Alternator output ?

pragmatist

pragmatist

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Treat me gently - I am a total neophyte when it comes to engines ? Last year we acquired a cat with two Volvo 2003 engines 29hp presumed to be original (1995). They each have a Volvo (Valeo) 50A 14V alternator - also presumed to be original. I would be very interested to know what we might expect from them in the real world. And to what degree engine revs will affect output. We have a mile or 2 down river (6 knot speed limit) before getting to the sea. How much charge might we expect to see in that distance, for example ? Don't know if it makes any difference but our battery bank is now 400Ah (and yes we do have/are upgrading our solar as well).

Hope some of you knowledgeable chaps can help !

TIA
Penny
 
Engine revs greatly affect the output of the alternator. The alternator usually has a smaller pulley than the crankshaft pulley, so it spins at higher revs than the engine. Typically, it will spin 2-3 times faster.

A typical alternator can produce around half its rated max output at perhaps 1500rpm (equal to an engine speed of say 500-750rpm). It can reach around 90% of its max rated output at perhaps 5000rpm (ie an engine speed of 1700-2500rpm).

However, the alternator can only produce as much current as the batteries can absorb. How much current they can absorb varies according to their state of charge, and the charge voltage reaching them. Your 100A of theoretical alternator output won't necessarily ever put 100A into your batteries. As the batteries become more charged, their ability to accept current decreases, so the charging current will drop as they approach full charge. With 400Ah of batteries, if they're say 30% discharged, you might see a charge current initially of 40-50A. The type of battery is also a factor; AGM batteries tend to accept a higher charge current.
 
Also, if you have diode splitter in the way, this will cause a voltage drop, so you may only be getting as little as 13v to the batteries.
 
An alternator controller might be a worthwhile consideration. This forces the alternator output up to a higher level, effectively converting an alternator into a battery charger. Where the latter controls charge based upon the effective voltage and current flow at the batteries, rather than just a tapering voltage level as the batteries get closer to there fully charged level. By the time the battery is much above 80% the alternator output signifcantly reduces, more so with a split charge diode splitter, and in theory an alternator alone might never achieve 100% charge level.

I personally don't worry too much as I always have shorepower alongside, and two massive engines when underway, but on myu boat before last where I had no shorepower and relied upon a solar panel alongside, maximising charge underway was always an important consideration, so I fitted a Sterling unit.

Not sure if this specific one is still available from Sterling, but worked a treat when I had one ...

Sterling Universal Advanced Regulator.pdf
 
Penny,

If you have a couple of miles on the river before you hoist sails that maybe equates to 20mins of engine time but add a bit for messing about so say 30mins engne running. If both alternators were contributing 25A each average over the period then you'd generate 25AmpHours (50A x 30mins) whilst you motored down river. So that equates to 6.25% of your 400ah bank capacity. However I'd say that was a bit hopeful. Standard automotive alternators do not produce much juice at low revolutions (compared to a high output alternator such as a Balmar). That, together with all the other factors mentioned above (SoC, charging regime, diodes, battery acceptance, Peukerts formula etc), will push down on those results. Maybe budget on 5% of capacity recharged whilst motoring down river. It's not until you have a decent alternator with a decent regulator and decent batteries that you start to see fast effective charging from an engine alternator(s).
 
How much charge goes in will depend on the voltage and the state of charge of the batteries.
If the voltage is 14.0V and not more like 14.4, the charge will be quite slow if the battery is more than half charged to start with. It takes about 12 hours to charge a battery at 14.4V, 14.0V is a bit more than trickle charge, it's what used to be used on vehicles or plant that ran for lots of hours and didn't have the batteries flattened while the engine wasn't running.

The exact type of battery will make some difference.

But in short, half an hour of engine running doesn't do much unless you are starting out with batteries which are very low.
 
Hi Penny, also, you are quite likely to be motoring more than just the mile or so down the river, so don't just think that is the only time you will have the engines running.
G
 
Thank you to all for your excellent help. As Crazy-Diamond says we will undoubtedly have the engines running at other times (especially to windward we think - yet to see how the cat performs :rolleyes: ).


pvb - the batteries are at present sealed lead acid as we already had 2 we'd already bought - rather wish now that we had gone for AGM.

Bobc - don't believe we have a diode splitter :unsure: but that is only one of the very many questions to be dug out with a new-to-us-boat ! We do have a VSR.

Robih -and superheat6k - Yes take the point entirely but we are considering the possibility of re-engining (given their age) but we've agreed that we will test things out this summer (Covid permitting) as we had virtually no use of the boat last year. One of those tricky ones - do you fiddle with what you've got, put up with it as it is or go for new everything and get the benefit of modern phased charging. So this year is going to be testing everything out. If we get stuck we do at least have a generator !

TernVI - yes afraid a trickle might be the end result. Our previous boat had a Perkins 4108 (38hp) with an old-style alternator and we didn't get much out of that - but then we didn't motor much either.

Be interested to see what methods of measuring your state of charge you use (oh dear I'll get lots of flack on this one !) - we have a SmartMeter and a volt meter at present and will have a NASA BM1 by the time the season starts. The solar, of course, has its own controller which reports charge, rate of charge and (purportedly) SOC.

Must stop - the puppy is biting me - so will continue at a later date and, again, thank you for all your comments and suggestions.
 
Penny, we have a Volvo 2003 in our 31 ft yacht. With deeply discharged 2 x 100Ah flooded lead acid batteries we see about 40A and 14.2v from the alternator to start with. However, the battery monitor shows a steady drop in current down to say 20A after just 30 minutes and continue to drop to single figures after an hour. This is down to two things, alternator temperature which reduces the current as it gets hotter and the internal resistance of the batteries. It took one 10 hour motoring trip across the channel in flat calm seas to fully charge the batteries.

So we use lots of solar instead, still takes a while just don't have to listen to the engine.

There are slightly better alternators available in the UK and I have one to fit but Covid put a stop to that. You can spend hundreds but I am going to try one of these along with bigger batteries later in the year.

12V 100 AMP UPRATED VOLVO PENTA ALTERNATOR A3673
 
For battery status a simple, and ideally calibrated, Voltmeter reading to two points of decimal works for me. Had a Nasa BM1 once - lasted a season or so, was never even close to accurate apart from after a week on steady solar charge (it couldn't report more than 100% of battery charge), and I was happy to bin it when it packed up.
 
There’s some good advice here but the problem is that it’s all generalities or experience relating to people’s own specific setup.

I have decades of experience with 12v systems as a user and I’ve learned a lot but the most important thing I’ve learned is that if you are a novice to get the best most highly recommended marine electrician you can find and get them to give you an opinion on your system. You might be fine, you might benefit from some small tweaks or you might be heading for expensive trouble without intervention.

By all means absorb the info here - it’s really great to get the shared forum wisdom - but I’d suggest you get a pro onboard before you take any action. It’s a wise investment in my experience.
 
You can always get a marine electrician to change your system or sell you more stuff, but most people are a bit vague about what exactly they want from their electrics. However much you invest in kit on board, it comes down to batteries being limited storage and charging sources being limited by hours of sun, engine hours or whatever. Until you know exactly how you will use the boat and what demands you are going make of the 12V system, you can't make sensible decisions about the system. You can have two couple with identical boats, one will find everything is OK for them, the other will need more batteries or more solar or a generator, because they have different ways of using the boat and different expectations of using power-hungry equipment.
Eventually on many smallish boats, you end up facing choices, if we want to keep running the fridge all week, we'll probably need to motor a bit more if the solar hasn't produced much. If we want to run the heater a lot in the winter as well as other stuff, we need to keep an eye on battery levels. We could spend a grand or two on upgrades, but the gains would be marginal. It's not like being on the national grid.
 
Rob_Webb said:
There’s some good advice here but the problem is that it’s all generalities or experience relating to people’s own specific setup.

I have decades of experience with 12v systems as a user and I’ve learned a lot but the most important thing I’ve learned is that if you are a novice to get the best most highly recommended marine electrician you can find and get them to give you an opinion on your system. You might be fine, you might benefit from some small tweaks or you might be heading for expensive trouble without intervention.

By all means absorb the info here - it’s really great to get the shared forum wisdom - but I’d suggest you get a pro onboard before you take any action. It’s a wise investment in my experience.
Click to expand...
I think getting a marine electrician on board is good advice. The real difficulty however is identifying those electricians who really know their onions. There are too many people masquerading as experts in the field who in reality are inadequately experienced and knowledgeable in what is a specialist subject. My new to me boat is evidence of this in that the former owner only employed reputable Marine electrical experts yet what I have found on board is lamentable. Done for speed and cheapness yet I’ve seen the bills that the former owner paid - top dollar.

So, finding the expert is the issue. There are a small number of people on these fora who have demonstrated their expertise, however finding the equivalent knowledge/expertise within proximity of one’s boat and willing/able to take on the job is the difficulty.
 
Once again thanks to you all and sorry not to have been back here - frantically busy and that's before we start fettling the boat ! Personally I prefer comments on here to trying to find a good electrician - having met and been disappointed many times previously. So thanks for all the wise words. Penny
 
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