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.

Just because an alternator regulator has a thermistor in it does not mean it will always protect your alternator from cooking itself. This alternator was charging a large bank of flooded batteries, almost 900Ah. It got so hot that it had cooked the stator as well as melting the seal out of the front bearing. An external regulator with alternator temp protection could have saved this customer from an expensive repair.

If you want to protect an alternator, to make it through the warranty period, reducing regulation voltage can work okay, but it did not here.. 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 a poor choice.

In a marine application, the regulator in these alternators is only about one thing, protecting the alternator from a melt down. Course as we can see that was not the case for this thermistor compensated alternator. Automotive alternators are not about charging performance nor do they 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 quite horribly.

So what does the charge voltage actually look like with a thermistor protected alt charging a deep cycle house bank? It looks something like this.

This alternator was 80A rated feeding a 400Ah bank. In just 13 minutes the alternator was surpassing 225F (Fluke 289 with temp probe). At that point voltage stopped climbing and became somewhat steady. Once voltage was held steady the amperage output started to decrease slightly thus allowing some cooling of the alternator. About 18 minutes later the voltage rose to just under 13.7V but the alt temp had increased again so it continued to bounce along somewhere between 13.64V and 13.69V. I data logged this voltage for 1.5 hours and it never exceeded 13.81V. Needless to say the batteries were getting chronically under charged.

Voltage is the pressure that allows current to flow into the batteries and without a sufficient charge voltage only so much current will flow into the battery based on its state of charge and terminal voltage. By reducing the terminal voltage you also reduce how much current the battery can accept.

The mooring sailed boat I created this graph from destroyed a set of AGM batteries, using a Hitachi internally regulated alternator, in just 18 months. Ouch!!!!!!!!!

This is a premium high performance "small case" alternator and it is Denso hairpin based. For a small case alt these do quite well. When ever possible, when charging massive banks, it is best to fit a large frame alternator designed specifically for heavy duty applications. The alternator pictured here is a 165A Balmar hairpin wound alternator. Its performance to size ratio is extremely impressive but you may not need this level of performance.

Despite everything I have just written the answer may actually be; no you don't. What you need will depend largely on use, bank size and desired daily Ah use when compared to maximum desired daily engine run time.

What should I do if I have a?

Dumb Regulator W/Low Voltage Set Point - If you have an old 13.6V - 13.8V regulator, and wet cells, ditch it and get a regulator that is a minimum of 14.2V - 14.4V or even 14.6V and you will charge a lot faster. Battery gassing generally begins above 14.3 volts, though is temp dependent, so a dumb reg that does 14.5V - 14.6V will cause your batteries to need water added more often. 14.4V would be better maintenance wise but 14.6V will charge even faster and is arguably going to fend of sulfation better. If you are a coastal cruiser doing 40 engine hours per year this is a perfectly adequate option. Best case here is to convert to external regulation.

Dumb Regulator - (Non thermistor model) This can do fine provided you have minimal voltage drop in the system and it regulates to around 14.4V or so and matches the absorption set point of your batteries. While a single absorption voltage is arguably less than ideal if you are not a full time cruiser an arrangement like this can work perfectly fine. Be aware that with a large bank you can literally burn up a "dumb regulated" alternator because it has no self protective features.

Super Dumb Regulator - (temp compensated) If you tie to a dock after each sail and shore charge, you can probably keep this alternator. If you actually cruise regularly, & deeply cycle the batteries convert it to external regulation or upgrade to a higher performance alternator & regulator.

Voltage Sensing Improvements: On top of the voltage gradient issues stock alternators usually suffer horribly from voltage drop issues between the alternator and battery bank. Many stock alternators can be set up or wired to provide external voltage sensing at the battery terminals as opposed to the back of the alternator itself. Please do not try this if you don't know what you are doing. Any good alternator shop can help you with this and it can make a big difference in charging performance. The only accurate way to get accurate voltage sensing is for the negative regulator lead and the positive sense lead to be direct wired to the battery bank.

How Do I Measure The Voltage Set Point Of My Existing Regulator?

With the batteries at 100% state of charge run the engine and test the voltage output at the B+/Output terminal of your alternator with the DVM neg lead to the case or neg terminal of the alternator. The voltage should be somewhere between 14.2V & 14.4V but I would not go much above above 14.6V with a "dumb" regulator. Many regulators on Universal engines were factory set at 15.0V to try and compensate for the horrible factory wiring.

For those who have a Yanmar with a Hitachi and don't want to spend $1000.00 plus on an alternator upgrade I recently developed this 80A medium duty Yanmar/Hitachi frame alternator that is already set up for external regulation. The CMI-80-ER is also an excellent value at $349.00

For boat owners with Motroloa/Prestolite/Leece-neville 8MR case alternators I have an article HERE that shows you how to convert your own alternator to external regulation.

For external regulators I highly recommended the Balmar MC-614H or the ARS-5H. Oh and don't forget about proper voltage sensing when wiring your new system. Alternators & Voltage Sensing Why is it Important?

Summary:

In short, smart regulators and performance alternators most definitely do have a place on cruising boats and they are tremendous tools. However, their use should ideally be well justified for the expense laid out.

The installation of a smart regulator should be considered in the total system design. Their use is very often justifiable, but occasionally it is not, especially for a weekend use coastal cruiser. It can just a big expense with marginal gains for the weekend / over-nighter who is charging at a dock all week.

A small bank of inexpensive flooded batteries, for a day boater or weekend cruiser, does not absolutely require a $1000.00 + charging system when a simple dumb regulator, with the proper voltage setting, can work suitably well. A good battery monitor, properly programmed, or another way to monitor SOC performance, such as the Balmar Smart Gauge is often a better place to start than a $1000.00 plus alternator upgrade.

If you want or need expensive high acceptance batteries, or a large bank, I would strongly suggest you buy an external regulator and a high output alternator. If your batteries are located in a hot engine compartment then you really should have an external regulator with battery temp compensation. Heat kills batteries and the voltage needs to be compensated when batteries are hot.

If you are a weekend boater, with flooded cells, at a dock all week, and your alternator is already putting out 14.4 volts or so (not a thermistor alternator), stick with it and you'll probably do just fine unless your bank is huge.

Good luck & happy boating!