarely if ever reach the proper absorption voltage, on cycling banks, and attaining & then holding these voltages is absolutely critical for battery health.



The reason the Yanmar/Hitachi alternators charge so slowly, and you rarely see anywhere close to rated output, except for a few minutes after a cold start, is because the regulator in them is actually dumber than dumb, at least from a deep cycle battery charging perspective.


If you want to protect an alternator, to make it through the warranty period, reducing regulation voltage can work okay. If you actually want to charge batteries, used in deep cycling applications, and do so in a healthy manner, alternators with internal thermistor regulation are really quite pathetic.


In a marine application the regulator in them is only about one thing, protecting the alternator from melt downs. It's not about charging performance nor does it care about your batteries. Slap it in a car, where the batteries are always full, and you're good to go. Put it in a boat with deeply cycled banks and they really perform horribly.