Add 2nd alternator for less charging time?

[Alec H]

My Yanmar 3JH5E, 39 hp with 4000 hours, came equipped with an 80 amp alternator. The 12v has been upgraded to 800 amp/hr house battery bank, no solar or wind gen. I was advised by the electrician who installed the new batteries and regulator, Victron stuff, to keep the batteries above 80% SOC, and recharge to 100% whenever possible. When I run the engine to charge batteries, I see that the rate of charge never exceeds 60 amps, I assume this is because the SOC is above 80%. However, I find I am running the engine longer than with my old batteries; usually two hours per day. SO, if I add an additional 80 amp alternator, could I expect to run the engine for half the time? I understand that adding an alternator will require additional V-belts. Solar and wind are costly; could this be a better way to save on engine hours?

[Cheechako]

Well it's hard to say definitely but yes, more alternator output should help. For say a 100 amp rated alternator, I would usually see maybe 85 amps for the first 10-15 mins only. Then the charge rate backs off. This was with an external high output regulator too. If you were using an internal regulator it may be nothing more than an automotive type which only charges high for a very short time... as that's all a car needs.
One of the fallacies of a large battery bank is without the ability to charge it, it doesn't help much.
It may be a lot easier for you to put on a good 100 amp alternator and regulator, unless you already have an extra V belt sheave on your motor. I don't know the Victron is it a high output regulator? Is the charge curve adjustable?
Charging to 100% is a losing battle, the charge curve drops off so far that you are doing little. Let's face it, your batteries live in a state of discharge 92% of the time anyway. They are only 100% for a few minutes even if you could get there. That's life.
Same alternator.
Different batteries + different regulator. Must be one. Probably the regulator? Or maybe it's just the bigger bank is getting drawn down further and requiring more?

[john61ct]

What specific model batteries?

Have you determined their max CAR C-rate when depleted to say 50%?

Increasing current in the early Bulk stage might save a few hours, but

the lower-amp stages as SoC climbs will still be 5-6 hours to get to 100% Full.

But only doing that a few times a week is "good enough" to prevent reduced longevity from chronic PSOC.

High C-rates are good for AGM longevity regardless of time savings, .2-.4C is the **minimum** reco by quality makers, 160-320A in your 800AH case.

[a64pilot]

OK first it’s exceedingly unlikely your fully charging your bank in just two hours, regardless of charge source size, even if at 80%, it’s far more likely that your slowly walking down your banks SOC.
You need Solar, or a Honda or other generator in my opinion, or run far longer than two hours.
Second yes a larger charge source will help, but maybe not as much as you would think, especially if you are recharging from 80% SOC.
Go here, and read the article on how fast can an AGM battery be charged.
https://marinehowto.com

Actually sit back and read all the articles about battery charging, and you will learn a lot.

[NYSail]

Originally Posted by Cheechako

Well it's hard to say definitely but yes, more alternator output should help. For say a 100 amp rated alternator, I would usually see maybe 85 amps for the first 10-15 mins only. Then the charge rate backs off. This was with an external high output regulator too. If you were using an internal regulator it may be nothing more than an automotive type which only charges high for a very short time... as that's all a car needs.
It may be a lot easier for you to put on a good 100 amp alternator and regulator, unless you already have an extra V belt sheave on your motor. I don't know the Victron is it a high output regulator? Is the charge curve adjustable?
Charging to 100% is a losing battle, the charge curve drops off so far that you are doing little. Let's face it, your batteries live in a state of discharge 95% of the time anyway. They are only 100% for a few minutes even if you could get there. That's life.
Same alternator.
Different batteries + different regulator. Must be one. Probably the regulator?

Factory settings on the balmar mc614 regulator has 19 minutes bulk, 19 minutes absorb then switches to float. You have to manually go in and program regulator to match battery specs. For instance my absorption time is now 3 hours and my batteries love it after 3 days on the hook. Of course if batteries don’t need that power it will go to float sooner. An internal regulator no matter how many will never fully charge your big bank I’m a reasonable time. But many experts on the subject will be chiming in soon I am sure

Greg

[john61ct]

Originally Posted by a64pilot

OK first it’s exceedingly unlikely your fully charging your bank in just two hours, regardless of charge source size, even if at 80%, it’s far more likely that your slowly walking down your banks SOC.

Yes, missed that claim. Very common for people to trust Full indicators that lie, drop to Float 3-4 hours too early.

No way anyone gets to true Full in under 4 hours with a lead bank when deep cycling, even the last 5% takes hours.

> You need Solar,

or Shore power

> or a Honda or other generator

no ICE source makes sense for the long tail, when only say 10A or less are being accepted by the bank.

[stormalong]

You do not say if your existing alternator has an internal regulator or an external, multi step (smart) regulator. A stock alternator with a stock regulator will never meet your charging needs. A second alternator is a good idea.

Adding solar is a good idea regardless of what you do with the alternator(s). Whatever charging profile you use the charging rate will drop off to very low current as the SOC nears 100%. There comes a point that running the engine is a waste of fuel and wear & tear on the engine. This is where solar helps. Solar will top up that SOC and keep it there without the noise, heat and ... of running the engine. Also solar is quite inexpensive today compared to 10 or 20 years ago. It is light weight, passive and usually out of the way. The only maintenance is to wash it occasionally.

[907Juice]

I just spent the money on a portable generator... yeah it is bigger and heavier, but it has multiple purposes. And probably cheaper once you add in extra pulley, belt, alternator, external module, etc. For the 2-5 day cruiser seems like a no brainer.

[smac999]

replace it with a ~150a, pulley kit, and external regulator. 2 small ones would be silly.

the 80% drain limit is silly. and you'll never charge them to 100% with an engine. normally you stop at 85-90% or so. starting from 50%. to get from 85-100 you need solar or a dock. or 5+ hours of motoring. durring which very little charging is happening and you are just wasting fuel.

[KP44]

I'd say keep it simple. Replace your alternator with a 100 amp or 150 amp alternator with external, updated regulator.

Or consider having two isolated systems. I have the alternator that came with the engine dedicated to the engine battery. I have a 150 amp alternator for the house bank of about 900 amp hours.
I thought it would be a nightmare running two systems but it has worked flawlessly for about 10 years. It gives the redundancy I always want for sailing in remote areas.

[MJH]

Originally Posted by Alec H

My Yanmar 3JH5E, 39 hp with 4000 hours, came equipped with an 80 amp alternator. The 12v has been upgraded to 800 amp/hr house battery bank, no solar or wind gen. I was advised by the electrician who installed the new batteries and regulator, Victron stuff, to keep the batteries above 80% SOC, and recharge to 100% whenever possible. When I run the engine to charge batteries, I see that the rate of charge never exceeds 60 amps, I assume this is because the SOC is above 80%. However, I find I am running the engine longer than with my old batteries; usually two hours per day. SO, if I add an additional 80 amp alternator, could I expect to run the engine for half the time? I understand that adding an alternator will require additional V-belts. Solar and wind are costly; could this be a better way to save on engine hours?

Are you sure you have an 800amp/hr. bank...that is equivalent to about 3-4 8D size batteries at about 160 lbs. each?

I believe the rule of thumb is an alternator capable of 25% of your total bank size which would be about 200A in your case. In addition to the new alternator you will need a serpentine belt modification kit to handle it which is not really a bad idea anyway. I priced the addition of a second alternator for my boat and the total price for parts alone was ridiculous. Besides, your engine already has 4,000 hours I don't think of that as a wise or timely investment. Most new engines already come with a much larger alternator and serpentine belt.

I have 4x140W=560W solar panels on my boat with a Balmar 110A alternator and regulator to support the 2x8D Gel battery house bank plus starting Gel battery. I would strongly recommend solar to assist in keeping that large house bank charged. How big, as much as you can afford and room to install...perhaps panels atop dodger and stern arch that will always be working versus the hanging lifeline type panels only occasionally used. I found solar panels to be reasonably priced but everything else associated with them drives up the total cost...do a lot of research first. Lastly, I found locating someone to build a stainless steel stern arch at a reasonable price very difficult but eventually did at half the price.

Good Luck.

~ ~ _/) ~ ~ MJH

[Maka]

I agree.
A generator for recharging batteries is the way to go.
You can perform other tasks with it while its charging batteries too.

[CMark1]

This is an example of a secondary alternator system.
Maybe a useless story.
Had the pleasure of delivering a 46' Swan years ago that originally had an an engine driven compressor for the refrigeration system (similar to what you vehicle AC system uses) When the owner upgraded to electrical freezers/fridges, they talked about installing a generator. The yard suggested that by going the generator route would take away a significant amount of internal storage, modify the boat up unnecessarily, and be a large financial cost. Plus another engine to maintain. They opted with installing a secondary 130A alternator in the space where the compressor lived originally, with an external regulator. The process I believe was much simpler. Bracket fabrication to the engine/engine bed. Secondary take off pulley to drive the alternator. Wiring the regulator/batteries. The owner was very happy with the setup and the overall cost was reduced. Cant tell you how long it took to charge but clean install.
thanks for coming to my tedtalk

spam alert
ps. if anyone is in the market, I've got a new 12V 130A Mastervolt w/ External Reg listed in classifieds.

[Pizzazz]

Drop the 80% SOC goal and you will be happier without any modifications. Also, note the charging voltage. If it is less than 14.4V you need either an external regulator or a different alternator.

The long of it is as follows. Wet batteries have high charging efficiency up to 80% SOC, then it drops down quickly. The best use for wet batteries is between 40 and 80% SOC. In this range you will have approx. 300AH useable capacity, that will take 5 hours to recharge. If you want faster charging, you have to decide if it is cheaper to add a second alternator or install a second belt. It depends on your setup, one larger alternator is always better.

In your current mode of use, charging only in the 80-95% range, a second alternator will not help at all.

[john61ct]

If you have a big expensive bank, far better to get them to 100% Full as frequently as possible.

Correct, more high-amp ICE charge sources do not help accomplish that, once you already have a high enough C rate to maximize the early Bulk stage.

But in most cases, a bit of solar for that final 4-5 hours after 80% SoC, will get the job done without great expense.