Alternator output power - excessive?

[skyflyer]

the PO of my boat proudly told me that he had upgraded the alternator to one with "120 amp" output, (I think standard fit was 90 Amp)

I'm just wondering why I need that?

The battery bank (2 x 190 Ah wet lead acids) only takes charge at a rate of about 20 A when discharged to 50% (I avoid discharging below this level). This current then rapidly decreases anyway. So even if both batteries are 50% discharge the most I need from my alternator is 40 Amps, surely?

I have an Adverc smart charger system which adapts alternator voltage output anyway by altering field coil current, to ensure rapid charging.

No harm in having more output in case I increase my battery bank capacity, I guess, but I have noticed quite a few posts over the years which suggest that alternator output is the b-all and end all of it.

Looking forward to an electric lesson!

Cheers

 

[Graham376]

Batteries will take what they need and I rarely see out alternator putting out more than 25-30 amps. Presumably, the previous owner of your boat also uprated the belts and pulleys?

 

[skyflyer]

No idea but he had the yard do it, so must be correct :ambivalence:

 

[knuterikt]

With 400Ah capacity batteries at 50% SOC I would expect charging at 60 to 80 A.
It would drop quickly. Running a generator at max capacity over some time will often damage it due to heat buildup.
I have LiFePo4 batteries that can accept higher amps over longer time. To avoid frying the generator we have regulator that measures generator temperature and throttles the generator to keep temperature bellow 96c.

 

[pvb]

the PO of my boat proudly told me that he had upgraded the alternator to one with "120 amp" output, (I think standard fit was 90 Amp)

I'm just wondering why I need that?

Lots of boat owners wrongly believe that a 120A alternator will push 120A into the batteries. It won't, unless the batteries can accept that current. In most cases, the batteries installed won't accept anywhere near that current.

Bigger battery banks can accept more charge, so adding extra batteries is often a good idea. If replacing batteries, AGM batteries can accept high charge currents, so can be a good choice.

 

[William_H]

As said 120 amp is the max capability of the alternator itself. Achieving this current output requires that the alternator be rotating at specified RPM under load and that you have a load which will take 120 amps. Almost certainly your batteries taking charge will never take that current but if you had ie an anchor winch or thruster then you might get to that current under ideal conditions. Then only for a short while. So yes it is good to have a high current alternator but the original was probably quite adequate. Less chance of the alternator getting hot under extreme load having a bigger one. Hopefully a longer life from bigger alternator. Not really aconcern anyway. ol'will

 

[VicS]

As already said 120 amps is unnecessarily large for the existing battery bank but according to the manuals the standard fit is only 35 amp, together with a 70 Ah starter battery. The previous owner obviously decided to go large rather than just larger.

It is not clear if the two batteries are combined into one bank or wired separately but I would suggest that they should be permanently wired as one bank and a smaller one added as an engine start battery.

I would expect to see a higher charge current than 20amps if they are healthy and 50% discharged . Perhaps this should be investigated.

If you do not have the original manual for the Adverc and cannot find one on line (I can't) there are some useful extracts regarding checking its operation in this this thread: http://www.ybw.com/forums/showthread.php?479336-Adverc

 

[sarabande]

Perhaps the PO ran a hairdryer or similar from an inverter and needed to feed the batteries to power the inverter that dried the hair of the SWMBO that cooked the meals for the owner who bought the big alternator ?

 

[lw395]

Some of the old-style alternators often found on boat engines not only have a low current rating, but need a lot of RPM to get there.
A more modern alternator rated at say 120A will often be able to give 90A or so at 1/3 max RPM.
I think the alternator on a boat can be in quite a hot environment compared to a car with a 30mph breeze running through it, so a bit of de-rating might be called for.
As well as charging, some people run windlasses, inverters, fridges etc.
Our boat, if the batteries are low after a couple of nights at anchor, we might put 50A into the batteries initially, plus run the fridge as we depart under power.

So, while you might not 'need' 120A, it might not be a bad thing.
And you can get one from a breakers for £20 or so...

NB when Isay a 'more modern alternator', I mean a typical fit for a car around 1995 to 2005 or so.
More modern than that and it might be 'smart', the regulation varied by the ECU to optiise fuel economy etc.
Avoid unless you know more than I do!

 

[prv]

For what it's worth, the standard fit on a Volvo D1-30 (28hp 3-cylinder) is 115amp. So your 120 doesn't seem outrageous to me unless it's been nailed to the side of a 1GM10 or something.

Pete

 

[Heckler]

Lots of boat owners wrongly believe that a 120A alternator will push 120A into the batteries. It won't, unless the batteries can accept that current. In most cases, the batteries installed won't accept anywhere near that current.

Bigger battery banks can accept more charge, so adding extra batteries is often a good idea. If replacing batteries, AGM batteries can accept high charge currents, so can be a good choice.

Not often we agree but we do on this, the alternator works as you say, so really pointless putting a big one on unless a specific LARGE load needs to be addressed. Wont do any harm though.

 

[VicS]

For what it's worth, the standard fit on a Volvo D1-30 (28hp 3-cylinder) is 115amp. So your 120 doesn't seem outrageous to me unless it's been nailed to the side of a 1GM10 or something.

Pete

A 3GM30 I think ... at least it was earlier in the year. Similar I guess to a D1-30 apart from the paint colour

 

[lw395]

Not often we agree but we do on this, the alternator works as you say, so really pointless putting a big one on unless a specific LARGE load needs to be addressed. Wont do any harm though.

That's vague and not very helpful.
It's more useful to look at it a bit more scientifically.
How big are the batteries?
How much might they be discharged?
How much of the time might a small, original type, alternator not be charging them as fast as a bigger, more modern alternator?

We found significant gains, because the original Yanmar fitted alternator tended to be power-limited for a significant period of time when running at low-medium rpm, as we tend to be motoring away from a mooring of something. A bigger alternator, regulated at a slightly higher voltage made a real difference for us. And that was with a quite small battery bank.
If you are only going to motor for an hour, getting more current in early in that hour makes a difference.
If you motor all day it will be insignificant of course.

With a smart alternator controller, there may be even bigger gains. My car seems to charge the battery at up to 14.9V at times. With stop-start, they are doing all sorts to get the charge back in when you decellarate or go down hills. I think they are persuading the battery that it 'will accept' a lot of amps at times!

 

[skyflyer]

If you are only going to motor for an hour, getting more current in early in that hour makes a difference.
If you motor all day it will be insignificant of course.

With a smart alternator controller, there may be even bigger gains. My car seems to charge the battery at up to 14.9V at times. With stop-start, they are doing all sorts to get the charge back in when you decellarate or go down hills. I think they are persuading the battery that it 'will accept' a lot of amps at times!

Hmm but that's my point - backed up by other comments above; you wont get more current as the batteries will take what the batteries take, so unless you have a very large battery bank that is very discharged the bigger alternator wont make any difference.

Similarly the load on the engine is a function of the alternator efficiency rather than it's size (I would suggest). You don't get something for nothing so the electrical power from the alternator has to come from the mechanical power of the engine. Maybe bigger alternators are more efficient, but my suspicion is that an alternator will be designed to be most efficient when running close to max output, so if you are drawing off (say) 40 Amps then a smaller (correctly sized?) alternator would maybe put LESS load on the engine than a 120A alternator?

Smart chargers change the output voltage by adjusting the alternator field voltage. Power is Volts x Amps so an increase in voltage for the same amp current gets more power to the battery - subject to the caveat of boiling them of course!

I hadn't considered the aspect of a temporary high load - but although my boat has a windlass, with an 80Amp CB (I think) I suspect although it CAN draw 80A most of the time it is nowhere near it. A starter motor(which seems pretty similar) doesn't draw a huge current when it isn't connected to the engine. The current comes when it is under load. Unless you are the type of sailor that hauls your 20 tonne boat to the anchor using the windlass and then breaks it out by force, I would guess that the windlass load isnt that great. Much like a starter motor load, the batteries can probably stand it for the relatively short time it is in use.

Just my tuppence worth, having started the thread here, but open to opposing views!

 

[skyflyer]

As already said 120 amps is unnecessarily large for the existing battery bank but according to the manuals the standard fit is only 35 amp, together with a 70 Ah starter battery.

What manuals? Not my boat!

It is not clear if the two batteries are combined into one bank or wired separately but I would suggest that they should be permanently wired as one bank and a smaller one added as an engine start battery.

They are wired through a 1-2-both-off switch. Many thousands of posts on tens of sailing forums have been written about the advantages and disadvantages of these so please everyone, let's not start that debate again but agree that it is largely a matter of personal preference. (like anchors!)

I would expect to see a higher charge current than 20amps if they are healthy and 50% discharged . Perhaps this should be investigated.

It probably is initially, to be fair, but I rarely start the engine and rush down below to check the current! Its generally down to 20 or maybe 30 A by the time I look at it a few minutes later. I avoid deep discharge - generally switching batteries once one is approx 50% (which is still ample to start the engine) .

If you do not have the original manual for the Adverc and cannot find one on line (I can't) there are some useful extracts regarding checking its operation in this this thread: http://www.ybw.com/forums/showthread.php?479336-Adverc

Useful link thanks, but I have the manual - its working perfectly too!

 

[lw395]

Hmm but that's my point - backed up by other comments above; you wont get more current as the batteries will take what the batteries take, so unless you have a very large battery bank that is very discharged the bigger alternator wont make any difference.

Similarly the load on the engine is a function of the alternator efficiency rather than it's size (I would suggest). You don't get something for nothing so the electrical power from the alternator has to come from the mechanical power of the engine. Maybe bigger alternators are more efficient, but my suspicion is that an alternator will be designed to be most efficient when running close to max output, so if you are drawing off (say) 40 Amps then a smaller (correctly sized?) alternator would maybe put LESS load on the engine than a 120A alternator?

Smart chargers change the output voltage by adjusting the alternator field voltage. Power is Volts x Amps so an increase in voltage for the same amp current gets more power to the battery - subject to the caveat of boiling them of course!

I hadn't considered the aspect of a temporary high load - but although my boat has a windlass, with an 80Amp CB (I think) I suspect although it CAN draw 80A most of the time it is nowhere near it. A starter motor(which seems pretty similar) doesn't draw a huge current when it isn't connected to the engine. The current comes when it is under load. Unless you are the type of sailor that hauls your 20 tonne boat to the anchor using the windlass and then breaks it out by force, I would guess that the windlass load isnt that great. Much like a starter motor load, the batteries can probably stand it for the relatively short time it is in use.

Just my tuppence worth, having started the thread here, but open to opposing views!

I've been there, done it, measured it, it made a difference.
The icing on the cake was selling the gen-u-whine original alternator for more than the bigger one cost.

 

[noelex]

Many standard alternators fitted to marine engines are poor. The Yanmar 80A alternator fitted to my old boat was a typical example. Although a genuine 80A output would be fine, these alternators are designed to shut down as the alternator temperature climbs and the normal regulation voltage is much too low. So 20-30A is a more typical sustained output.

Replace the above with a large frame externally regulated alternator and the output can be much greater.

For the OP, a 380AHr of battery bank should be accepting more than 40A when 50% discharged. I suspect the charging voltages are a bit low, the batteries are a bit old or perhaps the external regulator is throttling back the output to protect the regulator from overheating. However, even if we accept this figure you need an output of 40A plus whatever house loads are in use. Personally, when motoring I always try to run high loads where possible including devices such as the electric kettle which will draw 1700w (150A @ 12v), but even if your demands are less you will need an alternator that can reliably (read long life) supply 40-50A at high alternator temperatures, and preferably at lowish revs. Many standard marine alternators will not do this despite their rating suggesting they will. An aftermarket good small frame 120A alternator is capable enough for these demands, but even with such an alternator there will be less reserve than you might imagine, even for your relatively modest needs.