stock yanmar (valeo) 125amp alternator charging lifepo4

[SOLAR SUPPORT]

It is a bottleneck with the resistance created by using thin cables for the alternator connection of the LFP service batteries, but; How safe this is is debatable. It is not possible to keep the voltage balanced with the resistive and heated energy cable between the alternator and LFP service batteries. The output voltage of the alternator becomes very high and this is not a good thing for the LA battery connected in parallel to the Battery Isolator. When charging LFP batteries with an alternator, you should use a battery isolator and at least one LA battery connected to it. The battery isolator and LA battery, which are constantly connected to your alternator, work as the fuse of the Alternator-LFP battery charging infrastructure. Lifepo4 batteries, which can be charged continuously with high current, cause overheating of alternators designed according to the charging current needs of batteries with LA chemistry.

Valeo125A and Mitsubishi115A fabricated alternators with internal regulators and thermal protection circuits initially produce a current equal to the label current when they are still cold. When the stator windings of the alternator heat up to around 130C, they gradually continue to operate continuously at 70% capacity. In environments where the air temperature is 25C° and above, if your engine room is closed and has sound-heat insulation, calculate that these alternators, operating continuously at 70% capacity, discharge around 800-1000 Watts of heat into their environment. When this heat energy is combined with the heat energy emitted by the engine running under load, some problems may occur. Therefore, you should cool the alternator and ventilate the engine compartment abundantly. The simplest solution to avoid such problems is to use a dc-dc charger, that is, a current limiter. If 30A is insufficient, a second dcdc can be added. You can place the newly produced high capacity and high efficiency DCDC chargers outside the engine room.

Don't want to deal with DcDc? To control the rotor excitation of fabricated alternators, you can work with an experienced alternator service technician to take the rotor excitation circuit outside the alternator and add a 90-100C° NC thermostat to this circuit. In this way, you protect the alternator and engine compartment from overheating. Thanks to this circuit, you can turn the alternator on and off at any time with a simple button. Even if you go this route, use a fan that ventilates the engine compartment well. Using a smart external regulator that works with the alternator is also an option. You should research which type of external regulator your alternator is compatible with.

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[CaptainRivet]

Originally Posted by SOLAR SUPPORT

It is a bottleneck with the resistance created by using thin cables for the alternator connection of the LFP service batteries, but; How safe this is is debatable. It is not possible to keep the voltage balanced with the resistive and heated energy cable between the alternator and LFP service batteries. The output voltage of the alternator becomes very high and this is not a good thing for the LA battery connected in parallel to the Battery Isolator. When charging LFP batteries with an alternator, you should use a battery isolator and at least one LA battery connected to it. The battery isolator and LA battery, which are constantly connected to your alternator, work as the fuse of the Alternator-LFP battery charging infrastructure. Lifepo4 batteries, which can be charged continuously with high current, cause overheating of alternators designed according to the charging current needs of batteries with LA chemistry.

Valeo125A and Mitsubishi115A fabricated alternators with internal regulators and thermal protection circuits initially produce a current equal to the label current when they are still cold. When the stator windings of the alternator heat up to around 130C, they gradually continue to operate continuously at 70% capacity. In environments where the air temperature is 25C° and above, if your engine room is closed and has sound-heat insulation, calculate that these alternators, operating continuously at 70% capacity, discharge around 800-1000 Watts of heat into their environment. When this heat energy is combined with the heat energy emitted by the engine running under load, some problems may occur. Therefore, you should cool the alternator and ventilate the engine compartment abundantly. The simplest solution to avoid such problems is to use a dc-dc charger, that is, a current limiter. If 30A is insufficient, a second dcdc can be added. You can place the newly produced high capacity and high efficiency DCDC chargers outside the engine room.

Don't want to deal with DcDc? To control the rotor excitation of fabricated alternators, you can work with an experienced alternator service technician to take the rotor excitation circuit outside the alternator and add a 90-100C° NC thermostat to this circuit. In this way, you protect the alternator and engine compartment from overheating. Thanks to this circuit, you can turn the alternator on and off at any time with a simple button. Even if you go this route, use a fan that ventilates the engine compartment well. Using a smart external regulator that works with the alternator is also an option. You should research which type of external regulator your alternator is compatible with.

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Using a too thin cable to create resistance is a big no go and dangerous, stay away. Yes that unintentionally saved a lot of alternator with LFP drop ins from death (mostly the ones that claim to exchange lead to LFP without addapting anything works) but should NOT made on purpose.

Much easier, cheaper and more effective then DC2DC charger with alternator with internal temp management like the Valeo125A and Mitsubishi115A is getting the Nordkyn VSR200 module which without modifing the alternator makes a fully regulated alternator with keeping the much better internal temp management that measues at the critical rotor temp instead the external case temp measures when modifing the alternator. 2nd if you have volvo you keep D+ which is not possible with external regulation and you need to hack the d+ additionally.
Yes you keep the regulator inside the alternator this way which produces some heat which cost you max 10A but thats worth it for a painless installation.
I saw up to 90A constant from a 115A Mitsubishi with the Nordkyn VSR200 regulator, most installation its at 80A if not too bad ventilated cramped engine room...the Nordkyn costs equivalent to 1 30A DC2DC but really protects the alternator as it also optimize the internal temp management and simply regulates it down when too hot.
Thats by the way also happening when you connecting it in parallel to an argofet but much more regulation down on the 115A Mitsubishi as its too conservative...the Valeo 125A i don't know but expect the same, means you don't need a DC2DC but when using without Nordkyn VSR200 you need a disconnect relay that the BMS or a BMV712 can cut when LFP battery is nearly full at 13.8-14V as the regulator is a lead one which gets to high for the LFP. And the high output is no problem for the FLA connected as the LFP takes the current and if relay cuts it the alternator sees only the argofet with lead and regulates down and these seconds with high current that the FLA can withstand.
DC2DC here with these alternator is wasted money, better get the VSR200 from Nordkyn, want more charge add a 2nd identical alternator (with Mitsubishi i highly recommend get a refurbished original one as the aftermarket ones are lacking in 95% the internal temp management you need, again Valeo125A i don't know in that detail but expect the same as this is how you save money in production) and connect it to the Nordkyn too as it can steer 2 alternators, even 3 if you want. Not individually !!! so the hotter of the 2 regulates the other one down too but thats normally marginal and for 5 A more a 2nd VSR200 makes economically no sense. Thats the cheapest way i know to get regulated real 160A charge, a refurbished 115A Mitsubishi is around 250Euro+2nd alternator kit and you have redudancy plus no issues with D+ if having a newer volvo with mdi box.

[donradcliffe]

Originally Posted by SOLAR SUPPORT

It is a bottleneck with the resistance created by using thin cables for the alternator connection of the LFP service batteries, but; How safe this is is debatable. It is not possible to keep the voltage balanced with the resistive and heated energy cable between the alternator and LFP service batteries. The output voltage of the alternator becomes very high and this is not a good thing for the LA battery connected in parallel to the Battery Isolator. When charging LFP batteries with an alternator, you should use a battery isolator and at least one LA battery connected to it. The battery isolator and LA battery, which are constantly connected to your alternator, work as the fuse of the Alternator-LFP battery charging infrastructure. Lifepo4 batteries, which can be charged continuously with high current, cause overheating of alternators designed according to the charging current needs of batteries with LA chemistry.

Valeo125A and Mitsubishi115A fabricated alternators with internal regulators and thermal protection circuits initially produce a current equal to the label current when they are still cold. When the stator windings of the alternator heat up to around 130C, they gradually continue to operate continuously at 70% capacity. In environments where the air temperature is 25C° and above, if your engine room is closed and has sound-heat insulation, calculate that these alternators, operating continuously at 70% capacity, discharge around 800-1000 Watts of heat into their environment. When this heat energy is combined with the heat energy emitted by the engine running under load, some problems may occur. Therefore, you should cool the alternator and ventilate the engine compartment abundantly. The simplest solution to avoid such problems is to use a dc-dc charger, that is, a current limiter. If 30A is insufficient, a second dcdc can be added. You can place the newly produced high capacity and high efficiency DCDC chargers outside the engine room.

Don't want to deal with DcDc? To control the rotor excitation of fabricated alternators, you can work with an experienced alternator service technician to take the rotor excitation circuit outside the alternator and add a 90-100C° NC thermostat to this circuit. In this way, you protect the alternator and engine compartment from overheating. Thanks to this circuit, you can turn the alternator on and off at any time with a simple button. Even if you go this route, use a fan that ventilates the engine compartment well. Using a smart external regulator that works with the alternator is also an option. You should research which type of external regulator your alternator is compatible with.

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First you say that the alternator output voltage can get very high, then you say it is an internally regulated alternator, which holds the alternator output voltage to some setpoint (typically 14 volts) which has been determined not to to damage LA batteries. Did your regulator break?

[CaptainRivet]

Originally Posted by donradcliffe

First you say that the alternator output voltage can get very high, then you say it is an internally regulated alternator, which holds the alternator output voltage to some setpoint (typically 14 volts) which has been determined not to to damage LA batteries. Did your regulator break?

He is mixing things up:
115A Mitsubishi and 125A Valeo are internally temperature regulated alternator with a charge regulator setup for AGM thats regulated via a sense wire and has avalanche diodes who can withstand a disconnect under full load (not to be used as this but if sh... happens it will 99.9% survive)

You can charge LFP directly with both but you need to take precautions:
1) you need to discoonect it at 13.8-14V so you are not overcharging the LFP as the AGM will go to 14.8-15.2V but already 14.4V float charging over longer time eg motorsailing would overcharge the LFP and trick out the simple built in BMS in drop ins. Also alternator is the wrong charge source to absorb the LFP, so take out off earlier before it can cause issues.
2) that you can disconnect it you need a surge protection. easiest way is to simply us an argofet and connect a disconnect relay and then LFP to one output and a lead starter to the other output. The BMS cuts then disconnect relay if 13.8-14V is reached and then the alternator sees only the lead and reduced output and will in short time absorb and float the lead.
Cheap solution and very reliable solution as you get full alternator output when LFP is connected and nothing can get happen if alternator gets disconnected.

If you wanna make it externally regulated you loose the internal temp regulation and maybebgain 5 till absolute max. 10A, not worth it.
Won't get more out, both can spin up to 16000RPM so regear to 1:3,3 and the Mitsubishi 115A deliveras 110A constant at 2000RPM engine and 7000RPM alternator.

I reorganized Mitsubishi worldwide as overall responsible but was also worldwide aftersales director for cars and trucks so i done all there in detail till last part number. after Daimler bought it and i have all internal documents about the 115A alternator which is a high reving heavy duty alternator for commercial trucks eg cooling in a small case. Its constructed to deliver 2/3 of its rating on its self cooling capabilities as eg the cooling trucks stand a long time with engine in idle without cooling air from driving but the compressors of cooling run and need power whichnthe alternator need to support. It can deliver its full rating at 9000RPM. Due to crank pulley size limitations on the 3 and small 4 cylinder marine engines its geared 1:2 but its optimal is 1:3,5.
The valeo is basically the copy from.valeo from it with different rectifiers

[SOLAR SUPPORT]

Originally Posted by donradcliffe

First you say that the alternator output voltage can get very high, then you say it is an internally regulated alternator, which holds the alternator output voltage to some setpoint (typically 14 volts) which has been determined not to to damage LA batteries. Did your regulator break?

Sorry, I wrote the first thing that came to my mind. There are two possible consequences of using a thin current-limiting cable between the alternator and the LFP service battery. These possibilities arise when you isolate the LA and LFP battery banks using a battery isolator to make LFP charging safe. If you take the regulator sense cable (RSC), which detects battery voltage, from the LA side, you cannot increase the LFP service battery bank to 14.4 volts. If you take the RSC voltage from the LFP battery bank, you will expose the LA battery to a charging voltage above 14.4, which will shorten the life of the LA battery. The reason for these imbalances is the cable you used between the alternator and the LFP service battery. Since this cable limits the current to be transmitted when the alternator is operating at full load, it causes a voltage drop proportional to the resistance it creates. Because the power cable you have used is not of the correct diameter for the current level that needs to be transmitted, it creates a current-limiting, voltage-reducing and heat-emitting resistance.

[CaptainRivet]

I cannot even believe reading this...big No Go to use an inappropriate and under dimensioned cable. Its not limiting the current, it just "burns off" current into loss and fools the lead regulator by introducing more resitance into another charge phase which then eg limits the current but you have no proper control over that.
That together with corroded contacts is just the major reason a lot standard alternator didn't destroy themselves in a short periode after installing LFP drop ins. But its not something you should replicate...
Install it like i described with argofet/cut off relay and after the LFP is cut off the lead regulator can do properly its job and charge/float the lead starter correctly.

[SOLAR SUPPORT]

Originally Posted by CaptainRivet

I cannot even believe reading this...big No Go to use an inappropriate and under dimensioned cable. Its not limiting the current, it just "burns off" current into loss and fools the lead regulator by introducing more resitance into another charge phase which then eg limits the current but you have no proper control over that.
That together with corroded contacts is just the major reason a lot standard alternator didn't destroy themselves in a short periode after installing LFP drop ins. But its not something you should replicate...
Install it like i described with argofet/cut off relay and after the LFP is cut off the lead regulator can do properly its job and charge/float the lead starter correctly.

To charge LFP batteries with an alternator, a power transmission cable of appropriate thickness must be used; I would not recommend the opposite. I think you're suggesting disabling the charging energy transmission cable between the LFP service batteries and the battery isolator using a large voltage sensitive relay. The relay you mentioned must turn off the charging before the Hvcutoff action of the BMS. Meanwhile, since the LA engine battery is connected to the same battery isolator, you think that the alternator and other electronics will not be affected by the overvoltage dump load. Meanwhile, you think that the high voltage dump load occurring in the alternator stator windings will be absorbed by the LA battery. It may be possible to measure this voltage peak level, which rises for a very short time. I'd like to think you installed this setup and made sure it didn't cause any problems.

The VS relay you use must be proportional to the alternator capacity and must be very expensive. In fact, the current in the rotor exciter circuit of the alternator is much lower, between 3~8 Amps, and it is possible to control it with a small and inexpensive voltage sensitive relay. When you cut this circuit, no high voltage wave occurs in the stator windings of the alternator. This is a very reliable on-off method for the alternator. By adding an NC 90/100 C° thermostat to the same circuit, you can also protect the alternator against overheating.

[CaptainRivet]

Originally Posted by SOLAR SUPPORT

To charge LFP batteries with an alternator, a power transmission cable of appropriate thickness must be used; I would not recommend the opposite. I think you're suggesting disabling the charging energy transmission cable between the LFP service batteries and the battery isolator using a large voltage sensitive relay. The relay you mentioned must turn off the charging before the Hvcutoff action of the BMS. Meanwhile, since the LA engine battery is connected to the same battery isolator, you think that the alternator and other electronics will not be affected by the overvoltage dump load. Meanwhile, you think that the high voltage dump load occurring in the alternator stator windings will be absorbed by the LA battery. It may be possible to measure this voltage peak level, which rises for a very short time. I'd like to think you installed this setup and made sure it didn't cause any problems.

The VS relay you use must be proportional to the alternator capacity and must be very expensive. In fact, the current in the rotor exciter circuit of the alternator is much lower, between 3~8 Amps, and it is possible to control it with a small and inexpensive voltage sensitive relay. When you cut this circuit, no high voltage wave occurs in the stator windings of the alternator. This is a very reliable on-off method for the alternator. By adding an NC 90/100 C° thermostat to the same circuit, you can also protect the alternator against overheating.

It just hurts....the 115A Mitzi and 125A already have an internal temperature protection that works well. You use the alternator as is with no modifications at all (which is important with the newer volvo engine with MDI box and D+!!!, modify your alternator and your volvo engine won't start anymore!!). I installed it a dozen times like that, serval others installed it according like this to my advice ...i don't assume, i know it works.

Argofet is a splitting diode based on mosfet so the voltage drop is minimal and no voltage sensing relay, nowhere used in this setup.

An appropriate sized cable(i would use 50sqmm) for engine rooms (110 degrees celcius isolation rating, oil and acid resistant isolation) needs to be rooted from 115A or 125AH alternator to where you place the argofet (connected to output). One output goes to your lead starter and the other output to the input of a relay (eg a Battery protect 220A, the 100A is a bit tight but also a simple mechanical relay can be used if your BMS can steer that) and the output the LFP you charge. The cutoff relay is steered by the BMS to cut off the LFP at 13.8V. It avoids to overcharge the LFP as the alternator can go under some circumstances in lead float mode (its a lead regulator!!) from 13.8V on (see Eric Bertschers article of overcharging a LFP by floating) and your BMS won't notice that and won't cut off (my electrodacus BMS will as its in control over the charge source) . Its not to protect from a BMS sudden shut off as you don't care as lead is always there as surge protector. Alternator can shut off any time you want or need in this setup.

In my case i have a BMV712 as last resort cut off too which also steers the alternator cut off relay in a last resort action (in series connected to remote/steering terminals of relay). If your BMS doesn't have the possibility to steer an external relay based on SoC or a voltage threshold then you can use the BMV 712 relay output to cut off the alternator at 13.8V.
The argofet isolates the lead from LFP and the lead works as surge protector when the LFP is cut off by the relay
Again nowwhere a ******** voltage sensing relay which should be only used with lead batteries only.

Also one additional hint: Normally that cable is already there conneting the aternator but also the starter with the same cable. You need to split that and root a cable for the starter directly your lead starter (i used the original installed cable). Then ypu route a second pair of cable from alternator to argofet. it must be rated for continious 110A on the length you are using.
On my FP Lavezzi the original cable used was a 20sqmm cable without fuse for a 5m run, i swapped it
A) for a 50sqmm one alternator to argofet with a 125A NH fuse.
B) for a 50sqmm starter one and connected a 150A breaker for windlass (not a cable fuse rating!! type due to surge of motors) as i have a 1500W starter@12V. This because its directly connected to LFP bank as i use a hybrid LFP starter and house battery and it needs to disconnect in a defect of the starter or the cable.
This is not controlled by BMS as i wanna decide myself if i discharge the LFP too low before i end up on the rocks...in a desaster event the main lfp bank fuse will blow eg an internal short.

[SOLAR SUPPORT]

On my own boat I have a 115A Mitz alternator that came with the new VP d1-30 engine. I didn't change my alternator. I took the rotor excitation circuit rotating inside the stator out of the alternator. My original alternator still works with its own internal regulator. In the Mediterranean, we experience temperatures around 40C° during the summer months. The alternator, which works at full capacity with the engine in the sound and heat insulated engine room of the boat, really heats up the environment.

That's why I added a thermostat on the alternator. Mitzi115A alternator produces around 80A current when its thermal protection is activated in the engine room, which is cooled by air taken from the outside environment. When the hottest outer part of the alternator reaches 100°C, the thermostat-rotor warning circuit opens and the alternator stops working and runs idle until its own temperature drops below 90°C.

My original battery isolator is not argofet and the internal regulator of my Mitzi alternator works with the voltage sense cable. Even if I had an Argofet insulator, it would still be necessary to use the voltage sense cable, because the internal regulator controls the alternator output voltage according to this voltage.


I did not use a standard type voltage sensitive relay used as a battery combiner. I used a small relay that is quite cheap ($20) but has a voltage adjustment of 0.1 volts and a capacity of 20A with a voltage display screen. This relay, which turns the alternator warning on and off, opens (charge off) when the voltage at the battery isolator output rises to 13.8V, waits for 10 minutes and close (continues charging). The cycle continues in this way. I did not change the charging cables of the existing installation, which have a cross section of 50 mm2. While the alternator is operating, there is a voltage drop of around 0.2~0.3 Volts between the battery isolator output of the main cable and the LFP battery input voltage. Cable isolation temperature is around 40C°.

When the small relay that turns the alternator on and off measures 13.8 Volts, the voltage level of the LFP battery group increases to 14.0-14.1 volts. It is not a problem if the LA engine battery does not reach 14.4 volts with the alternator. While balancing the LFP batteries with solar energy at 14.5 volts, I use my manual switch that connects the LA engine battery in parallel with the LFP batteries.

[SOLAR SUPPORT]

My Last post need a correction :

"This relay, which turns the alternator warning on and off, opens (charge off) when the voltage at the battery isolator output rises to 13.8V, waits for 10 minutes and close (continues charging)."

"When the small relay that turns the alternator on and off measures 13.8 Volts, the voltage level of the LFP battery group increases to 14.0-14.1 volts. It is not a problem if the LA engine battery does not reach 14.4 volts with the alternator."


Corrected:

This relay, which turns the alternator warning on and off, opens (charge off) when the voltage at the battery isolator output rises to 14.2V, waits for 10 minutes and close (continues charging).

When the small relay that turns the alternator on and off measures 14.2 Volts, the voltage level of the LFP battery group increases to 14.0 volts. It is not a problem if the LA engine battery does not reach 14.4 volts with the alternator

[CaptainRivet]

The mitzi regulates itself, you don't need to intervene and switch it off when some external part gets past 100 degrees....its engineered to protect itself, just reduce output if necessary and that protection is very conservative to last 500k miles in hot african countries which is the core market for the fuso cooling trucks that alternator is originally developped for.

Thats why Eric from Nordkyn developped the VSR 200 module (which you can also use on the valeo) to regulate it via the sense wire as in a lot of cases you can run it with more output then the conservative original temp protection would allow getting 90A+. And well a remanfactured one is only 250Euro, so if i simply replace it after 7 years beacsue i stress it a bit more but get significantly more output thats worth it...and if finally someone aftermarket would reproduce the original 2nd alternator volvo kit (which is 2 pieces metal worth 100Euro and 2 pulleys) you would have the possibility to get reliable simple 160A cont charge for below 500Euro with additional backup, regear to 3:1 and your close to 200A but 160A output close to idle

[SOLAR SUPPORT]

Originally Posted by CaptainRivet

The mitzi regulates itself, you don't need to intervene and switch it off when some external part gets past 100 degrees....its engineered to protect itself, just reduce output if necessary and that protection is very conservative to last 500k miles in hot african countries which is the core market for the fuso cooling trucks that alternator is originally developped for.

An alternator emitting heat over 100C in an engine compartment insulated with heat and sound insulation increases the average temperature of this volume. While charging my old LA AGM batteries with 90A on a summer day with the Mitzi115A alternator, I measured a temperature of 130C° from the stator windings of the alternator. The engine and alternator of trucks operating in hot climates are cooled much better while in motion than boat engines. The engine compartments of our boats generally have heat and sound insulation, and air circulation is normally quite low. Yes, the engine is cooled with sea water, but; There are parts in the engine compartment that are sensitive to high temperatures. It's like a seawater filter in a plastic container. Battery isolator, rubber hoses and wooden panels are also sensitive to extreme heat. That's why we evacuate the hot air from the engine compartment using additional fans and try to cool this room. Even if the alternator has a structure that can withstand higher temperatures, the discharge of heat energy it emits to the environment is a negative side effect and we do not want this to be excessive.

[CaptainRivet]

Originally Posted by SOLAR SUPPORT

An alternator emitting heat over 100C in an engine compartment insulated with heat and sound insulation increases the average temperature of this volume. While charging my old LA AGM batteries with 90A on a summer day with the Mitzi115A alternator, I measured a temperature of 130C° from the stator windings of the alternator. The engine and alternator of trucks operating in hot climates are cooled much better while in motion than boat engines. The engine compartments of our boats generally have heat and sound insulation, and air circulation is normally quite low. Yes, the engine is cooled with sea water, but; There are parts in the engine compartment that are sensitive to high temperatures. It's like a seawater filter in a plastic container. Battery isolator, rubber hoses and wooden panels are also sensitive to extreme heat. That's why we evacuate the hot air from the engine compartment using additional fans and try to cool this room. Even if the alternator has a structure that can withstand higher temperatures, the discharge of heat energy it emits to the environment is a negative side effect and we do not want this to be excessive.

Read again...
the Mitzi is developed for cooling trucks means its often NOT moving for a long time but engine and alternator runs on engine idling to support the load of the cooling compressors which need to run otherwise cargo is rotten...the fuso being small and compact has a very badly ventilated engine room often blocked by sand. As i wrote it constructed to deliver 2/3 of its rating relying on its self cooling capacity, not any additional ventilation by eg driving.
130 degrees at stator is fine, the isolation lacquer of the windings as critical part can do 160 degrees celcius. It has internal temp protection means in a very badly ventilated engine room its limiting itself to what it can do, perfectly ventilated more.

Evacuting air with additional fans you just put more load on the alternator then necessary.
Its limiting itself and the engine its on will also survive the heat its emssioning. If not then its a misconception you need correct by using the VSR200 and generally reduce the ouput so its emitting in general less. And not ful.on full off full on like you do.
Reduce its output by the 10A really capable fans use to evacute enough air and the alternator produces less heat.
Also regearing is a solution as its emiting less heat as better cooled and not heating up further. Its mostly geared 1:2 till 1:2.5 as for marine engine the crank pulley size is limited to be able to install in as many boats as possible...its optimal gearing in is 1:3.3 for marine engines turning max 3500RPM

[SOLAR SUPPORT]

We are struggling with the low efficiency of classical alternators. The alternator, which has an average efficiency of 50%, distributes increasing heat energy to the environment in direct proportion to the electrical energy it produces. This is a problem for a small, enclosed engine room with insulation and less ventilation, and the alternator that is very resistant to heating does not minimize the problem. Increasing the alternator speed is not a solution to the problem, it increases the problem. If a fan that requires electrical energy must be used to ventilate the engine room, you have to do so or suffer the consequences. Since there are alternators with an efficiency of over 90% that can be used on land, it is possible to increase the efficiency of alternators designed for cars by producing them specifically for use in boats. Alternators with increased efficiency will undoubtedly not be very cheap as the ones with low efficiency. However, I think that alternators that have higher efficiency and discharge less heat into the environment will be preferred on boats or hybrid vehicles. Let's remember that the primary charging source for many boats is solar energy. So if a low-efficiency alternator is your primary charging source, you may want to run it at its limits.

[CaptainRivet]

Originally Posted by SOLAR SUPPORT

We are struggling with the low efficiency of classical alternators. The alternator, which has an average efficiency of 50%, distributes increasing heat energy to the environment in direct proportion to the electrical energy it produces. This is a problem for a small, enclosed engine room with insulation and less ventilation, and the alternator that is very resistant to heating does not minimize the problem. Increasing the alternator speed is not a solution to the problem, it increases the problem. If a fan that requires electrical energy must be used to ventilate the engine room, you have to do so or suffer the consequences. Since there are alternators with an efficiency of over 90% that can be used on land, it is possible to increase the efficiency of alternators designed for cars by producing them specifically for use in boats. Alternators with increased efficiency will undoubtedly not be very cheap as the ones with low efficiency. However, I think that alternators that have higher efficiency and discharge less heat into the environment will be preferred on boats or hybrid vehicles. Let's remember that the primary charging source for many boats is solar energy. So if a low-efficiency alternator is your primary charging source, you may want to run it at its limits.

You always anwser to specific problems with general theories but don't get that the root cause is a differnet animal.
The 115A Mitsubishi and 125AH valeo is NOT an average dumb automotive alternator found on many marine engines, its one optimsed to work on 2/3 of its rating relative to RPM relying solely on its self cooling capabilities and is temperature regulated.
You have the factory stock alternator on your stock engine and these are by the manufacturer rated for you engine room to work also with a big AGM bank.
Is it overheating then its a either a faulty alternator or a missconstruction by factory and you have to tackle the root cause which is reducing heat and thats done by either constantly reducing output of the alternator or re-gearing it higher (so its turning at a higher speed and gets more self cooling).
Installing fans putting a plaster on a wound without trearting the wound...
If adding fans resulting in an steady not overheating engine room and lowering the output of the alternator by the same currents the fans are using doesn’t result in the same result, then the alternator is NOT the heat source that make the engine room overheat, then its the engine itself. Then in contrary it makes sense to re-gear the 115A mitzi alternator that can do permanent 14000RPM constantly (optimum is 9000 at WOT) if stock 1:2 till 1:2.5 to 1:3,5 or as big as your crank pulley allows to regear (mostly its 1:3) as you cannot go smaller on the alternator pulley as this is already minimum so you get enough friction. This is specific to this 2 alternators, their best efficiency is higher reving up the curve but thats not possible as limited by crank pulley size, its still better then most other small case standard automotive ones. This gives more output at all RPM of the alternator at lower temps and add more fans to suck out more air from the engine room. If thats not possible reduce the output of the alternator and add more solar or hydrogen to create more energy harvest outside the engine room.
But toggling on/off the alternator at its max is worst solution you can do as it brings more heat into the engine room as you would reduce its output for the same AH created.
The only thing where a fan is beneficial is if the back of the alternator is located at the exhaust or heat exchanger (so the alternator sucks in the hottest air in the engine room far above the rest of average engine room temp) and the fan plus eg tube sucks in air from outside of engine room directly onto the back of the alternator and a heat shield blocks the hot air from exhaust or heat exchanger...but again then its better to relocate the alternator to a different position if possible (eg where a 2nd one would be with factory kit).