Do I need a new Alternator?

[ToddS]

I'll be the first to admit that I'm a bit deficient when it comes to electrical know-how on my boat... learning though. Anyhow, I've had it for around a year and a half, and recently installed new AGM batteries: a pair of parallel group 31 (house) and a group 27 (starter). I just noticed (after only a couple uses this season) that when I run the engine, my voltmeter at the circuit breaker panel reads 13.5-13-6 but I'm pretty sure that in the past (last year) it would read somewhere in the low-to-mid 14s when the engine was running. I checked the specs on the batteries, and they want to charge in the 14s. Further proof that I'm correct to be worried about 13.5 is that even after running the engine/alternator for a while (30-60 minutes), the batteries only read in the low-to-mid 12s for voltage, (up from high 11s when I start the engine) rather than around 12.8+ where they ought to be. I don't want to damage my new batteries. The alternator is a 60amp (original) alternator on a Yanmar 3YM30 engine, and as far as I can see the wiring everywhere seems to be in good condition, no corrosion or lose fittings that I could spot. Things I found in investigating/troubleshooting:

1. Same results, whether I've got the house, or starting (or both) batteries switched on.
2. Same results whether I run the engine at say 1200RPMS or 3400RPMS (just for testing... I don't normally run it at that speed)... and neutral vs in-gear also makes no difference.
3. I tried with everything switched off (no extra loads like fridge, or pumps, etc.) still no difference.
4. I measured both at the panel itself, and also used a multi-meter to test across the back of the alternator when it was running... same (or negligible difference)... so I don't think it is voltage loss over the harness/wiring.
5. I have a mooring, not a slip, so no electrical hook-up, but recall when launching at the boatyard weeks ago I DID use shore power, and using my on-board charger was charging batteries fine at somewhere in the low-mid 14s, and the batteries would go up to where they should be.

It has only been a couple of charging cycles like this, but I'm sure I'll damage my batteries if I keep charging at too-low a voltage. I don't know a ton about alternators, but I'm pretty sure mine is internally regulated, eliminating an external regulator being the source of the problem. To the best of my knowledge everything is pretty much factory stock (Beneteau 373) in terms of equipment in the electrical system on-board (I'm owner #2, but #1 kept things in great shape, and generally altered VERY little).

Is there more troubleshooting I need to do, or is the next step to buy/install a new alternator? It's about 15 years old (though only a few hundred hours), so I'm okay slapping on a new one if that's what's best. I suspect paying someone (northeastern USA) to disassemble/rebuild it will cost me nearly as much as buying a new one. I suppose I could keep the old one as a "spare" and tinker with it myself to see if it is repairable... I'm generally pretty handy even though alternators aren't an area I've dabbled before. What votes the sailors of the interwebs? Buy a new one? Do more testing? I'm a fool? Fix the old one? If I'm to buy a new one... I don't need a bigger one (IMHO), but... any advice on replacements to steer toward or away from? Thanks!

 

[GBGraham]

I would say no at this point 13.5v can be the full load cutting in voltage. I'm probably stating an obvious, is you alternator drive belt tight and in good condition?
I could be tight but worn and bottomed out in the vee of the pulley. A good sign of this happening is hot pulley's after a shortish run, also a rusty alternator pulley is a sign of slip.
If this is good, remove and take to a repairer that can run the alternator up on a bench tester, there may be a stator or diode failing. Does your system have a alternator charging light? If so, does it extinguish fully, i.e. no glowing.

 

[Maine Sail]

1- You're grossly over discharging your new bank. The lowest bottom voltage you should see, under your typical house loads, is about 12.20V if you want long life from the bank. Your batteries should never be in the 11's as standard usage practice.... A good digital volt meter, that reads to the hundredths position, and senses the house bank + & - terminals directly, is a good tool for monitoring your bottom voltage. An analog panel meter is not the proper tool for this.

2- Your alternator is rated cold and will not pump out anywhere close to 60A when hot. A battery monitor that shows current can be a big help here in understanding what is going on with your charging.

3- Your alternator is basically a car alternator and as such was really never intended for charging large house banks or AGM batteries. There is nothing to "fix" and it is just doing what it was intended to do. This article goes into much more detail: Automotive Alternators vs. Deep Cycle Batteries

4- If you're on a mooring you'll need solar unless you can run an externally regulated alternator for at least 6+ hours each time you discharge the bank. In otherwords an alternator alone is not sufficient to get your AGM's back to 100% SOC after each discharge due to lack of engine run time. Solar has the time to address the last 15% of the SoC curve..

5- A pair of group 31 AGM's, with refrigeration on-board, is a very small bank for that boat.

Rx:
Bigger Bank
External Alternator Regulation / Alternator
Solar
Battery Monitor or volt meter with good accuracy to the hundredths.
No more discharging below 12.2V even with typical house loads such as the refrigeration running

 

[ToddS]

Thanks for the responses so far... I can answer/clarify some points in the discussion so far.

Replies to GBGraham's comments/questions:
The belt "seemed" fairly tight while I noticed this (out anchored for a few days) this week, but just to be sure, one of the first things I did was to get it nice and tight and that made zero difference (I should have mentioned that). Also, the belt isn't frayed at all, or melted at all, or cracked at all, or creating excessive dust or squealing or flapping at all and generally looks like new... as do the pullies. Zero rust anywhere on or near the pullies or anywhere on the engine... (maybe just a tiny bit on one engine mount, but nothing major, and I've addressed the cause, and repainted the mount a bit over a year ago per the suggestion of the (very impressed) marine surveyor I hired when buying the boat. I suppose a belt is cheap.easy enough that I can try swapping to a new one just in case the wear isn't clearly visible... I'll do that just to eliminate easy stuff, but it seems pretty unlikely. At the electrical panel itself, I have both an analog (which I don't use) and digital (which I do use) voltmeter. The digital one is always on (though measures only to the 10th, not 100th), and additionally I do have a good, hand-held digital multimeter (voltmeter) which I keep on the boat, which I used in this case, and measures to the 100th. I only listed rounded numbers in my post because I'm not on the boat, and couldn't remember EXACT numbers in my head, days later, and didn't write down exact numbers either. This particular problem is a problem of an entire volt (or so), not 100ths though, so for this conversation, tenths will probably be sufficient. I do not have an alternator charging light to see if it extinguishes fully, but when I start the alternator, the voltage goes up from say 11.8 to about 12.6... so it is not ENTIRELY not working at all... just not nearly enough to be effective.


Replies to Maine Sail's comments/questions:

1. I would normally charge my batteries if they got below 12.1 or 12.2.... but when the POST-CHARGING voltage is only 12.2, it got down to the high 11s... So I did my best (charging them back up to 12.2-12.4) with what I had a the time, then came home and wrote this post. You're right though, I wouldn't normally do that. (See above for 10th vs 100th discussion)
2. I tested across the back of the alternator both hot and cold, and it voltage never exceeded 13.6, and while I don't have a measurement of current INSTALLED on my boat, my hand-held multimeter does show current. (though I don't have those numbers in my head or easy access to the boat where I live.
3. I skimmed that artcle (looks great, thanks!) and will read in more detail, but 13.6 output voltage when it is ice cold this week vs charging up to 13V last season doesn't seem to be explainable by just "that alternator/system wasn't designed for boats"... it was used for 14 years before I got it... and last year for me... and adding an additional 92AH (ish) battery I wouldn't think would cause this drop in alternator output. I misspoke earlier saying I had a pair of group 31s, I actually have a pair of group 27s... splitting hairs maybe, but for accuracy's sake, I figured I'd correct myself here. I replaced an old 31 with two 27s.
4. and 5. - Probably true... but I use the electrical system on the boat very little. Mostly just used a few nights per month on weekends, and only for all-LED lights briefly in the evenings and some miserly freshwater pump usage and charging cell phones (or chartplotter and instruments underway). I'm not using air conditioning, or microwaves, or heating water, or watching TVs. I admittedly do have a refrigerator on the boat, but never use it on my normal weekend visits, and so it gets used for a few days around the 4th of July, and around a week in August (running the engine quite a few hours each week). Water temps in the 60s and air temps in the 70s this trip shouldn't be straining the fridge TOO much though compared with someone in the tropics. I have one tiny solar panel that came installed on the boat... but it's very old/tired/worn, and I generally consider it to be netting me approximately nothing. upgrading that is on my list already... but the alternator is more pressing right now (in my opinion).

I'll look into a better, externally-regulated alternator... by the math in that article, they seem to say I need a minimum of 40% of my bank amp hours, which would theoretically be 73Amps... which was above 40% of my old bank, but I probably need an 80 to meet their suggestion. Still not sure why the sudden drop in performance though... and whether a "poorly designed system" explains away my symptoms this summer vs. the past summers...

 

[Maine Sail]

I would normally charge my batteries if they got below 12.1 or 12.2....

They should not go below 12.2V under typical house loads if you want them to last.....

but when the POST-CHARGING voltage is only 12.2, it got down to the high 11s...

This is because your bank is down towards 80% DoD and bulk charging takes both amperage and time to increase the battery voltage. The deeper you go the longer it takes and the hotter the alternator gets.

So I did my best (charging them back up to 12.2-12.4) with what I had a the time, then came home and wrote this post.

This is grossly insufficient charging especially if the voltage was measured when charging. If this is kept up the batteries won't last very long.

I tested across the back of the alternator both hot and cold, and it voltage never exceeded 13.6

The article in the above post explains this. Cold you're dealing with bulk voltage limitations and then hot you're dealing with the alternators temp gradient.

and while I don't have a measurement of current INSTALLED on my boat, my hand-held multimeter does show current. (though I don't have those numbers in my head or easy access to the boat where I live.

If you don't have a DC clamp meter, capable of at least 100A DC on board, it would be a good investment and can make troubleshooting a heck of a lot easier..

I skimmed that artcle (looks great, thanks!) and will read in more detail, but 13.6 output voltage when it is ice cold this week

At your depth of discharge (11's), voltage does not just magically jump to the 14's as many folks incorrectly assume it does. It takes both current and time to increase the battery voltage. Of course this demand on the alternator builds heat in the alternator and it then reduces its target voltage, a catch-22...

vs charging up to 13V last season doesn't seem to be explainable by just "that alternator/system wasn't designed for boats"...

13V is not a charging voltage, did you mean 14? If so, 14V is still insufficient for battery health.

AGM batteries should attain 14.4V minimum and depending upon brand as high as 14.8V. Eat Penn's do best with 14.6V temp compensated.. This voltage should then be held constant at this level for multiple hours to "absorb" the battery.

Focus on the bulk-times/duration's in the article below and how long and how much current it takes to attain an absorption level voltage. If your batteries come up to target voltage quickly, from the 11's, the batteries would be considered sulfated toast.

How Fast Can an AGM Battery be Charged?

it was used for 14 years before I got it... and last year for me... and adding an additional 92AH (ish) battery I wouldn't think would cause this drop in alternator output.

Used for 14 years on a mooring, with PSoC cycling (partial state of charge cycling) into the 11's, or tied to a dock after each use charging? Yes the additional 92Ah of capacity, at approx 80% DoD, is going to extend your bulk charging time to absorption voltage.

I misspoke earlier saying I had a pair of group 31s, I actually have a pair of group 27s... splitting hairs maybe, but for accuracy's sake, I figured I'd correct myself here. I replaced an old 31 with two 27s.

If you're running refrigeration this bank is tiny, especially for your PSoC usage which will cause your bank to slowly walk both capacity and SoC downward over time. Even without the fridge your bank will gradually walk down with a stock alternator.

Probably true... but I use the electrical system on the boat very little. Mostly just used a few nights per month on weekends, and only for all-LED lights briefly in the evenings and some miserly freshwater pump usage and charging cell phones (or chartplotter and instruments underway). I'm not using air conditioning, or microwaves, or heating water, or watching TVs. I admittedly do have a refrigerator on the boat, but never use it on my normal weekend visits, and so it gets used for a few days around the 4th of July, and around a week in August (running the engine quite a few hours each week). Water temps in the 60s and air temps in the 70s this trip shouldn't be straining the fridge TOO much though compared with someone in the tropics.

You 11ish range volts is telling you the actual story of what's going on and is typical of mooring sailed boats suffering from PSoC abuse.. This is where a battery monitor can really help you understand what is actually going on with the system. These days we have customers who can easily exceed refrigeration energy usage just charging computers, phones and tablets.

I have one tiny solar panel that came installed on the boat... but it's very old/tired/worn, and I generally consider it to be netting me approximately nothing. upgrading that is on my list already... but the alternator is more pressing right now (in my opinion).

Doing the solar should really be job 1. Your alternator, even a high performance one, is not going to get your batteries back to 100% SoC, which they desperately need regularly, or they can barely last a season.

I'll look into a better, externally-regulated alternator... by the math in that article, they seem to say I need a minimum of 40% of my bank amp hours, which would theoretically be 73Amps...

Again, your battery voltage is suggesting you need a larger bank and more frequent and better charging as well as solar for "topping up" between uses.

Also, the only 92Ah G-27 AGM I know of is the East Penn product (Deka, Duracell, Sam's Club, West Marine, NAPA etc.) and these batteries do not deal with or tolerate extended PSoC use well at all. They are not a deep cycle AGM but rather a "dual-purpose" design.

In the May 2015 and August 2015 Practical Sailor PSoC testing the East Penn AGM batteries lost 1% of Ah capacity for each PSoC cycle to 11.7V or roughly 30% of their Ah capacity in 30 deep PSoC cycles. I would urge you to read that article so you have a good understanding of what and how PSoC cycling can affect the bank.

Fighting Sulfation in AGM's - May 2015

but I probably need an 80 to meet their suggestion.

Focus on the healthier aspect of charging that external regulation can give you in the short run times we do on sailboats. 80-100A will run on your existing belt but the external regulator will provide much healthier charging than the factory automotive alternator can.

Still not sure why the sudden drop in performance though... and whether a "poorly designed system" explains away my symptoms this summer vs. the past summers...

Without knowng more about the system, diode isolators etc., etc., all we know is that you indeed added more Ah capacity. You then allowed the bank to fall into the upper 11's.... That takes a lot of amperage and a lot of time to recover from this depth of discharge.

Just last week we had a trawler customer have his underwater lights turn on without him knowing it (a glitch in the wireless remote). By the time we got to the boat the bank was at 11.3V..

It took over an hour of charging at 90-93A just to attain 13.0V (while charging) this is called "bulk" charging. He did not get to absorption voltage (14.6V) for a long while after that and this is with a high performance charging system. If this had been his stock Hitachi alternator his time to absorption voltage, well..... Much, much, much longer.......

 

[ToddS]

Maine Sail: I truly appreciate your input. I hope I don't come across as argumentative, because I am completely aware that you (and many) are far, far more knowledgeable when it comes to boat electrical systems than I am. I only question things or "go-back-and-forth" to better understand things, and to make sure I'm giving all the necessary details I can add to help get to the best conclusion at the end of the discussion. I agree, and think I'm getting everything you're saying (and I'll study that article a bit more)... The only piece I disagree with somewhat is that my bank is inadequately sized... sure, bigger would be nice, and most folks would go that way, but I'm pretty sure I can take the rest of the advice I'm getting, and get by with a 200AH bank and a 100(ish)AH starter with my very conservative use. The fridge is rarely ever used, and have generally and happily lived entirely with an icebox and NO fridge for the past 20 years... and taking that mostly out of the equation I'm pretty sure I can get by for now on approximately what I have installed, keeping an upgrade in mind for the more-distant future. That being said...addressing solar, upping my alternator output, going to external regulation, getting a 100+ Amp clamp meter (my non-clamp multimeter maxes at only 20A) are all great advice.

Like it or not, available money and available time will to some extent limit what I can fix in the next month or two, versus what I will fix in the fall.

Feel free to tell me if this is just plain wrong, but I'm tempted to start by getting a "bigger" capacity alternator (80-100A) that's externally regulated (feel free to advise on specific models/traits to steer toward or away from)... installing the alternator and regulator seems like something within my DIY capabilities, and something I can do while moored/anchored ASAP, and in short time. For the short-term... I can run it way more than usual (noting of course as you mentioned, the diminished capabilities of alternators when hot make this a less-than-ideal approach... but still better than what I currently can do) to get the batteries back up to where they should be ASAP. I can easily give up the fridge and keep the draining to absolute minimums until this is all resolved. a handful of 1.5W LED bulbs, and a few uses of the chartplotter while motoring should be fairly small and easy to keep up with. Fixing the solar situation is a bit trickier, and far more expensive once you get into custom fabricated stainless mounts or biminis, with all the associated panels, hardware, wiring, regulators, etc to make it right... and living hours away, working full-time, and the parent of young children, this exceeds what I have the ability to handle until we haul in October. Right now I have a (crappy) 24" square (approx) semi-flexible solar panel that's about 15 years old which came with the boat last year... useless... I don't have much room on deck to just swap that out with a bigger/newer version of the same... plus the one that's there is on the cabin top below my boom... with a prevailing wind from the south... so almost always under shadows of spars through the middle of the day... again... bad. Also I'm in New England, which makes for lots less sun than the tropics... though in some ways that reduces the need for power as well (less fridge, no A/C, etc.) When I do the solar, I want to do it right. The alternator swap will cost me hundreds and take minutes (maybe hours at most) I'd imagine... I can certainly withstand that level of cost and time commitment. But not several thousands of dollars, several days of work, and lots of trips on and off the boat like doing the solar panels right. Like it or not, I don't see getting though the solar project beyond just doing some research, until after hauling in a few months. Not that you're wrong about addressing that at all... it's just a limitation based on my reality. So... any other advice from you (or anyone else) before I start my cyber-shopping for clamp meters, alternators, and external regulators? I'm shelving major solar upgrades for a few months.

 

[kloudie1]

An option (maybe temporary) would be to use a small, say, 1000 watt (at 120V), alternator generator to run your fixed battery charger.(assuming your fixed charger is 50 amperes or less, total, @14V) If your fixed charger is a good one, it will top off the batteries more quickly than the alternator option.. There are some quiet, small, high quality ones available from Honda, Ryobi (Home Depot) , and others for around $400.. You'll need to get a CO alarm and an adapter to get from the generator's outlet to the boat inlet connector, and a safe way to handle gasoline on board. There are cheaper options but they are LOUD.. and not suitable for hour long running in an anchorage..

 

[LeslieTroyer]

@kloudie1. I’m not sure why you think a shore charger is faster than an alternator?? Properly setup with a regulator that is not brain dead can produce way more than most shore chargers. Even derating my 95A Alternator 40% due to a small belt it still puts out more than the 40A Charger.

I do run a Honda 2000 to charge but only cause it is much quieter and I don’t like running the engine @ 1500 no load.

Les

 

[jssailem]

Since you are going cyber shopping, I would explore MaineSail's site. ( https://shop.marinehowto.com/t/alternators--regulators I am pleased with the alternator I purchased from his site for my boat. It has upgraded my ability to charge my batteries, and with the external regulator I get more control of the charging system. Connecting the alternator to the battery has made the system more efficient.
The alternator swap was about 60 minutes. The Regulator was more involved. Both were at the dock which made getting additional wire and nuts/bolts as needed convenient. As mentioned by MaineSail the proper install of the alternator is the job of the installer. This includes the correct sizing of the bolts. A poor install can affect your performance.

 

[Benny17441]

Yes, automobile type alternator not adequate for fancy deep discharge batteries. In conjunction with a good inboard charger the alternator can adequately work but if you are going to strictly rely on the alternator you will need to upgrade or go back to the cheap batteries.

 

[kloudie1]

Les, I meant compared to his current (little pun) status..

 

[Stu Jackson]

Feel free to tell me if this is just plain wrong, but I'm tempted to start by getting a "bigger" capacity alternator (80-100A) that's externally regulated (feel free to advise on specific models/traits to steer toward or away from)... installing the alternator and regulator seems like something within my DIY capabilities, and something I can do while moored/anchored ASAP, and in short time. For the short-term... I can run it way more than usual (noting of course as you mentioned, the diminished capabilities of alternators when hot make this a less-than-ideal approach... but still better than what I currently can do) to get the batteries back up to where they should be ASAP. I can easily give up the fridge and keep the draining to absolute minimums until this is all resolved. a handful of 1.5W LED bulbs, and a few uses of the chartplotter while motoring should be fairly small and easy to keep up with. Fixing the solar situation is a bit trickier, and far more expensive once you get into custom fabricated stainless mounts or biminis, with all the associated panels, hardware, wiring, regulators, etc to make it right... and living hours away, working full-time, and the parent of young children, this exceeds what I have the ability to handle until we haul in October. Right now I have a (crappy) 24" square (approx) semi-flexible solar panel that's about 15 years old which came with the boat last year... useless... I don't have much room on deck to just swap that out with a bigger/newer version of the same... plus the one that's there is on the cabin top below my boom...

Todd, the first thing that's missing is an ENERGY Budget. You talk about turning stuff off. That's bassackwards.
The All-Important Energy Budget:

http://c34.org/bbs/index.php/topic,3976.0.html

Second, why oh why did you buy AGMs? In addition to Maine Sail's fine posts here, you might want to read these, too:

AGM Batteries - Making The Choice (from Maine Sail)

https://forums.sailboatowners.com/index.php?threads/agm-batteries-making-the-choice.124973/


AGM Battery Issues and the Blue Seas Dual Circuit Switch (from Maine Sail) "DARN AGM Batteries"
https://forums.sailboatowners.com/index.php?threads/darn-agm-batteries.133773/

Additional Observations on the Limits of AGMs http://c34.org/bbs/index.php/topic,5977.msg63004.html#msg63004

I'll be the first to admit that I'm a bit deficient when it comes to electrical know-how on my boat... learning though.

Good. You can use a lot of the information in this next link, which includes many links to Maine Sail's material.
Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html

None of us was born an electrician.

 

[jssailem]

Stu you may not have been born an electrician, but I often think you have a genetic gift for the subject.

 

[Ward H]

Since you are going cyber shopping, I would explore MaineSail's site. ( https://shop.marinehowto.com/t/alternators--regulators I am pleased with the alternator I purchased from his site for my boat. It has upgraded my ability to charge my batteries, and with the external regulator I get more control of the charging system. Connecting the alternator to the battery has made the system more efficient.

Ditto +1 on buying from Maine Sail. Purchase and install advice as well as follow up customer service is great and very important to a DIYr

 

[arf145]

I don't usually wade in on electrical stuff, especially when MaineSail, who's a professional, and others who are very knowledgeable have already commented. But I think the OP has fixated on the alternator as a simple fix, when I suspect it just isn't going to take care of things in a mooring situation. The problem is time under charge, which is required for lead acid batteries to get back to 100%, no matter what you're pumping juice at them with. Are you really going to run the engine for hours each time you use the batteries? I'd look at the solar, which doesn't need to be all that expensive or all that complicated.

 

[ToddS]

I don't usually wade in on electrical stuff, especially when MaineSail, who's a professional, and others who are very knowledgeable have already commented. But I think the OP has fixated on the alternator as a simple fix, when I suspect it just isn't going to take care of things in a mooring situation. The problem is time under charge, which is required for lead acid batteries to get back to 100%, no matter what you're pumping juice at them with. Are you really going to run the engine for hours each time you use the batteries? I'd look at the solar, which doesn't need to be all that expensive or all that complicated.

In reading your response, and everyone else's, along with the articles others recommended, I think you're right. I was originally thinking alternator because I've been sailing for 20 years using only alternator to charge the batteries, so "I've just always done it that way"... Which is a bad mindset to have. I'm somewhat convinced on solar, though finding space to put it is obviously much tougher than the alternator. I have not a lot of open space on deck, two kids who love to wander the decks and break things, an open transom with no space there for solar power... So in all likelihood anything more than a partially-shaded 50 watt charger is going to involve modifying or building a dodger and/or a new bimini. The price of the panels themselves aren't nearly as scary as redoing dodgers and biminis... But yes... You're right... At least some of my focus should be on solar as well.

 

[ToddS]

@Maine Sail : So I pretty much spent my entire day at work yesterday digesting and researching stuff you (and to some extent others) said, and have taken much of it to heart. So given my current situation (no pun intended):

• 180AH house bank
• 90AH starting battery
• Yanmar 3YM30 deisel
• Sail mostly weekends off mooring in northeastern USA climate
• More willing than most sailors to be stingy with power consumption
• limited budget, and time (I suppose that's true of most everyone though)

My temptation is to:

• Buy and install a CMI-80-ER - 80A (or possibly CMI-90-ERH - 90A) Hitachi Replacement Externally Regulated Alternator with a Balmer ARS-5H regulator, plus a temp sensor... I'm fairly sure my tachometer runs off the alternator, but presumably the hookup/swap of this alternator will be pretty straightforward replacing my stock one. Fairly certain my belts are 3/8th... definitely single V... not sure how big I can go with alternator without modifications there, and I'm not at the boat currently to assess with certainty.
• Buy and install a solar panel or two and hook up (via a charge controller) directly to my house bank... once I figure out where (and how much) I can fit. I know i have room for something like a flexible 50A solar panel immediately, but more than one (or any 100's, or anything rigid) will require some bigger project work on my part which likely will be measured in months, certainly not days.
• Work the new alternator pretty hard initially (once it is set up) to get the batteries fully charged, and skip the fridge until everything is resolved. (I probably only use the fridge 10 days a year, so this isn't a big deal for me).

My "Energy Budget" is pretty light normally:
Interior lights - 1Amp X 3 hours = 3Ah
Chartplotter - 1Amp X 1 hour = 1Ah
Freshwater Pump 5Amp X .5 hours = 2.5Ah
Cell phones charging - 1Amp X 6 hours =6Ah
Refrigeration - 3Amp X 24 hours = 72Ah

Even if I add in a few other tiny energy uses (washdown pump for 10 minutes once per week, propane solenoid for 20-30 minutes per day.) I'm still under 15Ah per day without the fridge. Probably 20-30Ah per week is typical. The refrigerator is tricky to budget for me, because most of the time it goes unused, but for the 10 days per year (out of 40 I'm on the boat) where I use the fridge I need to boost SOME parts of my plan, but I'm not sure I need to supersize banks/everything to accommodate this as though I'm a live-aboard. A small 50 watt solar panel could keep up with my needs with zero alternator use for all but two or three times a year I turn on my fridge. Ironically, the only reason I started using the fridge was to cook a few more meals onboard to save money on eating out at restaurants... so I'm spending thousands to save hundreds. $20 worth of ice per year, and a 50W solar panel would be enough to keep my alternator doing essentially nothing at all without the fridge. Obviously I'm joking a little bit, and I know that's not exactly 100% true, since engine starting draws some and needs to replenish, and sometimes it might rain for two weeks straight making solar less useful, etc... But anyway... Do I sound like I'm on the right track with that alternator upgrade, and solar once I figure out physical size I can accommodate? I may even install 1 solar panel fixed, and have a 2nd one stowed away that I bring out just for the 2 weeks or so where I cruise and want to run the fridge...

 

[Maine Sail]

I get that you consider your use "light" but you're not accounting for PSoC walk-down due to never getting back to 100% SoC. With each successive use of the boat your battery gets lower and lower and lower...

We've not seen a 3YM30 with a 3/8" alternator belt. Most ever Yanmar sailboat aux engine uses a 1/2" "A" section belt.

So a 3YM30 should have a 1/2" wide belt and either the CMI-90-ERH or CMI-80-ER can work. The CMI-80-ER is a bit more of a "drop-in", because it is built on the Hitachi frame platform. The CMI-90-ERH may require a bit more tweaking to get alignment just perfect, but they both fit a Yanmar.

Either alternator can be shipped with a 3/8" belt, but again, a 3YM30 should have a 1/2" "A" section belt. I just looked at an invoice for one of our customers, with a 3YM30, and we had installed an "A" section (1/2" or 13mm top width) 37.5" belt.

When you do the alternator conversion, do yourself a favor, and do not reinstall the plastic alternator belt cover. They don't allow for proper alternator cooling and are only there due to lawyers and dolts who stick their fingers into a running belt.

You may need to "Belt Manage" either one to minimize belt dust, the alts won't mind this at all as it means they will run cooler and all things last longer when they run cooler.. We would always advise the use of at least an alternator temp sensor. The ARS-5H is an excellent budget regulator.

The benefit of an external regulator, over the factory Hitachi alternator, is in actually getting to a "healthy" absorption voltage which your AGM batteries desperately need.