Alternator Voltage at low revs

ianc1200

ianc1200

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I've had a problem with the above and any thoughts would be welcome. I've got a Nanni 5 cyl. 62hp with two alternators, 90 & 110 amp. These go to a Sterling Alternator to Battery charger, charging a domestic bank of 4 No. 120amp hr batteries, and one engine starting battery.
At 1500 revs the starter battery measures 13.6V, output from the alternators are 14.15V & 14.2V, the Sterling charger has starter output 13.7V and domestic output 14.4V, but the engine panel voltmeter is reading 12.3V. Other than the odd low reading of the panel voltmeter all pretty well OK.
At 800 revs, tickover, though, starter battery reading 11.9V, alternator outputs both 12.1V, Sterling starter output 12.01 (and falling) domestic output 13.63, panel voltmeter 11.0.

I've swapped the starter battery with one from the boatyard (no change),had all earth connections off and cleaned up, all positive connections onto all batteries, and at a loss as to why the panel voltmeter is reading a volt less, and more importantly the starter battery is becoming discharged.

I should get some new shunts to measure the amps going into both banks tomorrow or Thursday, but my question is what sort of output should I get from the alternators at tickover?

regards

IanC
 
Charge voltage

Firstly yes the panel meter especiallyb if it is an analogue type may well be inaccurate. Check compare with digital voltmeter then another DVM. You will find which is correct.

An alternator produces voltage by passing a magnetic field past coils. The faster the passing the more voltage. The magnetic field is adjusted by varying the current by the regulator. Obviously at some point with max current hence max magnetic field the alternator is too slow to produce the voltage required. All this assuming the alternator is fully serviceable. If it is a situation that has always been then probably the alternator is running too slow. If it is a situation that has suddenly appeared it would be worthwhile getting the alternator bench checked. It might have a failed diode or poor brush contact. (this latter giving less than max. field current).
You can calculate the speed of the alternator at 800 engine RPM by measuring the pulley relative diameters.If you find data on the alternator you can compare.They are always set up to give the alternator more speed than the crankshaft. It may be possible to fit a larger pulley on the crankshaft or a smaller pulley on the alternator. `Often a car alternator is set to run slower (bigger pulley) because the petrol engine can reach much higher RPM than a diesel. There is a physical limit to how fast an alternator can run before it flies apart. Of course a smaller pulley on the alternator may be more inclined to slip under heavy load. good luck olewill
 
Having had a big think about this I'm wondering whether the key switch on the panel is the cause; the panel voltmeter is reading the voltage via the key switch (& it's about 1 volt less than the feed from the starter motor) and the key switch also controls the field connection to one of the alternators (the other being "self excited" if that is the correct term) & so also a "dodgy" field connection caused by the key switch would affect one of the alternators.
I don't suspect the alternators; they were both rebuilt & tested last year, and the Sterling Battery to Alternator panel should indicate a problem with the alternators (as it did last year which is why they were rebuilt).

Thanks for the thoughts William, will take time to consider, & have the afternoon off to recheck all to see if I can locate the problem.

regards

IanC
 
The often small moving coil voltmeters they put in instrument panel arent worth having IMO. Try checking the voltages with a digi meter. The moving coil meter on my switchboard reads a good volt lower than the digi meter. Yours sounds very similar.

It sounds from your first para as if the voltage drop in circuit is only of the order of 0.1 or 0.15 volts and that wouldnt worry me.

At 1500 rpm, the voltage on the starter battery is lower than that on the domestics. It sounds to me as if the charger is still putting amps into the domestics which have been discharged a bit or are feeding some power using circuits whilst the engine starter is on float voltage of 13.6 or so indicating it is fully charged. Eventually the domestics should drop back to 13.6 or so.

At 800 rpm neither alternator is charging. Your domestic bank reads 13.6 because thats the "overvoltage" that is left on the plates from when the alternators were running and at float voltage - it will decay down to 12.7, or even lower if you are discharging the bank. The starter battery at a bare 12v suggests either some current draw ( ie something other than the engine is connected to it) or a part discharged battery.

As William says, you may well have to alter pulley size to speed up the alternators is you want them to charge at idle speed - but why do you? And what do you want new shunts for?
 
I can't see much wrong with the readings you have obtained.

The way the Sterling unit works is to "fool" the alternators into thinking the battery voltage is lower than it really is, thereby getting them to chuck out more and charge faster.
So, it is to be expected that the alternator side will read low because that's what the Sterling unit intends.

On the output side of the Sterling, it boosts the voltage to enable faster charging of the batteries and overcome diode splitter losses. It seems to be doing that - at least at 1500rpm, not so effectively at 800rpm.
The only thing I can think is you may need smaller pulleys on the alternators to get them to spin faster at low revs.

Read the blurb and it explains it all much better than I can, but the readings you've obtained seem about what I wopuld expect.

Don't expect precise readings from analogue dash voltmeters. They seem to be close enough to your digital readings?
 
Thanks to all who replied, just spent the afternoon checking out what's going on at the dashboard with the digital voltmeter. The panel voltmeter is pretty accurate, and as this usually reads a lot more, say 13.5V, I was concerned. However the more I run the engine, slowly the panel voltmeter is coming up. Added to this, when on tickover, even with the reading about 11.2V, a clamp ammeter is telling me about 20 amps is going into the starter battery.
Still think something is amiss, somewhere there's a current draw in the engine starting circuit I'm not aware about - perhaps the engine stop control? - but basically it works and if anything the starter battery will need changing at some point soon.

The shunts which have arrived today are so I can use a battery management panel (the Sterling one) so I can monitor the volts/amps in the starter & the domestic circuits without having the entire wheelhouse floor up!

Thanks again for the replies

IanC
 
Julian

I rang Neil this morning & asked should I telephone Julian at work for advice, I was told not a good idea!
Still all the vital details are as noted above & would be pleased at any thoughts you have, but following on from your thoughts on another thread I have a Smartguage ready to be fitted which I going to compare with the Sterling Amphour meter.

regards

IanC
 
our alternator (120A on a VP D2-55) barely charges at 800rpm. Needs 1500rpm to make a dent and 2000rpm for full output.
 
ianc1200 said:
Julian

I rang Neil this morning & asked should I telephone Julian at work for advice, I was told not a good idea!
Click to expand...

Days are busy at present
Evenings are a possibility.
I wrote something last night but don't seem to have posted it!
The gist is that in order to get decent charge at low revs you need big pulleys on the engine. Even mine with a 10"pulley only does about 10amps at tick-over.

I always work on the KISS principle no added gizmo's as if the system is designed built correctly they are not needed.
I know this wont solve it in the short term but its something to think about long term.
 
I've been reading this with interest, but feel a need to discharge my duty of care!

I think the original mention of altering pulley sizes, to obtain sufficinet alternator revs at engine tick over, was in a hypothetical sense only. This is a bad idea because does your engine no good to sit at tick over, especially under little or no load, for extended periods...
 
tobble said:
I've been reading this with interest, but feel a need to discharge my duty of care!

I think the original mention of altering pulley sizes, to obtain sufficinet alternator revs at engine tick over, was in a hypothetical sense only. This is a bad idea because does your engine no good to sit at tick over, especially under little or no load, for extended periods...
Click to expand...

not hypothetical at all been doing it for years with great success:
Totally agree about running engines at tick-over however in general inland marine engines work way below their maximum revs all the time.
Now another thing that gets flagged up is maximum speed for the alternator this is normally about 10,000rpm, on my engine the max revs are about 2200 so in order to get even close to the maximum speed for the alternator I would need a 4:1 pulley ratio, so if the alternator pulley is 2.5" the crank pulley can be 10". This has many advantages better grip on the belt and better charging at low revs, much of my boating is done between tick-over and 1100rpm, only getting above that when on a tideway, but the alternator spins and charges as if it was at 2200 rpm.
The largest conversion I did was on a JP3 1200rpm max just ran the alternator belt round the flywheel (23") got a 8:1 ratio charged from tickover (400rpm)
 
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Job done; however it happened, the keyswitch was wired up so when the engine was running the glow plugs were on, and when you turned the key to heat they turned off! Can't imagine how this came about, wasn't like it at the end of last season, neither I or the boatyard have had any reason to do work behind the dashboard for some time.

Thanks for all the thoughts

IanC
 
ianc1200 said:
Job done; however it happened, the keyswitch was wired up so when the engine was running the glow plugs were on, and when you turned the key to heat they turned off! Can't imagine how this came about, wasn't like it at the end of last season, neither I or the boatyard have had any reason to do work behind the dashboard for some time.

Thanks for all the thoughts

IanC
Click to expand...

There is not a chance of getting that diagnosed over the interweb.
It does however beg the question how long has it been like that
 
Julian

I think the clue was my clamp ammeter telling me there was 20 amps going into the starter battery at tickover & 50 amps or so at a higher revs. Been pondering this all day & convinced myself it was something to do with glowplugs (5 cylinder perhaps = 50-75 amps?), but usually when you come up with the answer to a problem like this somehow you're always wrong. Couldn't believe it when I put the multimeter on the keyswitch lugs and found my answer could have been correct. When I worked out which lug was the correct one, and started the engine, & saw the panel voltmeter give it's usual reading of 13.3-13.5V (it's measuring the starter/engine circuit, not the domestics) I was almost deflated with nothing left to solve.
However, can now install the new shunts & get the amp hour meter going, install the Smartguage, and get on with the real job of varnishing.

Off to the other boat this weekend at Bradwell, coming for a sail?

IanC
 
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