The alternator pictured here helped to destroy a very expensive bank of TPPL AGM batteries in under two years. These batteries need an absorption voltage of 14.7V. The highest voltage I recorded on this vessel was 13.58V at the battery terminals. At this rate it would have taken days not hours to fully charge these batteries. Both voltage drop and a "thermistor" auto type alternator were to blame.
Internal automotive type voltage regulators are one of the most often misunderstood devices out there. They are simple but they also vary quite a bit in how they regulate or limit voltage. All a voltage regulator does is limit voltage but some go a step further and try to protect the battery or protect the alternator itself.
Just because an alternator comes standard on a marine engine does not mean it is a marine duty or high performance alternator. In most every case these alternators are nothing more than a light duty automotive alternator slapped onto a marine engine.
Unfortunately, for boaters, automotive batteries are vastly different in the way they need to charge, when compared to a deeply-cycled marine battery bank. The auto battery sits in a hot engine bay, think summer time stop & go traffic, and the alternator is in there too.
The auto industry and battery makers know that heat is a prime assassin of batteries. If the charge voltage is not compensated for when the batteries are hot they can become dangerously over charged.
In order to address these issues, in a rather Band-Aid like fashion, alternator manufacturers such as Hitachi, Mitsubishi, Denso, Delco, Paris Rhone/Valeo etc., etc. began adding a simple thermistor to the voltage regulation circuit as automobile load demands went up throughout the 70's, 80's and 90's.. The internal thermistor causes the regulators target voltage to decrease, as the alternator temp increases. The Hitachi voltage reduction is one of the most severe I've measured.
Sadly these reductions in the regulation limit voltage occur based on alternator or ambient alternator temperature regardless of whether the alternator is too hot or not. Take for example a small engine bay on a typical sailboat, it is not uncommon to measure engine room temps of 145F or more.
If we assume 145F with a typical Yanmar / Hitachi alternator, based on the thermistor temp gradient built into the regulation circuit, we can see that at 68F it should produce 14.4V +/- 0.3V (BTW +/- 0.3V is horribly SLOPPY). However, for every degree rise above 20C/68F, it reduces the regulation voltage limit by -.01V per degree Celsius. An engine bay temp of 145F is 63C. This 63C is an ambient temp increase of 43 degrees Celsius beyond the thermistor baseline of 20C / 68F.
43 X .01V = -.43V
With just an engine room temp of 145F the highest regulation voltage we can attain is 13.97V. Keep in mind I have not even included for any heat generated by the alternator itself here, just the engine room temp.
This Band-Aid approach is marginally acceptable for an automotive battery, which is accepting a few amps of charge current in a 130F engine space, but absolutely horrible for a deep cycled battery at 70F accepting more current than the alternator can produce, while trying to get to a proper and healthy absorption voltage.
In an automotive application this feature serves to protect the battery, but in a marine application it serves to protect the alternator, (reducing charge voltage reduces accepted current the battery can take) but then serves to murder the batteries.
Not all marine alternators use thermistor limited regulation circuits but more and more do as the years go on. Alternators such as the Motrolola style units on early Westerbeke and Universal engines, or the Leece-Neville 8MR series have simple "dumb regulators".
Lets examine at some of my crude definitions as applied to marine (auto -type) alternators:
Dumb-Regulated - A simple dumb regulated alternator does BULK and ABSORPTION and that is it. There is no thermistor protection for over heating or battery temp compensation and they do "burn-up" quite frequently when used to drive large banks. If the voltage regulation point is 14.4V it will bring the bank to 14.4V and simply hold it there. If the regulator senses that it's below 14.4V the alternator is in BULK mode putting out all the current it can. A healthy and long absorption charge is actually good for deeply cycled marine batteries and is the part of charging that serves to reconvert lead sulfate.
Super-Dumb-Regulated - Basically the same as above but they have an additional thermistor self protection feature that reduces alternator regulation voltage based on alternator ambient temperature. These alternators are a horrible match for deeply cycled batteries because they chronically under charge them, especially with the short duration engine run times sailors prefer when out cruising. When you reduce the voltage limit of the alternator you also greatly extend charging times.
Lowering the regulation voltage limit has the net outcome of reducing current output thus allowing the alternator to now cool off a bit and hopefully not cook itself. Thermistor type alternators are just cheap automotive alternators. Reducing voltage using a temp gradient is the least expensive way to get these alternators through the warranty period and not destroy hot automotive batteries in a few months. As applied to the marine market the alternator maker could care less about your batteries or how fast you destroy them.
Certainly with 20+ hour motor runs, which are about as rare as fish with legs, getting back to 98-100% SOC is possible even with a "Super-Dumb" alternator regulator but this is many, many, many hours longer than it would take with a simple dumb-regulator or with a true smart-regulator.
What about external regulation?
Smart Regulation - Smart-regulators handle the alternator a bit differently, in terms of protecting the alternator or reducing terminal voltage to temp compensate the battery. Balmar and some other smart regulators, reduce the field voltage, which in turn reduces alternator current output. They can do this based on a high-limit for alternator temperature while still keeping the voltage limit set point exactly the same. The smart regulator does not begin reducing field voltage UNTIL the alternator hits the high-limit.
For example, the current output of a 100A alternator may be reduced to just 70A, to keep it below 230F, but the pre-set voltage limit still remains at 14.4V, or where ever you set it at for your batteries. By temp protecting the alternator it does take slightly longer to get to the designed target voltage, but the target voltage has not changed or been reduced based on ambient engine room temps. Only the time to get to the limit voltage is slightly extended.
Contrast this to a Yanmar / Hitachi alternator which reduces the regulation voltage limit based on ambient alternator temp. With smart-regulators the voltage limit is not reduced just current output via the voltage applied to the brushes. Reducing current output slows charging a bit but nowhere as much as reducing the limit voltage based on ambient alternator or engine room temp. While reducing the current output does extend bulk charging time you still get a healthy absorption stage and you still get to the target voltage significantly faster. This type of regulation is more expensive and thus why cheap automotive type alternators don't use it.