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Post Info TOPIC: Low Alternator Output at Idle


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Low Alternator Output at Idle


What are the alternator and crankshaft pulley diameters on my Beta 38 WOC K35616? I am trying to figure out why charging to the house battery bank is not stronger at idle or slightly above. I do have a Balmar MC-614 regulating the alternator so fully realize that I must also understand exactly what that is doing, but I do not have any delays or belt load manager reductions programmed into it and so expect it to ramp the field fairly quickly in attempt to reach the target charging voltage programmed for AGMs. What I see instead is a current DRAIN of about -2A on the battery at idle (likely power for the engine control panel and Balmar), which will increase to about 9-15 A net charge if I increase RPM from ~800 RPM idle to ~1100 RPM. According to the ISKRA data sheet (attached), if I have the right reference, my 120 A alternator should be able to output (at full field) roughly 50 A even at idle if I assume the alternator RPM to be about twice the engine RPM or about 1600/800 RPM. So I'm just trying here to sharpen up my understanding of the engine/alternator side of this before diving more deeply into the Balmar workings. 



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Your engine should idle at about 900 RPM, not 800.

You have the wrong reference. The alternator that we supply is a Mahle AAK. Iskra/Letrika sold out to Mahle some years ago.



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Thank you. I was aware of the Mahle ownership of Iskra, but assumed, apparently incorrectly, that the product designs and ranges are roughly the same. Which Mahle/Letrika AAK part is the 120A optional one supplied on my engine, so that I can reference the correct data? In any case, it looks like any of the AAK range should also be able to easily deliver >20A at idle, of course assuming full field from the regulator.

And, yes, I see that idle listed on my test certificate is 865 RPM; I was just glancing at the panel tachometer from down in the boat, but should get a strobe tach to get more precise numbers.

Still relevant is the pulley diameter question. Are you able to tell the main crankshaft and alternator pulley diameters for my engine, since it must be the ratio of those that relates alternator RPM to engine RPM?



-- Edited by Blue Moon on Wednesday 26th of August 2020 05:21:09 PM



-- Edited by Blue Moon on Wednesday 26th of August 2020 05:21:31 PM

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The crank pulley is 130mm.
Alternator pulley is 80mm.
We can supply a smaller alternator pulley which will call for the tach to be re-calibrated.

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Thanks for those numbers, which bring the picture into focus. With my factory idle speed of 865 RPM and crank/alternator pulley ratio of 130/80=1.63, my alternator should be turning 1405 RPM at engine idle. n0 for the Mahle 120A AAK alternator is 1400 RPM, meaning that no output current should be expected from it at that RPM. So it seems that, by system design, there would be no charging at idle for my engine. Is this correct?

Is the alternator pulley supplied with the optional 120A alternator that same diameter as the one supplied on the standard 70A alternator? n0 for the 70A alternator is a good bit lower, which means that one would charge at engine idle with the 80 mm pulley. Was the choice of 80 mm pulley for the 120A alternator intentional; that charging would not begin at engine idle, or was that just a carryover from the 70A alternator pulley, guaranteed to work for belt length and adjustment but, unfortunately, not for charging at idle?

Would the bend radius for the serpentine belt still be bg enough if dropping to a 70 mm alternator pulley?

Changing the alternator pulley shouldn't be too difficult. What is involved in recalibrating the tachometer? Could I do that on the boat? Does it require special tools or access?



-- Edited by Blue Moon on Monday 31st of August 2020 07:41:49 AM

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We can supply a 62mm pulley at no charge. You will just have to pay for the shipping.
We can also supply the VDO "tool" to adjust the tach in the same package. You need access to the back of the panel plus a photo tach to check the calibration.

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OK. Ill call the office to make arrangements. Thank you for the continued great support.



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