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Old 22-06-2011, 17:20   #1
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Alternator Problems?????

Trying to get a handle on where our problem may lie. Abouit two months ago we noticed that our starter battery wasn't getting a good charge, the house bank was just fine. After much machination we realized the starter battery was shot (so we replaced) because it wasn't getting a charge from the alternator. Further investigation reveled that one of the posts(diodes) of our multi battery isolator wasn't working, the post for the starter battery. So we figured we simply had to replace the isolator. WE did so a month ago and everything worked great for a month. Two days ago we noted that the house bank was not getting a charge, upon investigation we found that one of the posts on the isolator was shot (the house bank post). The previous isolator had worked just fine for several years, managing multiple banks. We are leaning towards some issue with the alternator causing these problems (shorting the isolator?). Any thoughts would be appreciated.
We have a 57hp Yanmar with an 80A Hitachi alternator tied into a two bank isolator. The house bank is 5, 100 Ah AGM's, and then a single starter battery.

Vicky & Ed
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Old 22-06-2011, 17:31   #2
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Re: Alternator Problems?????

if is perko--is the switch and it happens a lot---i had 3 go in 5 yrs so i had my electrician place heavy duty toggles for each battery bank and the invertor(built in) so i wouldnt have that problem ever again..... look at that before alternator, which should not be reason the switch failed. mebbe a connection failure, but not the alternator....
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Old 22-06-2011, 17:36   #3
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Re: Alternator Problems?????

He's not talking about a battery switch, he's talking about a diode isolator that splits the alternator charge to go to two different battery banks. I doubt it is the alternator, unless it is putting out excessive voltage or it is puts out more amps than the isolator is designed to handle, which would be easy to spot on your voltmeter or amp meter. Most likely it is just a bad isolator problem. The diodes in them are prone to overheating, hence the huge heat sink they are mounted to. However, I've seen some boats with the isolator mounted in a very hot spot, like in the engine room, which is guaranteed to not be good for the isolator.

Besides, isolators are not the best way to charge two battery banks due to loss from heat and problems with getting the right amount of charge current. Better to use something like a battery combiner (Yandina), or a trickle charger like the Echo Charge or the Trik-L-Start.
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Old 22-06-2011, 18:07   #4
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Re: Alternator Problems?????

I would surmise that the charging isolator is probably too small for the output of the alternator. There are different sizes.
- - But is you want to go "modern" you can replace it with a charging relay such as:
SI-Series Automatic Charging Relay - Blue Sea Systems
- - You would wire the output of the alternator to one battery bank and then connect the charging relay between it and the other battery bank.
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Old 22-06-2011, 18:11   #5
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Re: Alternator Problems?????

+1 what Kettlewell said.

Lose the isolator. Route all onboard charging sources (battery charger, alternator, wind, solar, etc.) directly to the house batteries. Be sure to include a healthy fuse with an ampere interrupt capacity (AIC) above 5000 amps. ANL and Class-T and MRBF terminal fuses are in compliance with this application.

Then, use an EchoCharge or DuoCharge to maintain the start battery. Start batteries require very little charging to keep full. House batteries, by contrast, are what soak up the available charging current. And, with 500AH of AGMs they will take all you can throw at them....easily 500 amps.

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Old 22-06-2011, 18:14   #6
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Re: Alternator Problems?????

You need to size the isolater so it is capable of taking the full output of the alternator to EACH battery--some so-called 100 amp isolaters will take 100 amps of input, but assume that only 50 amps are going to each bank.
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Old 22-06-2011, 18:24   #7
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Re: Alternator Problems?????

You need to size the isolater so it is capable of taking the full output of the alternator to EACH battery
I always assumed that if you have a 100 amp alternator you could get a 100 amp isolator and be fine, though as always I probably would oversize up to 150 just to be safe. I always assumed that the standard way to rate them was that either leg of the isolator could take the full load it is rated for, like it says here.
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