What size alternator?

[Sans Bateau]

I have 2 x 105 AH service batteries and I am about to change my engine starter battery which is independent of the the other two. The new battery will be again about 100AH and about 700CCA to support the electric windlass.

Question is do I need to upgrade my 50amp alternator? The charging system is assisted by an Adverc manager thingy. The Adverc web site seems to suggest I should, but what are peoples experiences, no point in putting bigger batteries on if I cant charge them other than plugged in ashore, which I try and avoid.

Oh! One other thing, any suggestions on a supplier? The engine is a VP2003.

 

[nickcred]

If it's any help we have 2 x 110 amp hour domestic and a I thinks its a75 amp hour starter battery and the Nanni 4150 39hp engine we have in the boat has a 100 amp alternator that does a grand job of keeping the batteries charged up even after a short run

 

[boatmike]

There is no clear answer to this. It depends on your current drain rather than battery size and also how long you intend to run the engine for. If you are expecting to put charge back in to two discharged 110 AH batteries that are say half discharged, your system will probably take about 4 hours to recover full charge at normal charge rates but if the boost is turned up on your smart charger you could well do it in 3. Don't make the common mistake of expecting a 50 Amp alternator to give you 50 Amps continuous charge though. In reality anything over 30 Amps is going to cook it. 30amps x3 hours= 90 Amp hours though so might be perfectly OK. Very high Amp alternators are only really necessary if you have a lot of current drain and only run your engine intermittently. If your existing alternator is OK why not leave it alone and spend the money on some solar panels instead?

 

[alan]

I read somewhere that about 1/3 of the capacity is a rough figure. I personally have 280Ah of batteries and a 70amp alternator.

In practice your engine start battery should take very little to get back to full charge and so I think 1/3 of your domestics should be about right. Are you sure you want to run your anchor windlass from your start battery??

I would think about 70A would be better for your capacity.

 

[Chris_Robb]

Steve, I have an almost identical set up to you, VP2003T, 1 x 100 ah engine battery, and 2 x 110AH dmostics, driven by the 50ah alternator plus Sterling controller.

I have a BM battery monitor on the domestics, so I can see exactly what charge is gong in to the domestics (not much goes to the engine battery as it is normally pretty full). If the batteries are down to 60% say, I will get a continuing charge of nearly 45amps, until the volts start to rise above 14.4, when the charge begins to fall off down to about 15amps at 14.8 volts, when I stop charging. On my last boat I had a 100 ah prestolite and adverc (both from adverc) and that used to start a charge at 80amps and drop back to 40 pretty quickly, and then down to similar figures as I get now (with the same battery config).

I would not be convinced that it would be worth the expense.

I also have a mains generator plus and 80ah sterling 4 stage charger. This seems to output after an initial rush down, to similar levels as the alternator.

Perhaps one of the elctrical gurus could explain why I did not see any real benefit from a 100ah alternator. I supect that if I doubled my battery bank, which I intend to do, then there would be an increase.

Good sailing!


Chris

 

[Chris_Robb]

[ QUOTE ]
Don't make the common mistake of expecting a 50 Amp alternator to give you 50 Amps continuous charge though. In reality anything over 30 Amps is going to cook it.

spend the money on some solar panels instead?

[/ QUOTE ]

Mike, - see my reply just before this - you say that I might cook my alternator by driving it so hard, - ie it outputs 40amp for quite a long time before dropping off. Is this a reason for getting a larger one? Also, if you have many more batteries, it is much easier to put the power in, and ins't that when a larger alternator pays off?

Solar panels; I have installed 2 x 85 watt this winter. Looks like this is supplying up to 8 apms on a 60% battery. (readings from the controler and the Nasa BM1) Perhaps I could leave the fridge on even when I am not on the boat???!! Instant cold lube!

 

[PeterGibbs]

Q - In practice your engine start battery should take very little to get back to full charge and so I think 1/3 of your domestics should be about right. Are you sure you want to run your anchor windlass from your start battery??


I think Alan is right. The actual drain from the windlass is sustained for 3-4 minutes? This does not demand that the starter battery is ramped to 100amph. And the windlass is much better connected to the deeper capacity service batteries - it is on mine.

Your 50 amph alternator with battery management is well suited to what you describe. Balmar gear is good and robust, but I wouldn't claim any alternator was "the" one!

PWG

 

[saltwater_gypsy]

If it aint broke don't fix it. There is acommon notion that fitting a larger alternator will miraculously charge your batteries in minutes. The fact is that if you measure the current going to the batteries , you will be lucky if it is more than 5 to 10A. You can't force charge into a battery and the ADVERC system is a way of coaxing more charge in.
If you have a lot of kit that you run when the engine is on like freezers, air conditioning, watermakers etc then you may need a larger alternator to deal with the load. For ordinary battery charging 55A is fine.

PS I read an interesting article on how the belt life of alternators is reduced with the electrical load. Food for thought!

 

[boatmike]

Will 40 Amp cook a 50 amp Alternator.... Well theoretically no I guess but every alternator shop I talk to suggests that fitting smart regulators to small alternators is a major cause of blowing them up. I think they are right but of course you only get these high charge rates when batteries are deeply discharged. However I guess unless you do deeply discharge them you don't need a smart regulator or big alternator anyway so thats a bit of a silly argument. On Peregrine, I have a 4 cylinder Vetus Mitsubishi which used to have a 60 amp Alt with a Stirling attached. I often made it work very hard and eventually it cooked. I replaced it with a 110 Amp Prestolite which also has the stirling fitted and frankly the charge rate never exceeds 50 amps but I am now relaxed as it is well within its capacity and does not seem to get hot. I also have 2 solar panels about the same size as yours which give me about 6 amps on a sunny day and about 3-4 amps even when dull. When on the boat I always have the fridge runing day and night, eberspacher et al whenever, and don't economise on power really and find it keeps everything topped up to a min of 12.2 volts most of the time. I think solar panels are really the answer to ensuring the batteries stay healthy. When off the boat I still leave them connected and nearly always return to the boat after idle periods to find the batteries registering over 13 volts. Would not be without them now!

 

[fisherman]

If I turn everything on the draw is 30 amps, so no point in going bigger than the 40 amp I have. You will find that it doesn't achieve the rated output until about 2000 revs at the alternator, no matter what size it is. Adverc is the answer.

 

[Sans Bateau]

Getting the picture now guys. I now don't feel there is any point in changing the alternator. If it fails, then yes go bigger. I would like to get some kind of battery monitoring system, something better than the NASA one, but along those lines, better still if I can monitor start and domestic separately.]

In the summer when spending more time on the boat than just a weekend I have a plug in 45watt solar panel that I think just keeps up with the fridge. I had a horrible thought Easter weekend at anchor for two night, Eber kicking in all evening, lights (LEDS and fluorescent), anchor light, fridge, water pump, shower pump, etc etc that I had pretty much drained the service batteries, however the digital volt meter on the chart plotter still read 12V when we left on the Monday.

I have a Vitronic multistage charger I leave on most of the time when berthed up, just keeping the batteries topped up.

In answer to the query about running the windlass from the starter battery, my theory is, the engine is always running when the windlass is used, the windlass motor runs for short periods, more like a starter motor than a fridge motor and I think I'm right in believing that the charging system will favour the starter battery before the domestic.

 

[Chris_Robb]

[ QUOTE ]
Getting the picture now guys. I now don't feel there is any point in changing the alternator. If it fails, then yes go bigger. I would like to get some kind of battery monitoring system, something better than the NASA one, but along those lines, better still if I can monitor start and domestic separately.]

In answer to the query about running the windlass from the starter battery, my theory is, the engine is always running when the windlass is used, the windlass motor runs for short periods, more like a starter motor than a fridge motor and I think I'm right in believing that the charging system will favour the starter battery before the domestic.

[/ QUOTE ]

There is no need to have a battery monitor looking at both domestic and engine battery. The engine batttery really does not need monitoring. The Nasa BM looks purely at the domestics, and it works a treat for me. No point in wasting money on one that will monitor both sets.

In terms of connecting the windlass to the engine battery, I think that is right, however the charge will go into the batteries in proportion the the resitance to charging - so if you are taking out a wacking great current, most of the charge will go into that battery.

 

[lw395]

I would only change if, in my useage of the boat, the existing alternator was struggling.
If your idea of the perfect cruising day is half an hours motoring at lowish revs between sailing, then more current to charge batts and run fridge may be useful. A more modern alternator will often have much more current at low revs than an older design, as I found when replacing the standard one on my 3GM with an ex-mondeo bosch.
If the existing alternator is generally getting the volts up to 14.3 or so in your normal use, then you don't need more current!

 

[boatmike]

Stirling do a very good battery monitor (digital) It's what I have. I also agree your philosophy regarding windlass power but don't forget you need very heavy cables.

 

[mustntgrumble]

I have 2003T with standard 50a alternator, & Stirling clever box. Electric windlass off engine start battery - 70ah, 900CCA - and domestics 4x 108ah. Never a problem with power in 5 years.
Not till engine got thirsty and drank some sea last week anyway /forums/images/graemlins/shocked.gif - pm me if you want any VP spares