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Old 11-24-2013, 08:36 PM   #1
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Alternator Output Connection?

I have twin FL 120's. In a typical installation, where are the alternators charging outputs connected? I assume the alternators are connected to the starting bank in some fashion. What I do not know is how, or where the connections are made in a "typical" installation.
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Old 11-24-2013, 08:51 PM   #2
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I have twin FL 120's. In a typical installation, where are the alternators charging outputs connected? I assume the alternators are connected to the starting bank in some fashion. What I do not know is how, or where the connections are made in a "typical" installation.
It's according to what your two alternators are set to charge. If one is dedicated to the house bank, it should connect there. If one is dedicated to the starting bank it may connect near the starter, or they both could connect that way. If they both connect that way you may have a battery combiner to share the charge. My two alternators are dedicated to different banks.
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Old 11-24-2013, 09:24 PM   #3
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Typical Battery Isolator Circuits - ARCO
Maybe this will help.
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Old 11-24-2013, 10:00 PM   #4
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It's not that simple...depends on who installed the engine.

My engine was installed by a typical engine knucklehead that sends the charge back through a tiny 10 gauge wire (maybe smaller) to the buss bars where it is split to include the starting circuit as well as feed the 1/2/both switch for charging purposes.

Bad design and I'm trying to rewire it to a dedicated start batt now but having trouble getting rid of a surge in the charge circuit that causes y gauges to oscillate.

Anything is possible and many engine harness installs really disappoint me with the electrical side of things.
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Old 11-24-2013, 10:46 PM   #5
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Gentlemen, you have renewed my faith in me. Thinking my question too stupid to publish, I was at first reluctant to do so. But it is apparent it was not. Thanks to Anthonyd for the link. That does indeed help. Might need to get one of those isolator thingies...and do some wire tracing on my boat.
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Old 11-24-2013, 11:02 PM   #6
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If the engine were installed just like it was shipped from the factory (and as others have noted, this doesn't usually happen) then the alternator output is wired to the heavy lug on the starter. From there it is wired with 2/0 or so wire to the starting battery.

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Old 11-25-2013, 12:16 AM   #7
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A common method from that era was to connect each alternator output to an amp gauge at the helm station, then back to the starter lug. #10 wire was often used which resulted in a significant voltage drop -- the alternator might be putting out 14.6 V but the battery is only seeing 13.8 V, which makes for low charging amps and long charging times. Effectively, the alternator thinks the battery is fully charged but the battery is really only about 70% charge.

In twin-engine installations 2 battery switches of the "off-1-both-2" variety were used so either battery could start either engine, or either engine could charge either or both batteries.

Some of the Taiwan Trawlers with Lehmans had a variation on this. Both starters were connected to a common terminal, and 2 "off-on" battery switches were used to connect either or both batteries to the starters. The house distribution panel was connected to an "off-1-both-2" switch so either or both battery banks could be used to provide power for the lights, refrigerator, etc.

Now a common method has one engine equipped with a high-output alternator connected directly to a large battery bank for the house loads, and the other engine has the original alternator charging a starting battery that provides power for starting either engine. An "off-on" battery switch can be used to connect the two battery banks if needed, to either use the house bank to start or use the other alternator to charge the house bank.

Connecting the alternators directly to the battery, battery buss, or starter lug with a #4 or #6 cable will significantly improve charging capability. This bypasses the amp gauge at the helm station, but that gauge can be replaced with a voltmeter.

A fuse at the battery buss connection is a good idea to protect against a short circuit.
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Old 11-25-2013, 06:28 AM   #8
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>Bad design and I'm trying to rewire it to a dedicated start batt now but having trouble getting rid of a surge in the charge circuit that causes y gauges to oscillate.<

Use a field cut off or wire it to an oil pressure sensing relay during engine start..

A surge can also come from a slow Bendex release , as the starter is spun by the engine.

>A fuse at the battery buss connection is a good idea to protect against a short circuit. <<<
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Old 11-25-2013, 06:41 AM   #9
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>Bad design and I'm trying to rewire it to a dedicated start batt now but having trouble getting rid of a surge in the charge circuit that causes y gauges to oscillate.<

Use a field cut off or wire it to an oil pressure sensing relay during engine start..

A surge can also come from a slow Bendex release , as the starter is spun by the engine.

>A fuse at the battery buss connection is a good idea to protect against a short circuit. <<<
Alternator uses a pressure switch...

Starter as just rebuilt...

Web reading/suggestions seem to think I need some sort of load on the battery as often the gauges don't oscillate for a minute or so then start...as if when the voltage stabilizes after start...also doesn't do it if I have start and house banks tied together.

sorry to steal away from the thread...this is driving me nuts.
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Old 11-25-2013, 08:24 AM   #10
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a common method from that era was to connect each alternator output to an amp gauge at the helm station, then back to the starter lug. #10 wire was often used which resulted in a significant voltage drop -- the alternator might be putting out 14.6 v but the battery is only seeing 13.8 v, which makes for low charging amps and long charging times. Effectively, the alternator thinks the battery is fully charged but the battery is really only about 70% charge.

In twin-engine installations 2 battery switches of the "off-1-both-2" variety were used so either battery could start either engine, or either engine could charge either or both batteries.

Some of the taiwan trawlers with lehmans had a variation on this. Both starters were connected to a common terminal, and 2 "off-on" battery switches were used to connect either or both batteries to the starters. The house distribution panel was connected to an "off-1-both-2" switch so either or both battery banks could be used to provide power for the lights, refrigerator, etc.

Now a common method has one engine equipped with a high-output alternator connected directly to a large battery bank for the house loads, and the other engine has the original alternator charging a starting battery that provides power for starting either engine. An "off-on" battery switch can be used to connect the two battery banks if needed, to either use the house bank to start or use the other alternator to charge the house bank.

Connecting the alternators directly to the battery, battery buss, or starter lug with a #4 or #6 cable will significantly improve charging capability. This bypasses the amp gauge at the helm station, but that gauge can be replaced with a voltmeter.

A fuse at the battery buss connection is a good idea to protect against a short circuit.
very informative!!
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