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Old 08-11-2013, 21:07   #16
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Re: Question about current flow through alternator

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Originally Posted by phorvati View Post
I'm not on the dock. What does dock have to do with anything?
Starting battery gets depleted VERY little during starting, unless you use it for purging air or priming diesel fuel lines.

100 amps for 15 seconds = 0.41 AH,
so to put that back it takes
0.4 Amps for 1 hour.
So any trickle charger will keep your starting bank happy. If you can set it up to automatic so you don't have to think about it, thats the best. Starting bank does need to be charged during engine runtime, and it does not need to be charged from the alternator.
I probably put at least 5AH every day into starting bank via aux. solar charger regardless of if i started the engine or not. So my starting bank has been happy for a decade or so. I've changed it once.

The problem is if you start your engine numerous times over a long period of time and don't put any AH back.

the engine uses power while running.. fuel pump, gauges etc. most will draw a few amps while running. some draw 10a +. go do a 12h all night motor trip and see what happens. not charging the engine battery is just a bad idea.
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Old 08-11-2013, 21:15   #17
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Re: Question about Current flow through Alternator

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Originally Posted by Dsanduril View Post
A picture (even crude) being worth a thousand words, I think we're mostly saying...

Attachment 70049

Can you confirm that the "current setup" is what you have?
now with the pic it's easier to see that the engine battery will actually drain into / charge the house bank when you are sitting at anchor once the house bank is lower. the engine battery acts as the charger for the house bank... not good at all.

properly wired the batteries are all behind there own diode.
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Old 09-11-2013, 05:55   #18
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Re: Question about Current flow through Alternator

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Originally Posted by smac999 View Post
now with the pic it's easier to see that the engine battery will actually drain into / charge the house bank when you are sitting at anchor once the house bank is lower. the engine battery acts as the charger for the house bank... not good at all.

properly wired the batteries are all behind there own diode.
Ahh- so that was sort of the root of my initial question! So you're saying that the current WILL flow through the alternator even when the engine is NOT on, right?
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Old 09-11-2013, 08:11   #19
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Re: Question about Current flow through Alternator

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Originally Posted by teneicm View Post
Ahh- so that was sort of the root of my initial question! So you're saying that the current WILL flow through the alternator even when the engine is NOT on, right?
No.

If you look at Dsundiral (sp) drawings, how your boat is wired now, the starter motor is wired to the starter, the starter to the alternator and the alternator to the diode. This is the same thing as a single wire from the starter battery to the diode. Diodes conduct only in one direction and depending on the material it is made of won't start conducting until there is a difference in voltage across it. If it is silicon, that is 0.7 volts. So if the start battery is 12.6 V and the house bank is 11.9 V the diode will allow the start battery to discharge into the house bank. This has nothing to do with the alternator, it is because there is effectively a single piece of wire between the start battery and the diode leading to the house bank.

The proposed drawing he made will work, but it does not match what you said you wanted to do. You wanted to leave the wire between the starter and alternator. Put that wire back in his drawing, and again you have the start battery directly connected to the house battery. Again it has nothing to do with the alternator.

Picture the posts on the alternator and starter as only being a piece of metal to tie wires together that are too short.
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Old 09-11-2013, 08:47   #20
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Re: Question about Current flow through Alternator

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Originally Posted by teneicm View Post
Ahh- so that was sort of the root of my initial question! So you're saying that the current WILL flow through the alternator even when the engine is NOT on, right?
that was sort of the question back. Are there two output posts on the alternator? One that goes back the starter, and the other to house? Or is there one output post with doubled up wiring?

One output post then there will definitely be flow. But since you say "through the alternator" it makes it sound like there are two separate connections on the alternator; one for the starter and one for the house. If that is the case it is likely a dual-output unit, and then there would be isolation in the alternator. Don't think I've ever seen a 40A dual-output unit, but could exist. So, if there are two output posts we'd need to know make/model of the alternator to evaluate.
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Old 09-11-2013, 09:04   #21
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Re: Question about Current flow through Alternator

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Originally Posted by Dsanduril View Post
that was sort of the question back. Are there two output posts on the alternator? One that goes back the starter, and the other to house? Or is there one output post with doubled up wiring?

One output post then there will definitely be flow. But since you say "through the alternator" it makes it sound like there are two separate connections on the alternator; one for the starter and one for the house. If that is the case it is likely a dual-output unit, and then there would be isolation in the alternator. Don't think I've ever seen a 40A dual-output unit, but could exist. So, if there are two output posts we'd need to know make/model of the alternator to evaluate.
Not sure if I'm right, but from his description I believe he has the original wiring like my Yanmar has where the alternator has a 10 gage wire from the alternator to the starter, so the alternator charges the battery through the one starter cable. A previous owner added a large wire with a diode to the house bank at the alternator. He needs to remove the wire between the alternator and starter to make any of the changes work.
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Old 10-11-2013, 17:52   #22
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Re: Question about Current flow through Alternator

OK- tackled this today. Re-routed the Mastervolt charger and the two alternators to come together into a large 4 post bus bar. The bus bar is also connected to the house battery bank as well as two cables that run to separate, fused ACRs. One ACR is connected to the port side start battery, the other ACR is connected to the starboard side start battery. Also disconnected the smaller gauge wires linking the starter motor to the single alternator post. Got rid of the diode isolator and the tangle of wires coming out of it.

Everything seems to be working ok.

The only somewhat odd thing was that when I had removed the small wire off the alternator/ start motor AND the large gauge wire off the alternator post, I still had 12.4 volts at the alternator post. One of the other wires coming out of the alternator (but not the main post) runs to small fuel pump, and it has voltage at all times, so there must be some source of electricity from the engine wiring harness/ ignition wiring.
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Old 11-11-2013, 17:51   #23
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Re: Question about Current flow through Alternator

if this is the case. it's probably coming from the engine battery. meaning your just connected the engine battery to the house battery. that power needs to be sourced out and fixed.

if there is no wires on the post there shouldn't be power.
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Old 11-11-2013, 18:03   #24
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Re: Question about Current flow through Alternator

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Originally Posted by teneicm View Post
The only somewhat odd thing was that when I had removed the small wire off the alternator/ start motor AND the large gauge wire off the alternator post, I still had 12.4 volts at the alternator post. One of the other wires coming out of the alternator (but not the main post) runs to small fuel pump, and it has voltage at all times, so there must be some source of electricity from the engine wiring harness/ ignition wiring.
Was the ignition key off when this was measured? Can't think of any reason there would be 12V at the alternator with the batteries disconnected and the ignition off. Can you tell what the post label is where this is measured? Not "61" by any chance? Or "F"?
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Old 12-11-2013, 08:49   #25
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Re: Question about Current flow through Alternator

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Was the ignition key off when this was measured? Can't think of any reason there would be 12V at the alternator with the batteries disconnected and the ignition off. Can you tell what the post label is where this is measured? Not "61" by any chance? Or "F"?
Seems the post is labeled "B". There is also a 3-pin wiring clip below the post. I need to figure out what those wires coming out of the clip are doing...
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Old 12-11-2013, 15:49   #26
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Well on a GM alternator the B post is the battery and is "hot" all the time as it's a direct to the battery connection, maybe this is the same?
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Old 12-11-2013, 19:36   #27
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Re: Question about Current flow through Alternator

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Originally Posted by teneicm View Post

The only somewhat odd thing was that when I had removed the small wire off the alternator/ start motor AND the large gauge wire off the alternator post, I still had 12.4 volts at the alternator post. One of the other wires coming out of the alternator (but not the main post) runs to small fuel pump, and it has voltage at all times, so there must be some source of electricity from the engine wiring harness/ ignition wiring.
Do you mean that there is voltage at the alternator with the house bank disconnected from it with a master switch?
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Old 13-11-2013, 06:51   #28
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Re: Question about Current flow through Alternator

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Do you mean that there is voltage at the alternator with the house bank disconnected from it with a master switch?
I don't think I had the house bank off at that point. But the only wires connected to the alternator were through the small alternator wiring harness that connected into the main engine wiring harness bundle. So the source of the voltage must have come from the engine wiring harness- unless somehow something was getting back fed through the negative side.
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Old 13-11-2013, 07:16   #29
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Re: Question about current flow through alternator

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the engine uses power while running.. fuel pump, gauges etc. most will draw a few amps while running. some draw 10a +. go do a 12h all night motor trip and see what happens. not charging the engine battery is just a bad idea.
All that is powered off house bank. My glowplugs, actually thermostart in case of 4-108 is also powered off house bank. Like i said originally, the ONLY thing powered off of starting bank is the starter motor, and that runs about 10-30 seconds when you start the motor.
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Old 13-11-2013, 08:14   #30
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Re: Question about Current flow through Alternator

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All that is powered off house bank. My glowplugs, actually thermostart in case of 4-108 is also powered off house bank. Like i said originally, the ONLY thing powered off of starting bank is the starter motor, and that runs about 10-30 seconds when you start the motor.

Very few boats are wired that way. Most sailboat aux engines take instrument, glow, fuel pump etc. from the starting circuit. You can re-wire it not to do that but now you are depending on two banks for the operation of the engine... In the event of a bank failure it would require an at-sea re-wire to switch banks...

On some of the new common rail engines or engines with pre-heaters etc. the consumption just to run the engine can exceed 10A..
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