Will a 150AH alternator work on a Universal M-25XPB engine?

[Onemoreproject]

I am trying to achieve a balanced recharging system for my batteries. I have 6 dead GC2 flooded lead acid batteries that I am replacing with AGM GC2 (6v Golf Cart) batteries. AGM’s will charge much faster, but will need a bigger alternator than 70AH to provide enough power. I have been reading that the alternator output should be a minimum of 20% of the total AH of your battery bank and that 25% to 40% is preferred. To provide that much charging ouput I would need at least a 135AH alternator. Does a 25hp Diesel engine have enough hp to drive an alternator that big? I may not be able to properly charge a 675AH battery bank and be limited to a smaller bank of just 4 GC2 battery bank of 450AH.

I am also increasing my solar from 220 watts to 320 watts. I use about 100AH a day.
When I had only 220 watts of solar that was not enough power to keep the batteries topped off. Would the charging efficiency of AGM batteries be able to absorb enough energy to keep my batteries topped off?

Your input is appreciated!

[Stu Jackson]

What is you daily energy budget?


Yes, your sizing ideas are within guidelines, but much of the "balance" of a SYSTEM is based on BOTH how much storage (battery bank size) you have as well as what recharging (alternator) sources are available.


I have a 21 hp diesel with a 100A alternator with a 400 ah house bank and a 100 ah daily load. Thus, I can go (2) 24 hour days to meet the 50% capacity of my house bank. My external regulator controls the alternator output to about 75% of its 100a because no small case alternator is made that sustain a continuous load at its maximum for any period of time.


All this means is that you MUST control your alternator output to avoid burning it up, which, in turn, means that you simply have to run the alternator longer to replace the power you use from the storage medium.


Don't confuse maximum Amp level replacement goals and recommendations based on battery chemistry with the ah required to be replaced regardless of storage medium/chemistry.


That's both an advantage and disadvantage of AGMs.

[Jon Hacking]

One, there are a few issues here.

First, in technical discussions, units matter. There are Amps (A) that measure, say, the instantaneous output of a charging device, or an instantaneous discharging load. And then there are Amp-hours (Ah) which include a time component, like how LONG your batteries can put out that discharging load. Battery capacity is measured in Ah, while alternator output is measured in Amps. Sorry if I'm sounding pedantic, but units matter, especially to us EEs.

I don't know your engine, but if it uses a standard 1/2" fan-belt, then the max current you can really suck down that belt is about 90A. Much more than that & you'll turn your fan-belt into black dust thinly spread over your engine-room. Wider Serpentine belts can handle more, but conversion kits are expensive, as they usually have to drive the water-pump as well.

As for your engine handling the load, 90A@14v=1,260W=1.7hp. For reference, so you can run these numbers yourself, 1KW=1.34hp. Alternators aren't 100% efficient, of course, so you have to round up smartly, but it will cost about 2hp out of your engine to pump 90A into your batteries, & ~3hp for 135A. You'll probably feel that with your 25hp engine, but only if/when you're trying to race into port ahead of a storm.

Perhaps more to the point, only LiFePO4 batteries will accept much charge when they're almost full. Even AGMs will get up to their "Acceptance" voltage pretty quickly, & then your regulator should back off the current to protect your batteries. Charging ANY lead/acid battery completely takes a LONG time, even with a powerful alternator. The alternator is rarely the limiting factor.

Perhaps I'm getting ahead of myself here (& please excuse me if you already know this). ALL of your charging sources should really use 3-stage charging. In the Bulk stage, the charger (alternator, MPPT, etc) pumps out as much current as it can, until the battery voltage gets up to its "Acceptance" voltage - which is temperature & chemistry dependent for lead/acid batteries, so your regulator should have a temperature sensor for your batteries. Then the charger should hold that Acceptance voltage until the battery is full, before switching down to a "Float" voltage that the batteries can sit at all day without damage.

The trick, of course, is determining when the battery is full. Most chargers just hold the Acceptance voltage for a certain amount of time, typically 2-hours, which is patently silly. This really requires that your charger (alternator regulator, MPPT, etc) monitor the *current into the battery*. When the battery first gets to its Acceptance voltage, the charger is still pumping out as much current as it can, but as the (lead/acid) battery gets more charged, it takes less & less current to maintain that Acceptance voltage.

When the current required to maintain the Acceptance voltage drops to ~1% of the Ah capacity of the battery, the battery is full, & the system should switch to float. In your case, with a 675Ah bank, when the current required to maintain the Acceptance voltage drops to ~7A, the bank is full & the system should switch to Float. This is called a "tail-current" charge, & it's the best way to charge any battery, IMO, & especially lead/acid batteries.

Hopefully, your MPPTs already do this. Alternators need a bit of help. Not only can an internal regulator only "sense" the voltage at the back of the alternator (which can be VERY different from the actual battery voltage) but internal regulators are really only designed to replace what was used to start your engine (~0.3Ah) & to keep the lights on. They only have a single set-point voltage, & that voltage is purposely set low enough that it won't damage your batteries if you have to motor all day. Sub-optimal, to say the least.

All of this is a long-winded way to say that you should really invest in an external alternator regulator, if you want to maximize your current into your batteries while still protecting your (expensive) batteries. I suspect that an external regulator on your current alternator will actually perform BETTER than upgrading your alternator.

There are several external regulators on the market, of course, but my favorite is the WakeSpeed WS500 (full disclosure: I have no financial interest here, but the chief designer is a friend, & I helped him optimize his code). Microprocessor based, separate sense leads for battery voltage & current (so it can do a tail-current charge), temperature sensors to protect both the battery & the alternator, preset profiles for all major battery types (as well as a custom profile for us geeks), CAN-bus & USB communications, etc, etc. It's expensive, but it's good kit.

But the other thing you're probably noticing is that a slow but constant current is one of the best ways to charge your batteries, & that's exactly what solar-panels do. With prices now down to under $0.50/W, I'd suggest doing what you can to maximize your solar. 320W won't quite keep up with 100Ah/day, even here in the tropics, but it will certainly help. In 2019, we tossed out our 4x120W panels & replaced them with 3x400W panels, since the panels were only $200 each here in the Philippines.

Good luck!

[CarlF]

Is there some reason you are switching to AGM? They can suffer from PSOC (partial state of charge) sulfation unless recharged to 100% regularly -something that is hard to do unless you plug into shore power most nights. PSOC can kill an AGM battery in a year or two. And, as you've discovered, they need to be charged at fairly high amperage to avoid sulfation. Many cruisers have decided that AGM's are inappropriate for a house bank (they are fine for a start bank that is never deep discharged)

Why not just replace your flooded golf cart batteries with a another set? How long did the current set last you?

Or bite the bullet and upgrade to lithium.

[Onemoreproject]

Thanks for a great explanation. I have a Hart external regulator and Baldor 70A alternator. On my last boat, I had a cutoff switch so that if needed max hp in big seas I could stop the alternator from stealing hp from the engine. If I can fit a 150A alternator on this little engine, I may do the same thing. I want to ugrade to AGMs because they can charge faster than flooded batteries, but maybe they aren’t the best choice for a cruising sailor that rarely plugs in at a marina? I am wondering if I would be better off with just 4 AGMs? Would the faster charging rate offset the need for a larger bank and I could get by with a 468AH bank? My other thought is that 2 100AH lithiums cost less than 6 AGMs. If I got 3 lithiums it would cost 500 more than 6 lithiums. What do you think about a 4 battery AGM bank or a 3 battery lithium bank?

[Onemoreproject]

Thank you for your insight. The reason I want to upgrade to AGM is because they charge much faster to eliminate PSOC. Same reason to upgrade to a bigger alternator is to eliminate PSOC. I am considering all options. Flooded with more solar, AGM or even lithium. Both of which would require upgrading alternator and external regulator. I just want an integrated system that will charge quickly and reliably for optimal long battery life. I am adding a 100w Soliban panel to my dodger to increase solar to 330AH. But instead, may add 2, 100w panels to hang on lifeline rails and flip up. Less shadowing in that location and will double my solar instead of increasing by ⅓. The flip up panels will require stainless upgrade from lifeline wire to tubing around the cockpit. Lifeline upgrade would be about $1000.00 in SS work. No free lunch in any of the above scenarios.

[sailorboy1]

Originally Posted by Onemoreproject

AGM’s will charge much faster, only down around 50%-70% state of charge .... maybe


To provide that much charging ouput I would need at least a 135AH alternator. plus the supporting belting system

I may not be able to properly charge a 675AH battery bank and be limited to a smaller bank of just 4 GC2 battery bank of 450AH. all I have have had for 6 years fulltime and I normally never below 75% state of charge

I am also increasing my solar from 220 watts to 320 watts. I use about 100AH a day. then definitely don't need more that the 4 battery system

When I had only 220 watts of solar that was not enough power to keep the batteries topped off. Would the charging efficiency of AGM batteries be able to absorb enough energy to keep my batteries topped off?doubtful, very doubtful

Your input is appreciated!

In my opinion - go with the 450AH bank, increase the solar as much as possible, forget the 600AH battery system and the higher alternator and save the extra $$$$ involved

In end it is better to be able to get the smaller bank fully recharged than to have the bigger bank

[michaeld]

We also burn about 100 Ah per day (refrigeration). My experience is that 350W solar is only adequate on sunny days to get our 400 Ah AGM bank back to 100% SOC and SOH. I plan to add another 175W panel.

[Jon Hacking]

Originally Posted by michaeld

We also burn about 100 Ah per day (refrigeration). My experience is that 350W solar is only adequate on sunny days to get our 400 Ah AGM bank back to 100% SOC and SOH. I plan to add another 175W panel.

The amount that you get out of solar panels is entirely dependent on where you are, & what your weather is.

As part of his schooling, we caused our son to do an energy budget for Ocelot.

One of the interesting things to drop out of that analysis is that dividing the wattage of your panels by about 3 will get you pretty close to how many Amp-hours (Ah) you'll get into your batteries on a sunny day in the tropics. This is for horizontally mounted, unshaded panels in free air. (Taxes, dealer prep, & destination charges extra, your mileage may vary, bla bla bla...)

This is a very useful number that solar panel vendors are unlikely to give you. Unfortunately, as I say, it depends strongly on where you are. Once you start moving out of the tropics, you'll get less into your batteries, especially in the winter, when most of the sunlight will reflect off the glass surface of the panels.

We talk more about this on our Solar Panels page (which needs a bit of updating, as we recently threw out our 4x120W panels, bought at $4/W, & replaced them with 3x400W panels, paying only $0.50/W!).

[Jon Hacking]

One, going with LiFePO4 is a whole new can of worms that you probably want to study pretty thoroughly before diving into. The main issue (for us) is that cell construction is still new enough & inexact enough that the cells don't share the charge & discharge duties as exactly as lead/acid cells. This can cause them to become imbalanced, with individual cells becoming too high or too low in voltage. Your Battery Management System (BMS) will usually monitor this, & will usually provide a bit of balancing current, & you can buy better, independent Active Balance Modules, but it's something folks coming from lead/acid batteries don't usually think about.

The good side, of course, is that they'll happily slurp up as much current as you can throw at them. Which means, of course, that switching to LiFePO4 is ALREADY an alternator upgrade, since your alternator will now produce much more usable current for your batteries. In fact, you need an external regulator that monitors the temperature of your alternator, or you risk it getting too hot & damaging itself, as alternators are not actually designed to put out their "rated" current for very long.

Our first LiFePO4 bank, 12x100Ah cells in a 3P(arallel)4S(eries) arrangement suffered a severe imbalance event after about 2.5 years. I later found that my BMS hadn't run a single balance cycle in all that time, so it's kind of amazing the cells lasted that long. I eventually found the single bad cell, but the company had died, so I extracted the 8 best cells & turned them into a pair of 1P4S 100Ah@12v start batteries (we're a cat).

The house bank we replaced with 8x270Ah cells wired into a pair of 1P4S batteries in parallel. Paralleling the batteries instead of the cells means that I need 2xBMSs, but it lets me monitor each cell individually, on small readouts next to my nav-desk & my other power controls (MPPTs, Ah, etc).

[Onemoreproject]

Thank you for all your various inputs. I decided that I would not go with Lithium because there are too many variables and I don’t know that there are competent people in Mexico to make sure that all the components are working correctly to support a lithium installation.
I did increase my solar capacity from 220w to 465 watts. Instead of 6 flooded 6v I am replacing them with just 4, 220AH Lifeline AGM batteries. I am getting a new controller, charger and battery monitor to be sure they are being maintained correctly. I’ve gone through 2 sets of flooded batteries in 4 years and dont want to repeat that again! Mostly because the people watching my boat did not take care of he batteries. I am keeping my 70A Alternator with external regulator. Mexico is very sunny and I doubt I’ll need a bigger alternator.
Thanks again everyone for sharing your knowledge.