Two alternators vs one big fat one?

[Saguday]

Any opinions from the cognoscenti about the virtues or otherwise of having two 80A alternators (say) rather than a single large one (120-150A for example)?

Our 54HP Yanmar drove a 110A Balmar alternator until it unfortunately departed for the great alternator party in the sky and we were faced with a bill of about £800 for a replacement. In the end we found a 95A one for half the price but really with a 550Ah house bank we need a bit more than that so given a bog standard 80A alternator can be had for less than half that would it make more sense to modify the engine mount and have two of those instead?

Is this easy/practical? Are there kits around to do it or is it a custom job?

At least if one gives up there is another one to limp along on.

 

[pvb]

Two alternators give you the advantage of redundancy. But adding a second alternator isn't easy. The bracketry involved can be complex, and has to be done properly or vibration will shake it all to bits. Yanmar may offer a second alternator bracket. I used a Volvo bracket kit to mount a second alternator on my old 2003T engine, it was an expensive kit but very well engineered.

On my last boat, I had a 660Ah bank supplied by a 90A second alternator with an Adverc booster, and never had any problems. They were simple maintenance-free batteries, and I rarely saw more than 70-80A charging current. If you have AGMs, they will accept charge more readily.

With only 550Ah, I don't think you need a complex solution. Had you thought of fitting a booster to the existing 95A alternator? Maybe a Sterling Alternator-to-Battery charger?

 

[PaulRainbow]

Some narrowboats use two alternators, one small one for the engine battery and a big one for the domestics, some have a 12v one and a 240v one. If you need more power for the domestic bank you could fit a second alternator. I'd wire one to the starter battery and the other to the domestic bank.

You then need some form of "split" charging to make the output from both alternators go to the domestic bank. The Victron Cyrix battery combiner is dual sensing. With a single alternator and two battery banks it will switch when either bank reaches a given voltage, usually switches when the engine starts and it senses the alternator output, but would equally switch when the domestic bank reaches the appropriate voltage due to, say, a mains charger or solar array only being connected to the domestic bank.

With twin alternators, one connected to each bank, it would switch when you start the engine and the output from both alternators would go to all batteries. An advantage with the Victron is that if either alternator failed, the remaining alternator would still charge all batteries.

 

[lw395]

Probably optimum is a second alternator with a second engine to drive it?

Seriously, the quuestion is hard to answer without knowing how often and why you see the need to use more than a single 'normal size' alternator will supply.

If you are often running the engine simply to charge the house batteries, maybe a generator is worthwhile.
Maybe the cash would be better spent on solar panels or reducing power use?

Can twin alternaors be cooled more effectively?
Will they split the load between them properly or will one work hard and die young?
I don't really buy the 'redundancy' idea, a car alternator can easily be carried as a spare.

 

[Deleted member 36384]

On another sailing web site, aimed specifically at 'Attainable Adventure Cruising' (https://www.morganscloud.com), they specifically state that twin alternators are not required and is now a redundant idea due to the reliability of modern alternators. As the web site is aimed specifically at recommending tried and tested methods / equipment that offer reliability, they might have a point worth considering. This is what they say on the matter: -

https://www.morganscloud.com/2013/11/06/10-tips-to-buy-and-install-a-liveaboards-alternator/

We have found that a large case alternator, down-rated to 80% of maximum output, lasts 1500-2000 hours of heavy liveaboard use before the coils or bearings fail, or both. This has been a remarkably consistent statistic over about 12,000 engine hours on our boats. I would guess that you will want to plan on about half that for a small case alternator, particularly if it’s not down-rated (see above). The nice thing is alternators usually give you warning with bearing squeal or slowly dropping output before dying completely.

When we detect either symptom, we swap in our spare—less than 30 minutes’ work—and have the old one rebuilt by a reputable alternator professional.

 

[prv]

Some narrowboats use two alternators, one small one for the engine battery and a big one for the domestics

That's a nice reliable and simple way of avoiding the question of split charging, but it always strikes me as a bit of a waste given how little charging is needed for the starter battery of a healthy engine. After a few seconds of running the energy taken has been put back in, and from then on one of your alternators is doing very little. Seems a pity when the service bank could be making good use of it.

Pete

 

[pvb]

That's a nice reliable and simple way of avoiding the question of split charging, but it always strikes me as a bit of a waste given how little charging is needed for the starter battery of a healthy engine. After a few seconds of running the energy taken has been put back in, and from then on one of your alternators is doing very little. Seems a pity when the service bank could be making good use of it.

But, as PaulRainbow went on to explain, you can combine the output from both alternators easily.

 

[FullCircle]

I have looked into this and have the Yanmar kit and a second 80a alternator in the shed. I haven't fitted it because there was some evidence required of necessity. So, the first thing I considered was getting the maximum out of the alternator I have. I fitted a Sterling Alternator to Battery unit which prioritises the start battery. It really runs the alternator hard and gets a lot more amps out of the alternator over my previous Adverc or the older split charge diode system. I also fitted AGM2 batteries, as these take a deeper discharge than normal Lead Acid, and can also take a higher rate of charge. So, you get more Ah out, and can put more Ah back in quicker. If you have Lead Acid bank, you will get approx 280Ah of useable leccy from 550Ah bank. With AGM, a possible 440Ah useable.
Then you need to look at the belt setup on the engine. It is recognised that a normal toothed V belt is good for about 80Ah alternators, over that, you need multi V like you have on your Balmar. Now consider having 2 of those big pulleys on the engine and the complicated setup of the new brackets. Gets expensive. I would bet that you have already removed most of the power draining lighting and Nav lighting in the boat with replacement LEDs, so you already have reduced power drain by a factor of 7 to 10.
As solar is so cheap now, you may as well get that free. No moving parts, no wear and generally reckoned to give 20 years service. Add a similarly cheap MPPT controller, and you are there fo rless than half what you spent on a cheap alternator. By the way, my cheap 80a Hitachi alternator was £63. And it works when the engine is off. So, I am ramping up the solar to approx 300w and plan another 150w when I push off.

In other words, I think you need to be a bit more lateral Neil!

 

[prv]

But, as PaulRainbow went on to explain, you can combine the output from both alternators easily.

Which is another possibility, but isn't the configuration I was addressing, as is apparent from the fact I didn't quote it

Some people do wire the starter and service sides as completely independent systems each with their own dedicated alternator, and I was giving my opinion on that.

Pete

 

[William_H]

What OP really needs is a record of amps actually supplied by the alternator in real life. I think the statement "550AH house needs a really big alternator" may be spurious. Much depends on the speed you want to recharge, the degree of discharge of the batteries and engine RPM. My suspicion being that a cheap 80 amp alternator may well do the job most of the time and if it is too small may get hot or take longer to recharge batteries. Even then you could well afford to carry a spare. Or can the old big alternator not be repaired?
Alternator mounts do give trouble due to vibration etc so you may be introducing more potential for trouble with additional alternator. good luck olewill

 

[geem]

Any opinions from the cognoscenti about the virtues or otherwise of having two 80A alternators (say) rather than a single large one (120-150A for example)?

Our 54HP Yanmar drove a 110A Balmar alternator until it unfortunately departed for the great alternator party in the sky and we were faced with a bill of about £800 for a replacement. In the end we found a 95A one for half the price but really with a 550Ah house bank we need a bit more than that so given a bog standard 80A alternator can be had for less than half that would it make more sense to modify the engine mount and have two of those instead?

Is this easy/practical? Are there kits around to do it or is it a custom job?

At least if one gives up there is another one to limp along on.

I really dont think your battery bank size warrants anything as elaborate as two alternators or even a particulary large alternator. We have an 800 amp hr domestic bank and use a bog standard 60 amp alternator at 24v so equal to 120amp at 12v. We never see more than 50 amps from the alternator even with a sterling reg. Our friends have the same boat with over 1000amp hr domestic bank, the same alternator and no smart regulator. Neither of us have charging issues from the alternator and we have the advantage of standard set up. Our alternators are £100 new. We just carry a spare one.

 

[Heckler]

I really dont think your battery bank size warrants anything as elaborate as two alternators or even a particulary large alternator. We have an 800 amp hr domestic bank and use a bog standard 60 amp alternator at 24v so equal to 120amp at 12v. We never see more than 50 amps from the alternator even with a sterling reg. Our friends have the same boat with over 1000amp hr domestic bank, the same alternator and no smart regulator. Neither of us have charging issues from the alternator and we have the advantage of standard set up. Our alternators are £100 new. We just carry a spare one.

Nail on head! We are similar, 4 X 110 amps plus the starter one. Standard Valeo alternator, 60 amps, with a Sterling reg. I actually suspect now that the Sterling is not necessary. Have only ever seen over 20 amps charging a couple of times after a couple of cloudy days on the hook.
Better spending his money on some solar panels. Our mate John had a Balmar and fancy reg, a waste of space in my opinion, all it did was chew belts!
Stu

 

[Saguday]

Thanks for the comments, even the slightly passive-aggressive ones concerning the paltry size of my battery bank... and yes, I do understand about solar panels.

Just to be clear how we're set up:

- 5 x 110Ah AGM house bank
- 1 x 110Ah AGM starter
- 2 x 130W solar panels into a multi-stage charger
- (until recently) Balmar 110A alternator
- (now) unknown European 95A alternator equivalent

Advice and received wisdom from various sources including alternator and charging specialists was that the alternator should be rated about 25% of the battery bank it's charging - our Island Packet came originally with 3 x 100Ah house batteries and 1 x 100Ah starter so the original-fit Balmar alternator at 110A is about right on that basis for a 400Ah bank.

I've subsequently added 2 more house batteries and a year ago replaced the originals to bring the total now up to 660Ah. If the Balmar had still been functional I wouldn't have been bothered about uprating it but since it's now completely seized (total of 670 engine hours since new, by the way) and I was forced into replacing it at what I consider to be significant expense, it seemed reasonable to look at any alternatives. By the way, never had any problems with belts on the Balmar.

Given a bog-standard 80A alternator is £100 or so it did therefore seem tempting to buy two of those and take the remaining £600 left over from the cost of a new Balmar unit to do some mechanical mods to fit them onto the Yanmar - credit to our diesel mechanic for suggesting the idea. I'd theoretically get 160A of charging into the 660Ah bank (near as dammit 25% but of course less in reality) and have some redundancy built in since if one failed for whatever reason there'd still be one available to provide basic charging. Plus they're cheap to replace and readily available if they do go. Hence the thread.

In the end the 95Ah alternator we had fitted does the job but per the above is arguably small for the size of bank it's charging. Yes, it just takes longer to charge the batteries but the whole point to me with a sailing boat is to reduce the number of hours running the engine just to charge batteries - it's noisy smelly and doesn't do the engine much good. The solar panels I fitted did make it possible to lie at anchor for several days without needing to run the engine when we were liveaboard, although that was in the US mid-atlantic states and there was considerably more sunshine. Adding more solar panels at some point is a given, but that's a different thread.

 

[pvb]

You can effectively ignore the starter battery - it won't take any appreciable charge. You can also effectively ignore the "advice and received wisdom" about needing an alternator rated in amps at 25% of the bank capacity in Ah. The important thing is to know how much current your 550Ah bank will accept. If you have a battery monitor, check how much current is flowing from your 95A alternator. If you're regularly seeing 80A or so, there may perhaps be justification for increasing the alternator capacity.

 

[Heckler]

You can effectively ignore the starter battery - it won't take any appreciable charge. You can also effectively ignore the "advice and received wisdom" about needing an alternator rated in amps at 25% of the bank capacity in Ah. The important thing is to know how much current your 550Ah bank will accept. If you have a battery monitor, check how much current is flowing from your 95A alternator. If you're regularly seeing 80A or so, there may perhaps be justification for increasing the alternator capacity.

Again another nail on head! I dont understand this obsession with putting big alternators etc on. If he isnt seeing a big charge, a bigger alternator wont make any difference. The two solar panels should be keeping a good charge going in anyway.
Stu

 

[Bobc]

Again another nail on head! I dont understand this obsession with putting big alternators etc on. If he isnt seeing a big charge, a bigger alternator wont make any difference. The two solar panels should be keeping a good charge going in anyway.
Stu

Quite. I have 6 x Trojan T5s, giving 625Ah as my house bank, and have a 60A alternator and a Sterling AtoB, which does the job perfectly well.

 

[lw395]

Again another nail on head! I dont understand this obsession with putting big alternators etc on. If he isnt seeing a big charge, a bigger alternator wont make any difference. The two solar panels should be keeping a good charge going in anyway.
Stu

There is a case for it on some boats, if there is a need to run high loads like thrusters or windlasses without draining the batteries. Particularly if you want to run these things at 13V instead of 11V.
Boats and boat usage vary a lot!

 

[pvb]

There is a case for it on some boats, if there is a need to run high loads like thrusters or windlasses without draining the batteries. Particularly if you want to run these things at 13V instead of 11V.
Boats and boat usage vary a lot!

Unless you're thinking of shipping, things like thrusters and windlasses on leisure boats are used for incredibly short periods, and any battery drain will rapidly be replaced by an ordinary system.

 

[dom]

Unless you're thinking of shipping, things like thrusters and windlasses on leisure boats are used for incredibly short periods, and any battery drain will rapidly be replaced by an ordinary system.

Agreed, but lw395 may be referring not just to the capacity loss, but to the greater power one can get from a thruster where the operational voltage is say 13.5V than 11.5V. I personally know boats like this, though it's basically a function of an under-specified thruster.

 

[pvb]

Agreed, but lw395 may be referring not just to the capacity loss, but to the greater power one can get from a thruster where the operational voltage is say 13.5V than 11.5V. I personally know boats like this, though it's basically a function of an under-specified thruster.

Low voltage at the thruster is usually a cabling issue, rather than a thruster issue. But thrusters (and often windlasses) are usually used with the engine running, so voltage should be good, even on a standard system.