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Alternator noise filter-necessary?

Old 07-07-2014, 08:23 AM
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Default Alternator noise filter-necessary?

I have 2 noise filters, one for each alternator on my boat. I'm replacing both alternators, and noticed the previous owner disconnected both filters at some point. In addition to a regular VHF marine radio, I'm installing a Garmin 547xs (w/ transducer). I'd like to get a consensus from people using similar equipment if they find the filters necessary for proper operation of the navigation equipment. Also, can anyone identify the make and model of the filter from the image? There's nothing to identify them, therefore I don't know how to connect them. Thanks for any help.
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Old 07-07-2014, 03:37 PM
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That looks like a battery isolator rather than a noise filter.

http://bluewatermarinesvc.com/html/bat_isolator.html
Old 07-07-2014, 09:28 PM
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Default Previous owner did same thing to me

Originally Posted by Lodka View Post
I have 2 noise filters, one for each alternator on my boat. I'm replacing both alternators, and noticed the previous owner disconnected both filters at some point. In addition to a regular VHF marine radio, I'm installing a Garmin 547xs (w/ transducer). I'd like to get a consensus from people using similar equipment if they find the filters necessary for proper operation of the navigation equipment. Also, can anyone identify the make and model of the filter from the image? There's nothing to identify them, therefore I don't know how to connect them. Thanks for any help.
First answer was correct, the things disconnected are diode isolators. The one connection is to the alternator and then the other two are to separate battery banks. That way, the alternator will supply current to each of the two battery banks IF the voltage of the alternator is higher than each of the battery banks. BUT there is a price to be paid for having the battery banks isolated from each other. If the alternator is putting out approximately 14.2 volts, there will be a voltage drop of about 0.7 volts across your diode isolator which drops the charging voltage down to about 13.5 volts. Instead of a healthy charging current, you get more of a float voltage.

When the previous owner bypassed the diode isolator, he raised the charging voltage back to 14.2 volts BUT the battery banks are no longer isolated (one faulty battery will take out both banks). In my case, the connection he made was lousy and the alternator was destroyed when the connection became poor due to the heat generated. The alternator will not survive an intermittent connection (clearly stated in the Yanmar manual).

Another problem is overcharging (maintaining 14.2 volts on battery is OK for a while BUT eventually you will overcharge and shorten the life of the batteries unless you have a smart regulator).
Old 07-07-2014, 10:30 PM
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Lots of automotive type alternators have a temperature sensor input to regulator. Cold alternator, 14.2 or so. Hot alternator, 13.8 or so. A crude yet sorta effective way to get a bulk charge and a float charge with one dumb unit.

Check your volts both cold and hot.
Old 07-08-2014, 12:29 AM
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the previous owner disconnected both filters at some point

it looks connected to me...
Old 07-08-2014, 09:29 AM
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Originally Posted by smac999 View Post
it looks connected to me...
Not. What you don't see in the picture is that the cables were cut off about 4 feet from each isolator. Unfortunately, he left the port alternator + output cable connected to the port isolator without a load, so that alternator was already cooked when I bought the boat.
Old 07-08-2014, 09:31 AM
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Originally Posted by JohnStateside View Post
That looks like a battery isolator rather than a noise filter.

http://bluewatermarinesvc.com/html/bat_isolator.html
Ah. I stand corrected. Thanks for the link; that looks exactly like what I've got.
Old 07-08-2014, 09:56 AM
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scgator2001, thanks for the details. I actually installed a battery switch last summer. On our end-of-season journey from our slip to the winter boat yard last November, the engines stalled, leaving us with 2 dead batteries. However, it wasn't until a couple of days ago that I saw that the previous owner had left the port alternator connected to the isolator that was disconnected at the battery bank end, so that was obviously the end of that alternator. The remaining functioning alternator then quit probably for exactly the reasons you mentioned. The previous owner's wiring practices were horrendous (wire nuts galore, SJ cord, lamp cord, speaker wire, whatever came to hand). I'm installing new Delco 10SI alternators, but I don't know if they have smart regulators, but I'm also installing good digital voltmeters. I hope this will bring an end to some really peculiar behavior in the engine electrical system. Sorry so long-winded...
Old 07-08-2014, 11:26 AM
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those are batt isolators. and most noise problems stem fro poor grounds
Old 07-08-2014, 11:43 AM
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Originally Posted by Lodka View Post
scgator2001, but I'm also installing good digital voltmeters. I hope this will bring an end to some really peculiar behavior in the engine electrical system....
Make sure you DO NOT move battery switches with the engine running. My manual says that will smoke the alternator regulator so I have to shut down the engine to change battery switch alignment (I have three switches on board for three separate battery banks).

Watching your new voltmeter and knowing what to expect will save you some money in the long run when your batteries last longer.
Old 07-08-2014, 12:22 PM
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Good advice about not hot-switching.

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