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Old 04-19-2012, 10:35 AM   #1
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Zero amps charging at idle RPM, good or bad?

here is a question that has bothered me a bit and it relates to some other alternator questions that I have posted previously:

My engine idle is 650 RPM, with 1:2 pulley ratio it will give me about zero amps available for battery charging with an Electrodyne G-150 alternator at about 1300 alternator RPM. An equivalent 150 amps Delco-Remy/Prestolite has about 50 amps output at the same alternator RPM.

I consider this "lack of charging" at idle as excellent since it does not put any load on the engine just after it has been started, thereby reducing the wear on the cold engine. As we all know, 80% of the wear on an engine happens when it is cold, so "no charging" seems to be a way to reduce this wear.

Others have commented on how good it is that their alternator is capable of putting out large charging amps (say 50-60 amps) at idle, and several manufacturers use this as an argument for how good their alternators are.

For a trawler, which uses the engine for a looooong time every time it's being used, the charging amps at idle seem to me as being a useless argument for evaluating a trawler's alternator, but I would like to ask the clever guy's on the forum about their considerations and experiences.

Thanks
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Old 04-19-2012, 10:56 AM   #2
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I consider this "lack of charging" at idle as excellent since it does not put any load on the engine just after it has been started, thereby reducing the wear on the cold engine. As we all know, 80% of the wear on an engine happens when it is cold, so "no charging" seems to be a way to reduce this wear.
I can't believe the "load" is significant in this regard.
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Old 04-19-2012, 06:38 PM   #3
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If you switch the field, you can 'activate' the alternator when you think the motor is warm enough.

Also, most 3 stage regulators have a 'soft start' feature which lets the motor run for a minute or two before bringing the alternator on, and this is in a ramp up type field excitation.
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Old 04-19-2012, 07:41 PM   #4
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The load would be 3-4 HP, so not really significant.

But I consider it relevant to consider the "zero" idle RPM load, just to be on the safe side. The alternators in question put out the same amps at cruise RPM where the majority (say 99%) of the charging will be done.

The "zero" amps alternator simplifies an nstallation a bit as well, since the field switch can be avoided.

My thinking about the smart regulator soft start-up, is that it is to prevent "shock starts" of the system, more than to allow the engine to warm up. The 1 minute warmup is a plus though, but I don't think that 1 minute is enough to warm up any engine.
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Old 04-20-2012, 03:05 AM   #5
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Charging amps and alternator output are seldom the same thing since the alternator may pick up some or all of the house or system load at idle and may have insufficient output to provide any charging current. Published alternator output figures should be treated as guidelines, not reality.
For example, you may have a 25A system load on the batteries when the engine is not running. When you start the engine, if the alternator can only produce 25A at idle all 25A will go to the house and the charging amps will show zero.
The only way to really monitor this is to measure both the alternator output and the battery input which requires an unusual level of instrumentation.
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Old 04-20-2012, 06:07 AM   #6
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That "cold start" wear is the cranking and first operation till the oil fills all the galleries.

Once you have oil pressure its not "starting" simply cold, no problem with wear if modestly loaded.

The Amps at idle is great for cops and and taxi cabs or cars with 1000W boom boxes , or large hydraulic pump loads to hip hop the car.

It is also a help for folks with large DC loads at idle , like a windlass or bow thruster.

There is NO perfect, but the alt load on a cold engine would not be a concern.

AMP output when hot at modest cruise speed would be important with AGM.

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Old 04-20-2012, 07:21 AM   #7
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Originally Posted by Singleprop View Post
here is a question that has bothered me a bit and it relates to some other alternator questions that I have posted previously:

My engine idle is 650 RPM, with 1:2 pulley ratio it will give me about zero amps available for battery charging with an Electrodyne G-150 alternator at about 1300 alternator RPM. An equivalent 150 amps Delco-Remy/Prestolite has about 50 amps output at the same alternator RPM.

I consider this "lack of charging" at idle as excellent since it does not put any load on the engine just after it has been started, thereby reducing the wear on the cold engine. As we all know, 80% of the wear on an engine happens when it is cold, so "no charging" seems to be a way to reduce this wear.


Others have commented on how good it is that their alternator is capable of putting out large charging amps (say 50-60 amps) at idle, and several manufacturers use this as an argument for how good their alternators are.

For a trawler, which uses the engine for a looooong time every time it's being used, the charging amps at idle seem to me as being a useless argument for evaluating a trawler's alternator, but I would like to ask the clever guy's on the forum about their considerations and experiences.

Thanks
I think you're trying to justify as "good" what most of us would consider a problem. If it were my boat, I would do something about it so I was charging the batteries when the engine was operating.



What does the manufacturer recommend?
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Old 04-20-2012, 11:56 AM   #8
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See if your belts are tight enough.

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Old 04-20-2012, 12:07 PM   #9
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I donít understand. Amps are caused by demand/draw, so if there is not amps demand/draw there is not amps to be read. The Eagle has a DC and AC amp gauge in the pilot house so I can see what the amps being used are. No amp draw/used no amp reading.

Why would you be concerned at the altenator if the batteries are fully charged and no draw. What am I missing.
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Old 04-20-2012, 10:18 PM   #10
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Originally Posted by rwidman View Post
I think you're trying to justify as "good" what most of us would consider a problem. If it were my boat, I would do something about it so I was charging the batteries when the engine was operating.



What does the manufacturer recommend?
No - I am arguing that no load on a cold engine is good, and that this load can be reduced by selecting an alternator that has no output at idle - not even if the battery requires charging.

But - I am also asking if this is a valid consideration.

None of the trawlers really need charging amps at idle anyway unless the main engine is used as a generator.

I am getting more and more convinced that I don't want any charging when the engine is at idle, i just cannot see the reason why I need it.
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Old 04-21-2012, 05:13 AM   #11
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Why would you be concerned at the altenator if the batteries are fully charged and no draw.

Even fully charged batts have a slight draw , what a charger considers "Float" voltage ,

which makes up for normal internal losses.

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Old 04-21-2012, 07:55 AM   #12
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I might worrry if I ever heard an alternater chugging my engine...like a high output (150 amp) on a 15hp or so sailboat engine...but to worry about a 60-100 amp alt on a 100+ hp engine to me is like reinforcing my deck for an upcoming meteor shower...
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Old 04-21-2012, 09:12 AM   #13
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The OP's question "good or bad" appears to have been answered, "Bad."

Over my career we had many idling vehicles with lights, radios and and other electronic gear expected to be on all the time. A dead battery was no fun so keeping things charging at 600- 800 RPM was essential. Ditto a boat waiting for a lock, bridge turn or clearing a long no wake zone.

Question asked and answered.
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Old 04-22-2012, 01:48 AM   #14
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Sunchaser,

That's an argument that I can understand and see the logic off. Thanks.
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