Right Sizing an Alternator
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Eric – another thoroughly thought-provoking article; many thanks.
I upgraded from a stock 80A to a 100A Balmar with an external regulator onto my 39HP Yanmar. However, I then found that the tachometer was reading much higher than with the old alternator. With the aid of the ‘Engine RPM’ app on an iPhone to record actual RPM I graphed the readings and found that they’re pretty consistently 30% higher across the rev range with the Balmar.
In practical terms this doesn’t seem to matter as I think more in terms of percentage than actual revs, but is this a common effect on upgrading, please?
Many thanks!
Iain
Hi Iain,
Unfortunately it is quite common to have the rpm end up off when changing an alternator and as you observed, it is most commonly by a fixed percentage. The main culprits are:
Note that the main culprits here will be critical parameters we will need to program to in the next article so there will be a bit more information there.
If your tach is a mechanical one, then just adjust it to what your phone is saying is the right rpm and you should be good to go (do it at a high enough rpm that the errors are small).
Eric
Hi Iain,
To add to Eric’s comment, most electronic tacs can also be readjusted. I have had to do this on two of the engines I have changed the alternator on. It’s a bit of a PITA but not too bad once I got the hang of it. Here’s a tip: https://www.morganscloud.com/jhhtips/check-tachometer-accuracy/
Hi Eric,
Thank you for this thought provoking article.
I recently upgraded my 160A (12V) alternator that was clearly running way too hot (~ 120C) when asked to produce a steady 100A via the 2 Orion XS DC-DC chargers feeding my LFP house bank. I tried decreasing the draw to 80 A total with little meaningful effect.
I replaced this 160A alternator with a a new 240A alternator which purrs along nicely at ~ 60C with a steady 100 A draw. (The old alternator has been serviced and will go into the spare parts roster.)
My engine does not seem to be too stressed by all this, running the alternator with a good wide serpentine belt (a Cummins QSB5.9 ~380HP) but, now that I am confident that I am no longer overtaxing the alternator, how would I know if I am overloading the engine?
Hi Evan,
We will go into this more in part 2 so let’s see if that answers your question and if not, I can try to help more. Even if you are not doing whitespace programming, derating will be covered and you can just pretend that you are doing that derating through whitespace with a constant output relative to rpm.
The good news for you is that your 6BT is a much bigger engine than the engines seen in most cruising boats so an alternator putting out 100A @12V does not strike me as likely to be a problem. I would definitely want to see what the biggest alternator that Cummins makes available for that engine is though to be sure.
Eric
Thanks Eric. I did not think it would be a problem as the engine comes stock with a 160A alternator; but, it would be good to know what parameters one can use to determine acceptable alternator loads on any given engine.
Hi Evan,
I agree it would be good to have a clean way of calculating the max safe alternator load, but Eric makes clear that’s simply not practically possible (start reading at “How Big Can We Go Without Hurting Reliability?”). And I’m pretty sure that anyone who tells you it is possible to calculate almost certainly does not have Eric’s qualifications and relevant experience, and so is making it up.
That said, Eric will be adding more useful tips on alternator sizing is Part 2.
Hi Evn,
I had a Cummins 6 natural aspirated and used a 200 amp alternator down rated to 175 for years without issues. If memory serves, before mounting it I checked with Cummins and the power required was far below the maximum front of crank shaft take off they allow so I’m pretty sure you will be fine.
That said, it seems a pity to downrate the alternator by over 50%, so if it were me I would check with Cummins and then either add another DC-DC at least or, better yet, go over to feeding the LFP bank directly assuming the BMS is smart enough to control charging properly: https://www.morganscloud.com/jhhtips/check-tachometer-accuracy/
Yes. 100A of charging from this engine is a little underwhelming.
I’ve considered this question in detail; but, I won’t derail this discussion by going into all the considerations here. To extract more from the alternator, a smart alternator regulator like the Wakespeed or Arco Zeus would be a much more elegant solution compared to my existing, relatively dumb Balmar.
In the meantime, though, I do have the luxury of a 9KW generator and 900W of solar to provide up to ~ 300A of additional charging capacity when I have a big house bank deficit. (House bank is LFP 960Ah @12VDC)
It’s not perfect but it’s working very well. Perfection, after all, can be the enemy of good… 😉
Hi Evan and John,
A slightly different perspective on this might be to focus on the generator charging. If I understand the comment right, of the 300A of additional charging, about 230A of it is from the generator with the rest from solar. 230A is about 3.1 kW even if we take a charger of 95% efficiency, that is still under 3.3 kW which is a pretty light load on a 9 kW generator. Max generator efficiency is probably at about 75% load (6.75 kW). At 3.3 kW, you will be burning ~20% more fuel per kW hour generated and are into the part of the curve where it gets worse quickly as the load goes down so if you drop to only 2kW load, you would burn ~40% more fuel per kW generated than if you had a setup that could do between 5.5 and 8 kW.
Obviously this makes lots of assumptions including what your batteries are capable of accepting but I thought it is worth providing the different perspective. In a lot of cases, the question comes down to whether normal engine running for propulsion provides enough charging already or whether you need additional charging and if that is needed, then really focusing on getting the run time down and efficiency up.
Regardless, increasing from 100A output from the alternator does seem appealing.
Eric
Thank you Eric.
I hope it’s OK to take this discussion a little off-topic. We were considering the alternator load’s effect on the engine performance.
You are right in noting that with ~ 230 Amps of AC charging, the generator is still underloaded to some extent. Having said that, my LFP batteries will maintain this level of acceptance until they are at a full SOC or I elect to turn the generator off. (The 2 batteries in parallel will , in theory, accept a maximum sustained charging current of 400A each or 800 Amps total.) This is quite a contrast and a significant improvement in efficiency from the previous setup when I was charging lead acid house bank batteries with longer generator run-times and steadily diminishing charging currents and diminishing loads for the generator.
I am usually running the generator at anchor so will try to add other loads like the water heater, induction / microwave cooking tasks, water maker, heat-pump heating or cooling, etc. I can often achieve 45-50 AC amp draws which gives the generator a decent workout.
On a day when we are underway for shorter distances and if the house bank is depleted to a level where the alternator will not have enough time to recharge, adding the generator input is a good adjunct with us achieving as much as 330A + whatever solar we can harvest.
All of these details add up to a situation where, in theory, I could take the next step and beef up the alternator output; but, it’s kind of a ‘nice to have’ rather than an essential improvement.
Assuming the engine would not be overburdened, increasing the load on the alternator would best be done (as John has suggested) with switching to direct charging of the house bank with the necessary addition of a proper smart ”SOC-responsive” alternator controller with better temperature and output monitoring and reporting. The DCDC chargers could be repurposed to charge the start and thruster / windlass banks.
It’s all quite do-able but maybe not that necessary?
Hi Evan,
Yes, the real question is what is important to you and what you want to optimize for. I like to optimize around reliability, performance (both of the system and the overall boat) and effort to install and maintain. Other people prefer other metrics like comfort, cost, noise, fuel range, etc. Only you can decide when you have optimized to balance the metrics correctly. I do believe that if your goal is to burn less diesel, there are measurable savings available in your system but that is true in most systems and in fact the best way is to simply motor a bit slower normally.
Depending on your system, your latest comment brings up another potential optimization that may or may not make sense. This would be to set up your hot water to heat using generator waste heat which is basically free as opposed to using resistive electrical heat which requires burning a lot of diesel. Depending on your system, this may well actually represent one of the biggest fuel savings of what we have talked about and could potentially be not too expensive.
Eric
Thank you Eric,
While burning less diesel is a worthy goal, it would not be the only metric of success.
I certainly would prefer to minimize the duration or frequency of generator run times while at anchor (an aesthetic choice more than an economic one) so maxing out solar contributions and optimizing charging while running the generator makes sense to me.
The LFP house bank has allowed for significantly shorter generator times due to much greater charge acceptance but there is a point of diminishing returns on how much benefit any given upgrade provides. (For example, if I had unlimited bulkhead real estate, the addition of another 100 Amps of AC chargers could be welcome but for the added expense and trouble, it seems that would be solidly in the diminishing returns column.)
Thank you for your thoughtful suggestions.
John, you suggest you are about to adjust previous comments on “white space” power in other articles. The analysis presented here seems to have been entirely based on an extreme case of alternator size. Also alternator only runs at highest power for a short while until battery acceptance decays. Therefore please do not overdo your editing: for most boats there is enormous “white space “ gain to be had: just by not having to warm the engine up to charge in harbour because it charges whilst you do your normal manoeuvring is a huge gain. I suggest the message is about appropriate choice in alternator size, given which the gains are huge. This extreme example is helpful but should not overinfluence the basic message.
Hi Richard,
Absolutely agree, and I won’t be changing any of that. Where I was wrong was in assuming that by harvesting that unused power we were in some way getting free power, whereas in reality the power harvested this way results in quite high fuel burn per kWh because alternators are not very electrically efficient and so is far from free. That matters when we are comparing ways of making electrical power. For example I’m currently rewriting my article on efficient generator use and have already edited my review of Integrel.
That said, for many of us, maybe most, an alternator properly regulated is the best solution for a whole bunch of reasons as Eric makes clear in his summary.
ok – I currently have the largest alternator I could fit on my Beta Marine 25hp motor (120a, because this motor can only fit a very small short-case alternator without going waaaaay outboard), controlled by a Balmar MC-614 regulator.
I have three very distinct use cases; in the first I am running the engine to move the boat, but it is lovely and sunny out and I do not need the electricity provided by the alternator at all and it would be most efficient to have little or no alternator load. in the second case I am moving the boat under motor, but it is rainy or gray and I would like to charge the batteries but slowly, balanced with engine thrust output, and in the third case I am not moving the boat under motor, either at anchor or sailing offshore, but it is gray or rainy and I want/need to charge the batteries as quickly as possible.
(I understand the third case would be best solved with a generator, but in a 10m sailboat I really don’t have the space for one. I may pick up a little portable Honda for this upcoming winter gunkholing in the PNW, but for now my diesel engine is my secondary charge source, after my 720w of solar and very-occasional low-amp shore power. my batteries are LiFePO4, so charging fast with high current is possible/preferable.)
is it possible, with the Wakespeed controller, to address each of these different use cases directly, possibly by changing profiles manually with a digital switch over nmea2k or something? I haven’t been able to determine this from the website/manual/etc.
Hi Drew,
Your first use case of no alternator output can be done in a few different ways depending on how your boat is setup. The simplest is a field disconnect on a manual switch. Since you have a lithium bank, you may be able to use your BMS to not command charge. Wakespeed does provide the ability to provide an input like this as well but I have not studied the best way to do it. The “Feature-in port” may well be easiest and if you are real tech savvy, you could do it over CAN.
In your second use case, assuming you are not motoring near maximum rpm, there is no need to prioritize propulsion power as can be seen from the graphs above where the total load is well under the engine power rating. Unless you have a truly preposterously large alternator, the shape of the prop curve ensures that there is plenty of power for both a cruisers alternator and the prop. If you do motor near max rpm, then the solution will be provided in the next article through whitespace programming. Yes, it is a bit more efficient to charge slowly but I suspect that it isn’t worth the trouble for most people. If you really want to do this, the Wakespeed is your best bet I believe.
Your final use case is definitely possible as it is just running the alternator at the maximum output allowed by temperature.
FYI, your alternator is high enough output for the engine size (~1.5 kW alternator on a 19 kW engine) that I would definitely want to be sure that the engine won’t be damaged by it. If it is a problem, whitespace programming could be used as a mitigation which will be described in part 2.
Eric
Hello Eric and John,
Will you include some knowledge on alternator heat management.
How to blow some fresh air in an efficient way to help the alternator keeping a healthy temperature?
On a blue water cruiser I find that you don’t motor to frequently but often for a long time ( few hours to a day)
Thank you
Denis
Hi Denis,
Not in part two, but I will be addressing that in a rewrite of my installation tips chapter, currently in progress.
Hi Denis!
I agree, when you motor, you motor for a long while sometimes.
I don’t know if you have an small engine compartment or more of an engine room. But, I’ll share what we did on Seamer, our old and trusty Colombia 56.
Early into owning the boat, we decided to ventilate our fairly sized engine room for that reason, meaning coolIng the engine room, and to circulate air as well. It removed a lot of smells from the boat, because the bilge is the bilge. It helped so much with the temperature in the boat while running the engine or the generator as well as reducing the cooling time for the whole living room and dining area. The engine room is under there.
We installed two in-line 12V DC blower fans from Sea-Flow (270 CFM for the curious). They are available on Amazon and surprisingly affordable. We have a switch for them near the electrical panels. We used insulated flexible vent pipes that lead the hot air all the way outside through a cowl vent. It’s not unsightly, it’s all hidden.
If you have a small compartment, It’s a little tougher. There is no point in having an out if you don’t have an in. In smaller boat, it’s trickier because it’s counter intuitive to make a hole for air intake because of the noise. I guess it may come down to choosing a lesser evil. When I look at the comment below, I see you could maybe pick Michal Palczynski’s brain about it.
Anyhow, all I can say is our engine room is definitely not as hot. The boat does not head up as much as a result.
I hope this can help to start troubleshoot any engine room temperature issue you may have. I’m curious about what John may have to say in the expected revamp chapter he is working on. I’ll be reading.
Still, cooling our engine room to a balmy 45 degrees C, roughly, did not fix our alternator overheating problem at all. Therefore, I invite you to really dig into the cause for the alternator overheating and look, like Eric wrote, at the recommended space and room temperature from the manufacturer.
If not mistaken, many alternator stop producing around 100-104 degrees C, which is very hot. Your engine is probably at 90, but your engine room is necessarily much lower. So, it’s worth trouble shooting the cause of the overheat. Maybe check at what temp your alternator cuts off production, the operating temp of your engine and actual room temperature under way after it’s properly heated up. That can give your clues. Perhaps, you’ve done all that work.
We discovered that our Balmar XT-170 with a double V belt would always overheat after 5-10 minutes, and I mean always, but only if we were trying to get a high output out of it, like 115 to 120 amp. For reference, we have a Detroit Diesel 453 engine, some 120 hp hidden in the beast. It’s honestly more of a reliable excavator. Last starter we bought came from a dump truck in Mexico. In short, for the alternator, the double V belt would slips causing the alternator to overheat very quickly. It was as simple as that, no matter how much we adjusted them or how new they were. With a reduced demand on output, 60 amp at cruising speed, the alternator now never ever stops providing, unless the batteries are full, of course. To figure that one out took us hours and hours of our lives, a good number of belts, several cans of anti-slip spray, lots of advice from handy friends and from Chat GPT (Chat GPT was seriously our biggest help, kidding, not kidding, it can summarize boil down entire forum threads). We initially suspected the room temp was the culprit or the fact that we were in the tropics, and that was not it.
The only way for us to stop the slippage and overheating at a higher requested output for our motoring stints would be to change from v-belt to serpentine, like in cars. But, we are happy at 60 amp per hour, even on a cloudy day and especially if we will motor a lot. We have 4 lithium batteries of 280 Amp/h. It’s nearly sufficient for our power hungry boat (induction cooking and electrical oven) with our solar panel park. When we are careful, we don’t need the generator (Northen Lights, 10Kw, overpowered for our boat if you ask me).
All this to say that in our case, proper investigation lead us to find that engine room temp was but only one side of the alternator heating up, or maybe not at all.
Best,
Marie on SV SEAMER
Hi Marie,
Interesting, I have never heard of a 2 stroke Detroit in a cruising yacht. Many of the commercial vessels I worked on had 71 series engines and even 92’s. I think they are the best sounding engine in the world for the first 5 minutes then I wish I had something a lot quieter. With the exception of having to deal with 2 runaways, they have been nearly flawless performers for me. At least the 71 series have multiple engine side pulleys to run accessories off of which provides lots of flexibility and ability to drive lots of power generation but I don’t know if the 53’s had that.
60 amps is a huge de-rate from 170 and seems to me like something is weird. A single v belt is usually good for something like 100A and if you run matched dual belts, you can get close to double that without issue. If the belt is slipping, that will lower alternator rpm which will make it hotter for a given output but it will also burn through belts really quick. Another trick if you can’t hear it slipping is to watch a tach output from the alternator as it will show lower rpm than it should when the belt is slipping. I kind of suspect that your alternator temperature ratio is separate from the belt slipping. What is your pulley ratio? Have you ever tried directly ducting cool air at the alternator in the direction of the alternator fan flow as a test to see if it makes a difference? Have you tried measuring temperatures in a bunch of places directly with a thermocouple or even an IR gun (these are much less accurate)?
Eric
Hi Eric, all very good observations.
This 2 stroke is an absolute beast. Reliable, extremely so. We have the original manuals, two huge bibles. It is indeed noisy; we’ve been planning on insulating the room since we first heard it, but alas, we are in Fiji now… It’s not happening for a while. We can indeed run two alternators. We have two actually. The other is a smaller one dedicated to the lead acid starter batteries.
Back to the Balmar. The engine room is 20x8x4. The temperature sensor is on the outside of the alternator. We’ve also heat-gunned it during our trouble shooting and it seemed not too far off.
One of the problem is our oil sputtering of an engine causes the belt to become oily. This engine spits oil, that’s how it’s happy. We think this contributes to the belts getting oily and just getting destroyed and slipping and causing the alternator to quickly overheat. With a lower alternator output, the belt does not slip nor does it get destroyed. My husband and I both agree we could further toy with it and probably get the amps higher or just test properly when it is that the slipping and problems starts. But, we are far less motivated to find the issue with our alternator and belt than most. It’s a power hungry boat with induction and electrical oven, but consequently, we have 2,4 kw of solar panels and if we don’t need to make water, we don’t need to start the generator. And that’s the thing, we have a generator, and a good one. We start it, then start the 40 gallons an hour water maker and we make sure we are also cooking and not much goes to waste from burning that fuel. And the Balmar, it works so well right now. It’s producing less than it could, but it works the long hours if need be.
I’m sure my husband will get bored in the coming weeks because we are not planning for a big passage anytime soon. We are taking it easy in Fiji to bring the bank accounts back to healthy levels and sleep, sleep a lot… 12 500 nm over 14-15 months was a lot!
Thank you so much for sharing your valuable opinion on here. It may very well be the case that our overheating alternator issue is separate from the belt issue. I don’t know. I’ll for sure come back to report when we get back into troubleshooting it!
Best,
Marie
Hi Marie,
I hadn’t thought about an oil leak but I could see how that might cause the behavior you are seeing. Normally a slipping belt would not last long at all and would make a racket but an oily one might have a high slip ratio and go unnoticed which could potentially cause heating issues. Also, the belt might be throwing a mist of oil which could have coated your alternator making it unable to dissipate its heat as quickly. This all brings back silly memories of elaborate aluminum foil contraptions to try to direct oil leaks into easier to clean containers than the drip pan. I even know of people who would put it back in the engine but I never did.
With regards to noise, be aware that most of the literature is based on 4 stroke engines. These engines make an average amount of mechanical noise but they have very noisy intakes and exhausts. You might be able to knock the noise level down a lot focusing on those 2 areas but intakes are hard to quiet down and often require the whole engine space to be insulated.
Eric
Hi Eric – thank you for this article.
I have 2 alternators on my 88hp Yanmar turbo diesel and I wanted to share my experience.
The rated amperage is 285 and 220, so 500 amps altogether. My system is 12 volts. One is Arco Zeus 275, and another Mark Grasser 220A with external rectifier.
Conclusion: That was quite time-consuming and expensive installation but we have 4kW+ of continuous charging current. Because we don’t have space for the generator that was the only way to go. Although the installation cost was high, it’s significantly cheaper compared to a good generator and chargers, and the increased fuel consumption is negligible. As we know, the most expensive thing on a yacht is space, and alternators take up very little of it. And I don’t think that many boats with generators are able to charge batteries quickier that 300 amps. I am not trying to show here that alternators will always be better than generator, but in our particular case they are.
I think that 300-350 amps is a maximum “reasonable” charging current and if I would need more power I would go to 24 or 48 volts.
I have two alternators setup since 5 years and I’m very happy.
Wow Michal, your numbers blow my mind. Are you not worried about the strain this set up puts on your engine?
Marie, I think that our engine is so oversized for our propulsion that we have lot of power for generating the power
Hi Michal,
I don’t want to speak for Marie but my guess is that she is referring to the stress on engine components as you are taking large amounts of power off the engine not off the flywheel but off the front end of the engine where it might not be designed for those loads. I too wonder about whether the power and therefore torque is high enough to cause any issues as outlined in the section titled “Reliability”.
Eric
I have this setup since 2020 and the engine has 3000 hours since then.I don’t see any difference how the engine performs, so I assume (now I knock on hardwood), that it is not hurting my engine. Often the alternators are running at 350A for more than 2 hours continuously and everything is fine.
Hi Eric, indeed, I was thinking stress on engine components as it might not be designed for it. Maybe it is, seems hard to find adequate information, as you have mentioned in your article.
Michal, it’s great if it’s been working this way since a while (2020!). It’s great to know that if need be, a Yanmar 88 hp can be pushed this way. I think it’s absolutely great that you shared it with this community. I would be a crime to keep this a secret!
It’s always a question of smart compromise on a sail boat. After all, if a generator is not possible for you considering the layout, getting that output out of the alternators on your main engine is nothing short of fantastic. On the other hand, maybe your engine will get old faster, but heck, it will also have basically doubled as a generator all that time. In my book, it must be worth it. My comment is entirely hypothetical because it may be just fine to use your engine this way according to Yanmar. You are right that they were not thniking about this 30 years ago because of LA batteries so it may be impossible to find out. Still, if ever it causes some premature wear (also knocking on wood for you), it could still very well be worth it in your case.
Best,
Marie
Hi Michal,
Thank you for the report. That is definitely a lot of power out of alternators.
Part 2 will provide a good alternative to the field switches you use to manage the performance issues although with a dollar cost attached. I would not personally be comfortable with having to remember to turn off alternator fields at specific times as stressful situations on not a lot of sleep can lead to silly human errors that may shut down an engine at a bad time but I realize other people think the risk is reasonable. Your issues of stalling at idle or unable to accelerate make sense. I am a little surprised that you can’t run below 2100rpm with both units operating and not stall, at max I would expect your setup to be drawing around 12kW from the engine and at these rpms, there is often around 25kW available above the prop load on a ~65kW engine. Do you have a CPP or autoprop that is changing this?
Have you tried to verify with Yanmar that the engine can take the belt loads you are imposing? In particular, I know Arco have some alternators optimized for high output at low rpm and if you run normal belt ratios, these put quite a bit of stress on the engine. These low rpm optimized alternators really require special care to prevent engine damage and should not be considered as direct replacements for similar power rated alternators with a more normal current vs rpm curve.
Going to a higher voltage will not necessarily mean that you can produce more power. The cabling will get smaller and you will have a little higher efficiency but the limits related to your engine won’t really change, it will be the same power with a higher voltage and lower current.
Eric
Hi Eric,
This is really interesting observation. Thank you. On 2100 rpm the propeller load is around 13kW. Plus, 12kW from alternators (at 350A I think this is less) and we have 25kW of load on the engine. I just checked in the engine manual and at this rpms It should be 40 kW available. So, this is quite a big margin. Our engine has 10k+ hours so probably some power horses are dead already https://s.w.org/images/core/emoji/16.0.1/svg/1f609.svg
Of course, I didn’t check with Yanmar what is the max load on the front side of the crankshaft. I think that 30 years ago nobody could imagine that will be useful because the LA batteries couldn’t absorb that power. But we are using that two alternators setup for almost 5 years and it has around 3000 hours on it. I don’t see that anything has changed since then.
Arco have low rpm alternator versions but, as I know only for 24v and 48v versions. Arco 275A @12v has only one option.
And by going to higher voltage, I meant thermal management. Above 200 amps, the amount of heat generated by a alternator is too high. And small alternators have problems and are running too hot if they are working hard longer.
I can’t wait for the part II.
Michal
Hi Michal,
Yes, something seems weird on why you are limited below 2100rpm. It is hard to say what it is. And you are right that at 350A charge (I was assuming both alternators could output max but if you really never see over 350A, that makes a big difference), the alternators should be drawing less, significantly under 10kW I would think. Maybe you are just being conservative on the rpm where you start to have problems? Or maybe you are overpropped and measured when punching into a big head sea? Or maybe the tach is off? While engines do lose a little power with a lot of hours, unless something is really wrong it is usually not much, they start to have a very blue exhaust and get hard to start before they lose much power.
Eric