Automotive Alternators vs. Deep Cycle Batteries

A Deep Cycle Battery Assassin

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.

TIP: Click on any image to make it larger.

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 circuits 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/Yanmar voltage reduction is one of the most severe I’ve measured.

Unfortunately these reductions in the regulation limit voltage occur based on alternator or ambient alternator temperature regardless of whether the alternator is too hot from being over worked or not. Take for example a small engine bay on a typical sailboat, it is not uncommon to measure engine room temps of 145°F or more.

If we assume 145°F with a typical Yanmar / Hitachi alternator, based on the thermistor temp gradient built into the regulation circuit, we can see that at 68°F it should produce 14.4V +/- 0.3V (BTW +/- 0.3V is horribly SLOPPY). However, for every degree rise above 20°C/68°F, it reduces the regulation voltage limit by -.01V per degree Celsius. An engine bay temp of 145°F correlates to 63°C. An ambient temp of 63°C is a temp increase of 43° Celsius beyond the thermistors baseline of 20°C / 68°F.

43° X .01V = -.43V

With just an engine room temp of 145°F 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 130°F engine space, but absolutely horrible for a deep cycled battery at 70°F 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 (automotive based) 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 charge large battery 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 based on rotor RPM and copper temperature. 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 alternators ambient temperature. These alternators are a poor 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. Voltage is the pressure that allows current to flow into the battery, low voltage = lower current and considerably slower charging.

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 basic inexpensive 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, they only care about making it through the warranty period.

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 an external smart-regulator set up correctly.

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 230°F, but the pre-set voltage limit still remains at 14.4V – 14.8V, or where ever you set it at for your particular 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 and the alternator is still climbing towards that voltage at the maximum safe temp it can do so. The only change in regulation voltage is that the time to get to the limit voltage is slightly extended so as not to cook the alternator.

Important: Not all external voltage regulators are created equal in regards to alternator temperature protection. Only Balmar regulators utilize an “Adaptive” alternator temp sensing algorithm. The Balmar regulators reduce the field output in small 5% increments until the sweet spot is attained. They then move back up in 5% increments to always maintain the maximum output of the alternator at all times. Other external regulators cut the field by 50% or 100%, wait for the alt to cool, and then go back to 100% output. This results in a ping-pong match between the regulator and alternator that looks like this.

100% > 0% > 100% > 0% > 100% > 0%


100% > 50% > 100% > 50% > 100% > 50%

Because Balmar regulators are always seeking the maximum safe running temp of the alternator they yield the best continuous output performance. Expensive? Perhaps, but you really do get what you pay for.

Contrast how a Balmar regulator maximizes alternator output to a Yanmar / Hitachi alternator which reduces the regulation voltage limit based on ambient alternator temp and there is no comparison in terms of charging performance. 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.

How is This So?

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 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 20°C baseline, which is 68°F (see red arrow & red box in image). If the alternators set point was 14.4v by the time the alternator gets to 200°F the limit voltage has been reduced to just 13.67V and this does not even consider any voltage drop in the system wiring between the back of the alternator and the battery bank. It is not at all uncommon to see an alternator charging large deep cycle banks at 225°F – 230°F so 200°F is actually being kind.

Proper Absorption Voltages:

The absorption stage and absorption voltage set point or limit are critically important to maintaining healthy batteries. Attaining the proper voltage and then keeping it at the desired set point, for the proper duration, means faster & healthier overall charging.  Thermistor type alternators rarely if ever reach the proper absorption voltage, on deep-cycling banks. If they can’t even get to the proper absorption voltage, due to heat, then holding these voltages for the correct duration is simply a pipe dream. The correct absorption voltage and absorption duration are one of the most important aspects of deep cycle battery charging.

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 not suitable for charging deep-cycling banks.

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.

n a marine application the regulator inside 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.


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 where even the thermistor regulation circuit could not.

If you want to protect an alternator, to make it through the warranty period, reducing regulation voltage can often 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.

New vs. Fried

In case the context of the last photo was lost, this is a new stator and a fried stator. This alt was also thermistor protected but the battery bank was simply too large, and regularly over discharged, for the alternator to adequately self protect itself. Again, in this case a properly installed external regulator with alternator temp sensor, would have saved this alternator.

Voltage Profile – Temp Protected Auto Alternator

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 and charging extremely slowly.

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. This of course is based on its state of charge, internal resistance and actual 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, expensive mistake. The owner blamed it on the AGM’s when in fact the AGM’s would have been fine had they been properly charged.

Voltage Profile – Smart Regulator

This boat has a large bank of Trojan flooded batteries and the alternator, with smart regulator, is sized at 20% of the banks Ah capacity (cold rating). With the smart regulator it took approx 75 minutes, at full output (though temp protected), of continually rising voltage during bulk, for the batteries to attain the absorption voltage limit of 14.8V. Once at 14.8V the voltage was held steady by the regulator while current began to decrease. By sizing your alternator to 20-25% of the banks capacity, using its hot rating, your can rather drastically reduce the bulk charge time.

This is what a healthy charge voltage profile looks like. The other bank, previously shown, never exceeded 13.81V in 1.5 hours yet this bank attained 14.8V at 75 minutes and then continued to hold the proper absorption voltage for these Trojan Batteries. These batteries were in their 8th year.

Do I Really Need a High Performance Alternator?

Massive Ah Banks: This is a premium high performance Balmar AT Series “small case” alternator. It’s a Denso hairpin based stator / rotor design, and for a small case alternator it is a real bad ass. Most sailboat AUX engines use small case alternators. When ever possible, when charging massive banks, it is best to try and fit a large frame alternator designed specifically for heavy duty applications. The alternator pictured here is a 165A Balmar hairpin wound alternator that can compete with many large frame units only in a small case design. Its performance to size ratio is extremely impressive, & pretty expensive.

Medium Ah Banks: Other options for higher performance small case alternators would include the Balmar 6 Series alternators or the Mark Grasser/CMI Stage One Premier. Both of these options are a bit less expensive than the Balmar AT series and are suitable for many medium sized banks.

Small Ah Banks: For a bank in the 150Ah to 350Ah range you may not need the performance of a Balmar AT, 6 Series or a MGDC Stage One Premire alternator. To fill this niche Compass Marine Inc. has developed the cost effective CMI-80-ER.

What you need for an alternator can still depend largely on your actual use. Despite everything I have just written above the answer may actually be; no you don’t need a high output alternator. What you need will depend largely on use, bank size and desired daily Ah consumption when compared to maximum desired daily engine run time. If you only day sail, and tie to the dock every night with shore power charging, converting the factory alt will gain you little. This is why I say it’s about use.

What should I do if I have a:

Dumb Regulator With 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 it 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 – (thermistor 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 – Stacked on top of the voltage gradient issues, stock alternators also suffer horribly from voltage-drop between the alternator and the battery bank. Some stock alternators can be set up or wired to provide better 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 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. This is not always possible with factory alternators. For more information on the role accurate alternator sensing makes to performance please read: Alternators & Voltage Sensing (LINK)

How do I Measure the Voltage Set Point/Limit of My Existing Regulator?

  • Make sure the batteries are at 100% state of charge
  • Make sure the alternator & engine room are cold/ambient temp
  • Turn off all DC loads & charge sources including solar
  • Start the engine and using a DVM test the voltage at the alternators B+/Output terminal & B- or case of the alternator
  • Wait for voltage to stabilize
  • Voltage should be somewhere around *14.2V & *14.4V

*Many regulators on Universal engines were factory set at 15.0V to try and compensate for the horrible factory wiring. Some Delco’s are as high as 14.8V. I do not recommend using single point voltages, with this high of a voltage set point, when charging into full batteries such after leaving a dock and shore charging.

When do I Need a High Performance Alternator & Regulator?

This is the Mark Grasser DC Solutions/CMI high performance small case alternator. These particular MGDC/CMI alternators are a great value and start at $539.00 for a 100A model (without the external regulator). The pictured alternator is a 160A model but with that type of amperage you then jump into a serpentine (multi-ribbed belt) kit.

Did I over Think My System?

In my opinion, a lot of coastal & weekend only boaters, with inexpensive automotive type G-24, 27 & 31 flooded batteries, who are tied to shore charging all week, can & do sometimes over think and over blow charging systems. There is nothing wrong with this, so long as your wallet can handle the punch.

With certain battery types though you really do need to design and install a proper charging system. The reality for me is that currently I see less than 25% of the boats out there that have AGM, GEL or AGM TPPL batteries, yet nearly 60% of the coastal/weekend cruisers have a fully gourmet 1.5K+ alternator charging systems. Many who are dockside most of the time, have no dire need for them. Money wasted? Some times yes, when based on actual use. Like anything be realistic about your usage.

When Should I Use External Regulation?

Flooded Cell Batteries – If the bank exceeds the hot rated alternator capacity by more than 80% I recommend a smart regulator & temp sensing of both the battery bank and alternator. Eg; a 400 Ah flooded bank is comfortable with about 100 amps in bulk so a 70 amp alternator, using its hot rating, would get external regulation with alternator temp sensing at the least.

TIP: I generally prefer to go one or two size larger with the alternator than is desired in output & then derate the max current output in the regulator. This allows the alternator to work less hard , run cooler & live a longer life. This can be done with Balmar regulators by using the Belt Manager feature.

AGM Batteries – I always recommend external regulation with alternator and battery temp sensing at a minimum. I have seen a number of dumb & super-dumb regulated alternators burned up by the high acceptance rates of AGM batteries. On battery temp compensation is a minimum requirement for all AGM batteries, and factory alternators don’t do this correctly.

GEL Batteries – I always use external regulation with alternator and battery temp sensing at a minimum. GEL’s require specific voltage ranges that dumb regulators just can’t accommodate very often. On battery temp compensation is a must have with GEL batteries.

TPPL AGM Batteries – Same as AGM except that some TPPL AGM batteries want to see a minimum charge current (MINIMUM not maximum) of 40% of rated Ah capacity or a 160A alternator for a 400Ah bank (hot rated output not cold). Charging at less than 40% of Ah capacity can impact the cycle life of TPPL AGM batteries. At a bare minimum you want to be hitting these batteries with 30% of Ah capacity or more in bulk charging current.

Dumb Regulator / Temp Compensated (Hitachi/Yanmar etc.) – These factory alternator regulators are horrible for deep cycling applications. Almost every Yanmar engine with a Hitachi alternator, if used regularly in a deep cycling application, should be converted to external regulation or convert to a new alternator & regulator.

Dumb Regulator / Low Voltage Set Point – If your alternator regulator is set to less than 14.2V it would be wise to invest in a better regulator.

Inaccurate Voltage Sensing – This piece of the charging puzzle can not be over looked. Many factory systems have horrendous voltage drop between the battery bank and alternator. These alternators are what is commonly referred to as self-sensed meaning they measure system voltage at the alternator BEFORE ANY VOLTAGE DROP has even occurred. This will cause the alternator to go into the absorption/CV (constant voltage)/ voltage limiting stage prematurely. External regulators correct for this by allowing positive and negative regulator wires (volt sensing) to be routed directly to the battery terminals. This can yield significant increases in charging performance. For more info on this see: Alternators & Voltage Sensing (LINK)

The CMI Yanmar Fit Externally Regulated Alternator

For those who have a Yanmar with a Hitachi factory alternator, who 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 can be paired with a Balmar ARS-5 or Balmar MC-614 and represents an excellent performance upgrade value.

For boat owners with Motorola/Prestolite/Leece-Neville 8MR case alternators I have an article HERE that shows you how to convert your own alternator to external regulation. I also now have a 105A version of this alternator, already converted to external regulation & ready to go: CMI-105-ER.

External Regulators: For external regulators I recommended the Balmar MC-614H or the ARS-5H. Don’t forget about proper voltage sensing when wiring your new system: Alternators & Voltage Sensing – Why it’s Important


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 a solid justifiable expense, but occasionally it is not, especially for the day use boater who ties to shore charging each night.

A small bank of inexpensive flooded batteries, for a day boater or weekend cruiser, does not always require a $1000.00 + charging system. A good battery monitor, properly programmed, or another way to monitor SOC performance, such as the Balmar Smartgauge should ideally be your first place to start discovering your actual usage.

If you want, need or desire expensive high acceptance batteries, or a very large Ah bank, I would strongly suggest you invest in 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.

As always please design your charging system as just that a system.

Good luck & happy boating!

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  1. Clay Sparkes May 23, 2017 at 3:00 pm - Reply

    This is such an informative article, written in such a way that it’s really easy to understand. Thanks very much.

  2. Tom lane July 23, 2017 at 1:25 pm - Reply

    My new ( 2015 ) Myanmar was ordered with serpentine belt and 120 amp alternator. It is a Valeo not Hitachi. Does it also have the thermistor?
    It consistently under charges as you describe for the Hitachis

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