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Old 30-05-2020, 15:06   #1
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Alternator connected to starter WTH

So the pandemic has given me time. I am trying to figure out and draw my electrical system. I just spent two hours in awkward positions before I realized that my 55 amp alternator is connected to my starter motor by a #8 wire. There is also a 1/0 cable that runs from the starter motor to a battery position switch (off, 1, 2, Com). The switch is connected to my house bank.

This has worked okay in the past but I am thinking the alternator connection should have a bigger wire, at the least.

Does connecting the alternator charging system through the starter make any difference?Click image for larger version

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Old 30-05-2020, 15:14   #2
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Re: Alternator connected to starter WTH

That will work but...

Scroll down most of this article and have a look at the schematic posted about an actual boat wiring diagram. I followed this when I redid mine and it works brilliant. This article was what convinced me to get rid of the 1,2, ALL switch and use an ACR to keep my starter battery topped off. All my charging goes to house. And my alternator has its own cable leading back to positive house bus. I use a Blue Seas parallel battery switch instead.

https://marinehowto.com/automatic-charging-relays/
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Old 30-05-2020, 15:15   #3
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Re: Alternator connected to starter WTH

I can't think of any reason for your alternator to be connected to your starter; the alternator would usually be connected to the battery via a wire harness, not taking power from the starter.
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Old 30-05-2020, 15:27   #4
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Re: Alternator connected to starter WTH

This is the way my engine is. I don't know if it is "best" but it is OK. Both the alternator wire and the battery wire (thru the switch) are on the starter terminal. This is the same as if the alternator wire and starter wire were both on the hot post of the battery. Alternator wire to starter can be lighter as it is shorter and likely does not carry as much current as the starter wire.
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Old 30-05-2020, 15:28   #5
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Re: Alternator connected to starter WTH

My factory alternator wiring is the same as described in the OP. Alternator feeds power to the starter terminal, then the heavy cabling to the battery switch on each engine. It's an easy way for the builder to keep the alternator run short before it joins an existing large cable that's got no load on it once the engine is running.
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Old 30-05-2020, 15:36   #6
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Re: Alternator connected to starter WTH

Is there a fuse between your alternator and starter?
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Old 30-05-2020, 15:43   #7
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Re: Alternator connected to starter WTH

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Originally Posted by OloteleMtn View Post
Is there a fuse between your alternator and starter?
Not on mine, at least not that I've ever noticed. Now that you mention it, there really should be.
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Old 30-05-2020, 18:40   #8
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Re: Alternator connected to starter WTH

mine either
if that fuse goes, I believe it would kill the diodes, just like disconnecting it from the battery, or like some older "break before make" battery switches.
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Old 30-05-2020, 18:51   #9
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Re: Alternator connected to starter WTH

Quote:
Originally Posted by Charlie View Post
So the pandemic has given me time. I am trying to figure out and draw my electrical system. I just spent two hours in awkward positions before I realized that my 55 amp alternator is connected to my starter motor by a #8 wire. There is also a 1/0 cable that runs from the starter motor to a battery position switch (off, 1, 2, Com). The switch is connected to my house bank.

This has worked okay in the past but I am thinking the alternator connection should have a bigger wire, at the least.

Does connecting the alternator charging system through the starter make any difference?Attachment 216239
This is a very very common way of connecting the alternator output by the engine manufacturer for the reasons listed upthread and entirely suitable for the average boat.

Once you start adding large battery banks, external regulators and such like, then this arrangement becomes less suitable.
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Old 30-05-2020, 20:08   #10
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Re: Alternator connected to starter WTH

I think your laz switch is drawn wrong. Likly the stater comes from the common. And the Batteries go to 1 and 2. Otherwise it is very strange.

The alternator is normal. And fine if you have a stock 50-60a alt.
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Old 30-05-2020, 20:12   #11
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Re: Alternator connected to starter WTH

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Originally Posted by OloteleMtn View Post
Is there a fuse between your alternator and starter?
There is no point fusing a 2’ wire. Just make sure it’s secure. The chances of it cutting and shorting are small. If it’s protected it can be 52” without fusing.
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Old 30-05-2020, 20:45   #12
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Re: Alternator connected to starter WTH

Quote:
Originally Posted by sanibel sailor View Post
This is the way my engine is. I don't know if it is "best" but it is OK. Both the alternator wire and the battery wire (thru the switch) are on the starter terminal. This is the same as if the alternator wire and starter wire were both on the hot post of the battery. Alternator wire to starter can be lighter as it is shorter and likely does not carry as much current as the starter wire.
Quote:
Originally Posted by Wotname View Post
This is a very very common way of connecting the alternator output by the engine manufacturer for the reasons listed upthread and entirely suitable for the average boat.

Once you start adding large battery banks, external regulators and such like, then this arrangement becomes less suitable.
This arrangement is pretty much standard for "as delivered from the factory". Curiously, the ampacity of 8 AWG wire with 90C insulation is 55A, the maximum available from your alternator. So the engine manufacturer is probably within spec. As Wottie says, it stops working well with big banks that load the alternator more heavily than just the starting battery, or when you swap in a larger alternator. There's nothing wrong with either A) running a large cable direct from alternator to battery connection/bus or B) replacing the manufacturer's wire with a larger cable (this does have one extra connection to corrode, but also simplifies the upgrade).

One advantage to the OEM installation, there are only two cables to the engine, the positive and the negative, everything else is contained in the wiring harness. Makes connecting/disconnecting from the battery simpler.
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Old 30-05-2020, 20:45   #13
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Re: Alternator connected to starter WTH

Wait! Maybe, may be some basic stuff, try this:


OEM 1-2-B Switch Wiring History http://c34.org/bbs/index.php/topic,4....html#msg30101
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Old 30-05-2020, 22:05   #14
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Re: Alternator connected to starter WTH

It is the way engine manufacturers wire their engines and builders simply add a large conductor from the battery switch to the starter. Easy, peasy...and cheap.

Problem is that the alternator output wire is not protected by a fuse or circuit breaker and two types of service are mixed: the alternator is a source and the starter is a load. Yes it works, but as discussed up-thread, when you start installing a real battery bank and start managing your charging sources and loads as they should be managed, this system gets revised ASAP.
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Old 30-05-2020, 22:14   #15
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Re: Alternator connected to starter WTH

As already said this is the way it would have come from the engine builder. As long as you don't have an external regulator and/or a high output alternator it is OK. Running the alternator output to the positive lug on the starter is a simple way to have an inexpensive way to share a heavier wire to the battery.

That design is intended to put the power back into the battery that was used to start the engine and supply power to run the engine instruments while the engine is running. It is totally inadequate to charge a house bank on a cruising sailboat. If you upgrade the charging system that has to be changed.
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