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 letâs 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… đ
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 âwhite spaceâ 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 âwhite space â 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 âFeature-in portâ 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 isnât 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 wonât 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