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Alternator question

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Iskra  95A alternator with Cargo 14.7V regulator.

Came to the boat today,  batteries near fully charged.  Started up and after 5 minutes noticed an alarm from blue Sea meter.   At 1200 rpm, alternator/battery voltage was 15.2V and charging at 1.2 A.  Dropped revs to 850 and voltage dropped to just under 15V still with 1.2 charge.

 

Diodes? Regulator?  Don't know.  Decided to change regulator first as that was relatively easy with alternator in situ.  If problem the same, swap alternator for known good one.  Before I started attacking anything thought I would just try it again.  Alternator volts now showing a shade under 14.4V and charging at 20A - i.e. everything normal. Current dropped fairly quickly to about 4A and voltage rose to just above 14.5V - i.e. everything normal.  Stopped and restarted a couple of times and all worked as normal. 

 

15 minutes later and voltage is 14.8V, current 1.8.  Increase revs to 1300 and voltage DROPS to 14.7V - current stays about the same.

 

Any suggestions to what is going on?

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My initial thoughts are that the maximum output voltage and current are both controlled by the regulator, so if either are too high it sounds like a regulator issue.  As to voltage dropping from 14.8 to 14.7 this may just be the regulator hunting, how many digits does your meter display after the decimal point?  If only one then you could get a 0.1 volt reading change for just a few mV change as the display will round to the nearest digit.   What is the normal charging voltage?

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2 minutes ago, Chewbacka said:

My initial thoughts are that the maximum output voltage and current are both controlled by the regulator, so if either are too high it sounds like a regulator issue.  As to voltage dropping from 14.8 to 14.7 this may just be the regulator hunting, how many digits does your meter display after the decimal point?  If only one then you could get a 0.1 volt reading change for just a few mV change as the display will round to the nearest digit.   What is the normal charging voltage?

Thanks for your prompt reply. 

Voltage displays to 2 decimal places, ammeter to 1.   Normal charging voltage with full batteries is around 14.6 - can vary from 14.5 to 14.7.

 

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Restarting the engine will drop some battery charge so when you restart the regulator goes back to charge rather than float. Probably nothing amiss, see if it does it again.

Coud be a connection that is not perfect fooling the regulator, check the isolator switch/s especially if the nasty red key ones.

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3 minutes ago, Boater Sam said:

Restarting the engine will drop some battery charge so when you restart the regulator goes back to charge rather than float. Probably nothing amiss, see if it does it again.

Coud be a connection that is not perfect fooling the regulator, check the isolator switch/s especially if the nasty red key ones.

All connections good.   Restarted after 15 minutes.  As expected, starts at 14.55V 8A,  changes over a couple of minutes to 14.8V 1.3A.

 

What bothers me is the behaviour after ini.tial start, and slightly higher than normal float voltage

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Could this have been a surface charge remaining from the battery charger rather than an alternator issue?  It seems all other alternator readings bar the first are perfectly fine. Certainly worth looking at what voltages you get on the various charger states I'd have thought.

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OK thanks to all for your replies.

 

I've changed the regulator and started up and all is behaving as it should.    Drained a bit through fridge and inverter over the last hour or so, then on starting amps were up around 60+, dropping fairly quickly to about 11A with voltage at 14.4.  I would now expect the current to drop to 3 or 4A and voltage rise a little over the next hour.

 

Batteries probably were not 100% charged, but getting close to it.  Best bit it that it started to rain two minutes after I finished.  Most annoying part was trying to find the little nuts off the back of the alternator which hid under the engine.

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I have a sneaky feeling that cold alternator regulators might wander a bit voltage wise until they warm up settle down. It would not normally be noticed because typically the batteries would be somewhat discharged and the self regulation for current would have pulled the voltage down a bit and by the time the regulator came into play it would have warmed up. Only a theory

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My initial thought was a dodgy connection on the sense wire, but that’s discounted with “all connections good”. So maybe a failing regulator, now fixed by replacement. On the other hand Tony B’s theory could be right too :)

 

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Normal. 14.7V 20A, that's what I read isn't it?

Fault condition. Overvoltage, undercurrent.

Firstly, that overvoltage can't be arriving at the batteries can it, or the current would rise with it, Mr Ohm says so. So there has to be an issue with the main feed to the battery I would have thought, and that would be a good place to start. Even that obvious (from my point of view) deduction doesn't make  a lot of sense because I'm pretty sure these are machine sensed alternators so the voltage should only go over the set point with a shorted regulator or a blown field diode, but in either case the current should have gone up with it. This is one of those where I can't see a theoretical basis to tie all the observed symptoms together.

 

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