Alternator Theory

[RichW]

If an alternator is capable of producing an output of 14.1 volts when connected to a set of fully charged batteries. Is it then the regulators job to maintain this voltage when charging the batteries?

Assuming that it is being asked to produce current within its designed specification, it is spining at a sufficient rate etc.

 

The reason I ask is that I am only seeing 14.1 volts on the output of the alternator just after I have removed the shore power charger. Once I have taken charge from the batteries when out cruising, I am seeing sarting off 13.36 volts and 13.68 volts after a day cruising.

 

 

Rich

[BEngo]

If an alternator is capable of producing an output of 14.1 volts when connected to a set of fully charged batteries. Is it then the regulators job to maintain this voltage when charging the batteries?

Assuming that it is being asked to produce current within its designed specification, it is spining at a sufficient rate etc.

 

The reason I ask is that I am only seeing 14.1 volts on the output of the alternator just after I have removed the shore power charger. Once I have taken charge from the batteries when out cruising, I am seeing sarting off 13.36 volts and 13.68 volts after a day cruising.

 

 

Rich

 

 

Broadly, yes that is the regulator's job. But you have to remember that the alternator and regulator may not be able to achieve the regulated voltage.

 

What happens is that as the alternator output voltage falls the regulator turns up the wick until either the regulated voltage is reached or the alternator is at full output current. Working into a fully charged battery the alternator will easily reach the regulated voltage (because there is not much current needed) so you see 14.1 volts. Working into a partly discharged battery it may not be possible to reach better than (in your case) 13.68 volts. If you wait long enough (on a good battery) the voltage will rise and eventually reach the regulated voltage. Charging batteries charge more slowly as they become more fully charged- it takes a long time to get the last few amp-hours in.

 

 

 

N

[mykaskin]

If an alternator is capable of producing an output of 14.1 volts when connected to a set of fully charged batteries. Is it then the regulators job to maintain this voltage when charging the batteries?

Assuming that it is being asked to produce current within its designed specification, it is spining at a sufficient rate etc.

 

The reason I ask is that I am only seeing 14.1 volts on the output of the alternator just after I have removed the shore power charger. Once I have taken charge from the batteries when out cruising, I am seeing sarting off 13.36 volts and 13.68 volts after a day cruising.

 

 

Rich

 

I suspect that as the battery voltage gets closer to the charge voltage charging reduces, and thus will take an exteremely long while to get to the charge voltage (several days probably). Some alternator controllers that can be purchased for better charging can increase the voltage to give a boost charge in order to get the battery to maximum level while ensuring the batteries don't boil (temp sensor, and timed charging).

 

Cheers,

 

Mike

[WotEver]

If an alternator is capable of producing an output of 14.1 volts when connected to a set of fully charged batteries. Is it then the regulators job to maintain this voltage when charging the batteries?

Assuming that it is being asked to produce current within its designed specification, it is spining at a sufficient rate etc.

 

The reason I ask is that I am only seeing 14.1 volts on the output of the alternator just after I have removed the shore power charger. Once I have taken charge from the batteries when out cruising, I am seeing sarting off 13.36 volts and 13.68 volts after a day cruising.

 

 

Rich

Two points. Firstly the 14.1V that you're seeing immediately after disconnecting the shore charger is the voltage that the batteries are currently sitting at, thanks to the healthy output from that charger. As they are allowed to rest, so that voltage will drop - to around 12.7V if you weren't running the engine.

 

If your alternator voltage never exceeds 13.7V even on fully charged batteries then a stong possibility is that your regulator is a 13.8V unit and that is it's design voltage.

 

How old and what make is the alternator?

 

Also, what happens to the battery voltage after 20 minutes if you disconnect the shore charger, do not start the engine, and put a small load on the batteries, like an internal light? If it remains close to 12.6V then the batteries are good, but if it drops way down then the batteries have come to the end of their useful life and the alternator will never get up to full voltage.

 

Tony

[Tony Brooks]

Broadly, yes that is the regulator's job. But you have to remember that the alternator and regulator may not be able to achieve the regulated voltage.

 

What happens is that as the alternator output voltage falls the regulator turns up the wick until either the regulated voltage is reached or the alternator is at full output current. Working into a fully charged battery the alternator will easily reach the regulated voltage (because there is not much current needed) so you see 14.1 volts. Working into a partly discharged battery it may not be possible to reach better than (in your case) 13.68 volts. If you wait long enough (on a good battery) the voltage will rise and eventually reach the regulated voltage. Charging batteries charge more slowly as they become more fully charged- it takes a long time to get the last few amp-hours in.

 

 

 

N

 

 

This may be considered pedantic by some but that is NOT what happens with the majority of (all?) regulators.

 

The volatge an alternator can produce is limited by the effect the various magnetic fields that are bouncing about inside the alternator has on the generating coils (the stator). The larger the current being produced the larger the magnetic fields so the stronger the limiting effect. The upshot of this is that with exceptionally deep discharged batteries the alternator may only be capable of producing around 13 volts. However as the batteries become more charged the current that volatge can push into the batteries falls. The falling current reduces the strength of those magnetic fields so the charging voltage starts to rise. So far the regulator can do didley squat because the alternator is designed to limit the volatge itself during this phase of charging so it becomes self regulating for current. (this is why people who allege a properly designed and fitted alternator can be burnt out by advanced charging systems are just plain wrong. - note the properly designed and installed bit). It also explains why people who say that if they sat up all night chatting with the lights on so the charging volatge was very high in the morning are also wrong. The charging current magnitude is the result of the difference at any time between the battery voltage and the charging volatge - full stop - so low battery charge = low battery voltage = large difference = high current = alternator automatically reducing the charging volatge to protect itself, thus limiting the volatge difference d thus the charging current.

 

If you remove the load from an alternator (do not do it!) and run it up with the regulator disconnected you can easily get over 100 volts out of a 12V one and it will also illuminate a mains 40W bulb. That level of voltage when applied to a battery would certainly give it a good charge but would also ruin it so something has to be done to prevent the alternator destroying the battery.

 

As the charging current falls the charging volatge rises to a level that someone designer has decreed is a level that is safe for the batteries. At that point the voltage regulator will stats to work to keep the charging volatge at that decreed safe level. Now that safe level is open to interpretation and may well alter from application to application and battery type to battery type.

 

All the time the alternator or charger is limiting the charging current it is known as the bulk phase of charging and nothing can produce any more charging current.

 

Once the regulator (be it the alternator's regulator or the electronics inside a charger starts to work we are in the acceptance phase and it is only now that techniques/add ons can be employed to raise the charging volatge and thus increase the charging current.

 

That is it as far as the standard alternator is concerned although chargers will hopefully regulate their charging volatge so the current flowing minimises charge times. This voltage would probably destroy a battery if left applied 24/7 so the chargers should step their volatge down when they think the battery is fully charged so the charging current just keeps pace with the batteries' self discharge. This is known as the float phase of charge.

 

Batteries that are kept on float charge for long periods end up with the acid inside themselves in areas of strong acid and areas of weak acid. It is advantageous if it can be mixed up again so a charger (not an alternator) may every so often raise the voltage to a figure above the "safe" level for a short period of time. This mixes the acid up and is known as the equalistaion phase. The higher volatge will also help to combat sulphation.

 

 

As I said, some will say pedantic. and the quote is a perfectly reasonable assumption based on observations but the regulator does so all until the charging voltage has risen to the regulator's setting.

[RLWP]

Thanks Tony, that's really helped my understanding

 

Richard

[RobinJ]

If an alternator is capable of producing an output of 14.1 volts when connected to a set of fully charged batteries. Is it then the regulators job to maintain this voltage when charging the batteries?

Assuming that it is being asked to produce current within its designed specification, it is spining at a sufficient rate etc.

 

The reason I ask is that I am only seeing 14.1 volts on the output of the alternator just after I have removed the shore power charger. Once I have taken charge from the batteries when out cruising, I am seeing sarting off 13.36 volts and 13.68 volts after a day cruising.

You do not say what engine you have, and most of the other points have been answered, but if the alternator is not spinning sufficiently fast, then it may not be capable of providing its full capacity.

If you are worried, I would start by checking the quiescent voltage on each battery, after they have been stood without charging or load for a while, disconnect them and check the voltage on each battery is around 12.6, indiciating they are reasonably healthy. Then ideally with a clip type meter, measure the output of the alternator at various revs. to see what current it is delivering at the particular charge voltage, the lower the voltage the higher the current?

[Arnot]

If an alternator is capable of producing an output of 14.1 volts when connected to a set of fully charged batteries. Is it then the regulators job to maintain this voltage when charging the batteries?

Assuming that it is being asked to produce current within its designed specification, it is spining at a sufficient rate etc.

 

The reason I ask is that I am only seeing 14.1 volts on the output of the alternator just after I have removed the shore power charger. Once I have taken charge from the batteries when out cruising, I am seeing sarting off 13.36 volts and 13.68 volts after a day cruising.

 

 

Rich

Assuming that the alternator you have is one of the common vehicle derived units such as the ubiquitous Lucas A127 the answer is that the regulator does not actually measure the output of the alternator. It is internally connected to three auxiliary diodes that provide the excitation supply for the rotor. This internal supply is not subject to the load that the batteries (and any other power usage) draw which comes through the positive side of the main rectifier.

 

As the output of the alternator increases, the voltage drop on the main rectifier increases but the voltage drop on the auxiliary diodes doesn't. Because it isn't connected to the output, the regulator does not know this has happened and therefore cannont correct for it. Hence the voltage on the output sags by this difference.

 

So; the pheonomenon you correctly observe whereby the higher the load the lower the ouput voltage is entirely normal on a vehicle type alternator. In this type of machine the regulator's job is not maintain the voltage output of the alternator precisely but to balance the load presented to the battery and prevent it discharging. The big difference in terms of operation is that in a car, the power is almost always used whilst the alternator is generating it's output and very little is required to replenish that used from the battery to start the engine (typically about 0.1Ah). In a narrow boat, conversely, while the engine is running and the alternator is generating output almost none is used at the time, it is all stored in the batteries for use later.

 

Installing a battery sensed regulator circumvents this problem (and a few more) by having a lead that does actually allow the regulator to "see" the alternator output. In fact, it is even better than this, it allows the regulator to "see" the input to the battery and maintain the voltage at this point.

 

Essentially this is the difference between an alternator intended for the supply of vehicle loads such as headlamps and heaters etc. and the sort of alternator ideal for narrow boat useage when it is essentially used as a rotary battery charger.

 

Hope this helps,

 

Regards

 

Arnot

[casper ghost]

Can I just ask, My typical A127 70 amp alternator, running when engine on tickover but off the flywheel so still quite fast, checked at batterys, which were partially discharged, volts read as 13.59. I've no clue when it come to this so is that reading normal or no use at all? How should I check what sort of charging i'm getting. Note this is before I upgrade the charge cable, which is at present rather thin with 5 feet of length between alternator and batterys.

Casp'

[paulcatchpole]

Can I just ask, My typical A127 70 amp alternator, running when engine on tickover but off the flywheel so still quite fast, checked at batterys, which were partially discharged, volts read as 13.59. I've no clue when it come to this so is that reading normal or no use at all? How should I check what sort of charging i'm getting. Note this is before I upgrade the charge cable, which is at present rather thin with 5 feet of length between alternator and batterys.

Casp'

 

It's not really a lot of use, Casp. When the batteries are discharged or near it, they'll be holding the voltage down, so hard to draw any conclusions from it!

 

If you run it until you think they're full, how high does the voltage get? 13.8 / 14.0 / 14.4 / higher?

 

No easy way of measuring how much charging you're getting via voltage alone, you'd need to stick a shunt or clamp-style ammeter on (or in-circuit) the charging cable to see.

 

PC

[casper ghost]

I didn't think it meant alot.. With my last engine I just ran the engine when the batterys seemed low and I new that 1.5 hours running would give more than enough power for the evening. At least I know the alternator is doing something..

Casp'

[Gibbo]

What you can do is wait until your batteries are next quite low then when you start charging take a voltage reading every 10 minutes or so. Jot them down and sketch a graph from them. You'll see the voltage rising fairly linearly then it will go more or less flat. That's the regulation voltage of your alternator.

 

Your maximum voltage will give us a clue as to how well you're charging them and we can take a stab at how long it will take.

 

But to know what charge rate you're getting you really need to borrow an ammeter.

[RichW]

Thank you for you replies.

 

The alternator is a 95A Iskra (machine sensed) on a Beta 43 which is 9 years old.

I think I have two main areas to address. Firstly the condition of the alternator brushes and slips rings, secondly the fact that it is probably not spinning fast enough.

The first is simple to fix, the second not so.

Due to the hull design and prop size ( 58ft with 18 x 11 prop) we only need to cruise at 900rpm, which with a 3:1 pulley ratio means the atlernator is spinning at 2700 rpm. I therefore doubt if the current flowing into the batteries is any where near 95A.

I probably also have the same problem with our Travel Power alternator.

 

I did some rough alternator voltage measurements when recharging the batteries from 50%, battery bank is 4 x 110 ah.

30 mins = 13.36v

45 mins = 13.43v

1 hr 15 mins = 13.44v

5 hrs = 13.68v

 

I suspect the alternator is a 13.8v unit similar to that on our previous boat which had a Beta 38 engine. On this an Adverc unit had been fitted and I suspect we need the same again.

 

I will get a DC clamp ammeter and make some measures in the next week or so.

 

Does anyone know what the maximum continuous rpm of the Iskra is?

I do not what to exceed it by increasing the pulley ratio to 4:1 and assuming we were on river 4 x 2800 (Beta 43 max rpm) would be an alternator rpm of 11200.

 

Hope I have not missed anything

 

Rich

Edited August 9, 2010 by RichW

[Semitrad]

We have a Beta 43 but I don't think we could ever get near 2800 rpm.

[WotEver]

Thank you for you replies.

 

The alternator is a 95A Iskra <snip> Does anyone know what the maximum continuous rpm of the Iskra is?

I do not what to exceed it by increasing the pulley ratio to 4:1 and assuming we were on river 4 x 2800 (Beta 43 max rpm) would be an alternator rpm of 11200.

From their site: http://www.iskra-ae.com/eng/alternators.php

 

AAG range 12,000 RPM / 13,500 RPM (constant/short time)

AAK range 13,000 RPM / 15,000 RPM

AAK compact range 15,000 RPM / 18,000 RPM

AAN Compact range 15,000 RPM / 18,000 RPM

 

So even the AAG would be okay, and that's assuming you could even get up to 2800 rpm with your prop.

 

Tony

[casper ghost]

900 rpm with a 18 x 11 prop is surely hardly moving, my 43' boat with a fine swim, had an 18x14 with a Sabb which hardly revved and was fairly slow, surely your engine is revving higher than that during normal cruising.

Casp'

[RichW]

900 rpm with a 18 x 11 prop is surely hardly moving, my 43' boat with a fine swim, had an 18x14 with a Sabb which hardly revved and was fairly slow, surely your engine is revving higher than that during normal cruising.

Casp'

 

Well unless the Beta tacho is telling lies, no.

But that brings another question to mind if the tacho is fed from the engine alternator which has a pulley ratio, how does it measure the actual engine rpm?

 

Rich

[Tony Brooks]

Well unless the Beta tacho is telling lies, no.

But that brings another question to mind if the tacho is fed from the engine alternator which has a pulley ratio, how does it measure the actual engine rpm?

 

Rich

 

The tachos normally have a method of setting it so whatever frequency of pulses it receives the tacho gives a true(ish) reading. One thing is sure and that is that it will not be a Beta tacho but one that Beta have sourced from elsewhere. The adjustment may be a set of dip switches of a button to push.

 

My feeling is that the alternator may have lost a phase but without further tests I could not be sure.

 

If the diode paired with the one that half wave rectifies the tacho output has shorted out I think the tacho may well be telling lies, as it would if the belt was slipping. If Arnot etc. say I am wrong then I am.

Edited August 9, 2010 by Tony Brooks

[Arnot]

Thank you for you replies.

 

The alternator is a 95A Iskra (machine sensed) on a Beta 43 which is 9 years old.

I think I have two main areas to address. Firstly the condition of the alternator brushes and slips rings, secondly the fact that it is probably not spinning fast enough.

The first is simple to fix, the second not so.

Due to the hull design and prop size ( 58ft with 18 x 11 prop) we only need to cruise at 900rpm, which with a 3:1 pulley ratio means the atlernator is spinning at 2700 rpm. I therefore doubt if the current flowing into the batteries is any where near 95A.

I probably also have the same problem with our Travel Power alternator.

 

I did some rough alternator voltage measurements when recharging the batteries from 50%, battery bank is 4 x 110 ah.

30 mins = 13.36v

45 mins = 13.43v

1 hr 15 mins = 13.44v

5 hrs = 13.68v

 

I suspect the alternator is a 13.8v unit similar to that on our previous boat which had a Beta 38 engine. On this an Adverc unit had been fitted and I suspect we need the same again.

 

I will get a DC clamp ammeter and make some measures in the next week or so.

 

Does anyone know what the maximum continuous rpm of the Iskra is?

I do not what to exceed it by increasing the pulley ratio to 4:1 and assuming we were on river 4 x 2800 (Beta 43 max rpm) would be an alternator rpm of 11200.

 

Hope I have not missed anything

 

Rich

On a nine year old alternator you are probably better not messing with the brushes and slip rings, they both wear a bit but my the time they are beyond use, ther bearings are well on their way and the stator insulation is starting to break down. All of which spells new alternator time but not until the red light comes on, it makes horrible grinding noises or the smoke escapes.

 

Just don't only change the brushes - you never manage to get them to run in the slip ring grooves the originals sat in and shortly after what seems like a succesful repair, unless you are lucky, the detritus caused by the bedding in of the new brushes and sometimes the copper wool machined off the slip rings by them starts to short circuit and it all goes into Chernobyl mode.

 

Re the measurements, these are very useful but where were the positive and negative measurement points?

 

i.e. was the positive probe of the meter connected to the output stud of the alternator or the positive terminal of the domestic batteries or somewhere else?

 

Was the negative probe of the meters connected to the domestic battery negative, the main earth terminal, the engine block, the alternator case or somewhere else?

 

Generally the Iskra alternators produce a reasonable output even at fairly low speeds but it is fair to say that the pulleys on the later engines have got a better belt system and higher gearing. My suspicion is that if you get the wiring and control systems well sorted you probably won't need to mess with the pulleys, if you do then it's best to get them from Beta, they do sell them as parts.

 

You need to be a bit careful using a clamp ammeter to measure alternator output. If you get it near the alternator, the stray magnetic fields tend to give false readings.

 

You need to check the tacho accuracy with an optical tacho. This may be a red herring but often both the domestic and starter alternators have the necessary output for a tacho and if it has been plugged into the wrong one it will give incorrect readings.

 

Regards

 

Arnot