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#535949 - 07/13/12 12:06 PM Re: Boat charging system, I learned something (the hard way) [Re: Frantically Relaxing]  
Joined: Aug 2004
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KD4PBS Offline
Admiral
KD4PBS  Offline
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Posts: 1,182
The alternator output feeds only the isolator. As such, both batteries will be charged regardless of what position the switch is in. If there is a dead diode in the isolator, then whichever battery is on that diode will not get charged, unless the switch is in "both".
I'd love to test it, but right now Nauti Buoy is dead... I have to put a starter on her. Left me stranded last weekend after an overnight, and thankfully a wonderful man towed us the 3 miles back to the dock.

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#535961 - 07/13/12 02:58 PM Re: Boat charging system, I learned something (the hard way) [Re: Frantically Relaxing]  
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Frantically Relaxing Offline
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Frantically Relaxing  Offline
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I'm in no rush! smile

but I WOULD love to see the results of a test!


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#535996 - 07/14/12 08:21 AM Re: Boat charging system, I learned something (the hard way) [Re: Frantically Relaxing]  
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Al Offline
Nautical Alchemist
Al  Offline
Nautical Alchemist
Grand Poobah

Joined: Jan 2003
Posts: 15,089
Vagabond Wanderer from Mich.
Diodes can fail-short or fail-open.

If an over current condition, a fail-open will likely occur. In a transient voltage condition, a fail-short can occur.

In my experience, most faults were fail-short as the diodes were often signal diodes and they would receive voltage transients more than over current conditions.

In a boat, who knows? A nearby lightning strike could cause either.


President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.6L Diesel

previous boats:
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180
#536061 - 07/15/12 02:30 PM Re: Boat charging system, I learned something (the hard way) [Re: Frantically Relaxing]  
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KD4PBS Offline
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KD4PBS  Offline
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I replaced the starter yesterday, so hopefully this week I can let you know. It's yard work day today. Oh, joy.

#536245 - 07/19/12 06:01 AM Re: Boat charging system, I learned something (the hard way) [Re: KD4PBS]  
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CharlesS Offline
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CharlesS  Offline
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Bryson City, NC. 28713
FR, I am joining this conversation late, however my boat has the system that Al posted in the pictures. It did not come that way, but was updated by the previous owner. It does not have a automatic combiner.

On mine, if the switch is on both, both batteries operate everything, and both charge. If the system is on battery A, A and B both charge, although it starts and runs on A. If it is set to battery B, A and B both charge, however it starts and runs on B. When you combine the batteries, your charge is coming from the A battery side of the isolator. To me it sounds like you have a faulty isolator.

RV's have used similiar isolators for 30 years, and they are troublesome. When they fail, the first problem one will notice is one battery is charging and the other is not.


#536249 - 07/19/12 09:44 AM Re: Boat charging system, I learned something (the hard way) [Re: Frantically Relaxing]  
Joined: Jan 2003
Posts: 15,089
Al Offline
Nautical Alchemist
Al  Offline
Nautical Alchemist
Grand Poobah

Joined: Jan 2003
Posts: 15,089
Vagabond Wanderer from Mich.
FR, do you want to test your isolator's diodes?

You would probably want to disconnect the terminals to get the most accurate measurement, and depending on your meter, it must have a "diode-ohm" position on it to accurately measure a diode, especially if it is a modern multi-meter. Most good name brand meters have this function.

Previous discussion on this topic about forward biasing at 0.7v (ad nauseum) means that if you measure a diode with an ohm meter on the meter's normal ohm position, it may not measure correctly on a modern meter.

The reason for this is ohmmeters use a test voltage to measure resistance, and due to today's more sensitive digital components, most modern meters use a very low voltage through the test leads. This voltage is usually not enough to forward bias a diode.

This is normally true for good modern multi-meters, although some older analog meters (and some cheap digital meters) use a 1.5V AA battery to do this, which is sufficient to forward bias a diode.

However, for those meters using a low voltage, they usually have an ohm-diode setting, which uses a higher voltage that can forward-bias the diode, giving you a good reading.

So to test a diode, measure with the ohm-diode setting if your meter has one, and measure first one way, then reverse the leads and measure the opposite way.

The diode should read very low resistance in one direction, and very high resistance in the other.

The specific values you see depend on the type of diode, so just look for a low resistance one way, and a high resistance the other way.

Silicone diodes (which is probably what you have in the isolator) should measure nearly (or) infinity in one direction and less than an ohm in the other.

Finally, make sure that both diodes should give you a similar reading. If one is way off, then you know for sure you have a bad diode in your isolator.


President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.6L Diesel

previous boats:
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180
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