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Old 18-09-2020, 05:36   #1
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Alternator current dropping slowly after start

Hello folks. I just noticed today after a 4 hour motoring trip that my batteries didnt charge during this period. After looking around I noticed that the battery monitor reports a net current of 30 amps going into the battery right after start. However shortly thereafter it starts dropping slowly but surely. After a few minutes is now back to a net current lost.. It is not a sudden drop but truly a slow declining curve. Does this ring a bell to anyone? what is worth looking into first? Thanks for any tips you might have
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Old 18-09-2020, 05:43   #2
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Re: Alternator current dropping slowly after start

I would suspect the alternator and/or it’s charge controller.

Balmar has a good trouble shooting guide in their installation manual for their charge controller.
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Old 18-09-2020, 05:47   #3
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Re: Alternator current dropping slowly after start

Many alternators will push lower voltage and also somewhat lower max current as they heat up, at least with an internal regulator. But going from +30 amps to net negative does sound like a problem to me.

How much power are you drawing when this happens and what's your alternator setup?
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Old 18-09-2020, 05:54   #4
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Re: Alternator current dropping slowly after start

Quote:
Originally Posted by crankysailor View Post
Hello folks. I just noticed today after a 4 hour motoring trip that my batteries didnt charge during this period. After looking around I noticed that the battery monitor reports a net current of 30 amps going into the battery right after start. However shortly thereafter it starts dropping slowly but surely. After a few minutes is now back to a net current lost.. It is not a sudden drop but truly a slow declining curve. Does this ring a bell to anyone? what is worth looking into first? Thanks for any tips you might have
This would be normal behavior as your batteries charge. As they charge the current drops, and once fully charged the alternator will drop to float or shut off.

More details would be needed to know if there is a problem or not. Alternator setup, other charge sources, battery type and capacity, and what sort of battery monitor.
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Old 18-09-2020, 07:21   #5
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Re: Alternator current dropping slowly after start

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Originally Posted by rslifkin View Post
Many alternators will push lower voltage and also somewhat lower max current as they heat up, at least with an internal regulator. But going from +30 amps to net negative does sound like a problem to me.

How much power are you drawing when this happens and what's your alternator setup?


about 3 amps would be in use more or less...just a small fridge.

I would gladly tell you what my alternator setup is if I knew what "an alternator setup" was
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Old 18-09-2020, 07:29   #6
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Alternator current dropping slowly after start

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Originally Posted by wholybee View Post
This would be normal behavior as your batteries charge. As they charge the current drops, and once fully charged the alternator will drop to float or shut off.



More details would be needed to know if there is a problem or not. Alternator setup, other charge sources, battery type and capacity, and what sort of battery monitor.


Well..I have solar power as well (and an old wind generator that does little else other than make a lot of noise)...the solar controller reports being in bulk phase and says it produces about 5 amps but now the sun is down so it is probably less now. I just connected shore power and the battery monitor reports happily that it is taking 30amps in...So the batteries were indeed in need of charge as the battery controller reported I suppose.

I have a victron blue power 30A charger (I assume that is for shore power) and a victron solar MPPT 75/15.. I assume there is a charger controller for the alternator somewhere but I am looking for it (new to me boat )
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Old 18-09-2020, 09:46   #7
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Re: Alternator current dropping slowly after start

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Well..I have solar power as well (and an old wind generator that does little else other than make a lot of noise)...the solar controller reports being in bulk phase and says it produces about 5 amps but now the sun is down so it is probably less now. I just connected shore power and the battery monitor reports happily that it is taking 30amps in...So the batteries were indeed in need of charge as the battery controller reported I suppose.

I have a victron blue power 30A charger (I assume that is for shore power) and a victron solar MPPT 75/15.. I assume there is a charger controller for the alternator somewhere but I am looking for it (new to me boat )
We have a 100 amp alternator on our engine and 200 amp charger, and 30 amps is normal for us in the absorption phase of charging.

How many amps your batteries will accept depends on the SOC and the higher the voltage the less amps they will accept. Solar and wind will bump up the voltage so unless they are turned off or bypassed or regulated they will interfere with alternator or shore power charging.

I have NEVER been able to get up to 100% on alternator alone, even when motoring for 24 hours or more. This is probably a good thing as it's the regulator's job to prevent damage to the batteries from overcharging.

Your setup might be fine and everything working correctly, you just need to understand how it works and how to work with it and how to care for it. For example, once per week I turn off my solar and wind and use the generator on the hook to get my battery bank to 100% for equalizing or "balancing" my AGMs.I learned the hard way that my batteries don't like it if I don't do this in this way.
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Old 18-09-2020, 12:57   #8
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Re: Alternator current dropping slowly after start

It does sound like an external regulator for the alternator thinks the alternator or the battery is too hot, if it has an external regulator. Tracking down what kind of alternator you have will help. If you aren't sure, try posting a photo here and others might recognize it. External regulators of



What is the voltage at the battery when all this is happening?



The solar charger can confuse the alternator regulator by putting a high enough voltage at the battery that the alternator thinks its done. This would only be on a fairly full battery bank and under little load. You can use the victron app to connect to the MPPT controller and disable the charger and see how things change, but I see you experience this same behavior at night.
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Old 18-09-2020, 15:47   #9
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Re: Alternator current dropping slowly after start

Given that we don’t know whether you have an external regulator or what voltage your alternator was putting out it’s kind of hard to fault find. However amps into battery rely on volts at battery i.e. charge volts have got to be greater than battery volts. Ohm was right about electricity. You were putting in 30amps so obviously this was the case but not enough. Alternator may have lost a diode at which point the performance will drop to about 58%. The solar and wind can confuse a battery charger into thinking the battery is charged and the charger will go to float but internally regulated alternators just put out a voltage unless they get hot. What size alternator do you have? 30amps into battery doesn’t mean the alternator isn’t producing it may be going somewhere else. I really think you need to get a few voltage readings like at the alternator, at the batteries, careful here to measure on the battery posts to make sure you know what being applied to the actual battery. Then measure the voltage on the cable terminals this will show up bad connections at the battery. Feeling the terminals will give indication of bad connections too. Bad connections give off heat. If the battery charger is charging good chance this isn’t the problem but just put your finger on the terminals while a good current is flowing. Problem is now the batteries have been charged you need to load your system up to check things. Careful if you turn your shore power off to turn back on. Had chicken in freezer when I forgot recently, not good. I have a little blow heater I run with our inverter which is great for load testing.
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Old 19-09-2020, 13:35   #10
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Re: Alternator current dropping slowly after start

Do we know what the voltage out of altenator is? Dis it stay at 12volts or was it 13 or higher?
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Old 19-09-2020, 16:19   #11
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Re: Alternator current dropping slowly after start

Alternator output should be at least 13.8volts but up to 14.5 volts. If a diode is lost it would drop to 12.5 volts or so. My Valeo 125amp when internally regulated was 14volts but I fitted a Balmar external regulator, shortened the cable runs and
improved the alternator cooling. I now run it at 14.3 volts and get 104amps with alternator temperature around 86°c. We have 360ah of LiFePO4 use about 20% in a day so it only takes about 40 minutes to replace. My biggest problem now is getting hot hot water.
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Old 20-09-2020, 04:27   #12
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Re: Alternator current dropping slowly after start

So I measured the voltage at the alternator and there was nothing...Ok that settles it I thought. Then later while the engine was running I noticed that the voltage on the battery monitor was back up to 14,2V.. It s starting to look like something "loose" inside the alternator I guess?
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Old 20-09-2020, 05:46   #13
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Re: Alternator current dropping slowly after start

I was going to post a few suggestions. But, you haven't even told told what type of alternator or regulator you have.

Have you tried the very very very basics yet, like checking that the belt is tight enough?

If you really want help with your problem you need to tell us the alternator and regulator as a minimum.
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Old 20-09-2020, 06:09   #14
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Alternator current dropping slowly after start

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Originally Posted by sailorboy1 View Post
I was going to post a few suggestions. But, you haven't even told told what type of alternator or regulator you have.

Have you tried the very very very basics yet, like checking that the belt is tight enough?

If you really want help with your problem you need to tell us the alternator and regulator as a minimum.


Yeah belt is fine and alternator ist cranking...

Well...I must admit I don't know what alternator "types" there are... to me. I see the alternator...it's got a cable that goes to the battery. There is nothing else in between that I can see. What else can I say? the brand? Hitachi...12V..80A...what exactly do.you mean by "type". And like I say there is no external regulator that I can see nor any other component in between it and the battery. Now this is all in a hard to get to part of the boat and it is behind some spaghetti mess courtesy of previous owners...So maybe I am just not seeing it, dunno.
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Old 20-09-2020, 06:38   #15
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Re: Alternator current dropping slowly after start

Ok, based on that, it's probably the factory original internally regulated alternator that came with the engine. The issue is most likely internal to the alternator then, so it's probably down to either having it rebuilt or replacing it. Especially if the system worked fine up until recently.
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