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Old 09-07-2010, 01:06 PM   #1
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Battery Isolators and Alternators

This weekend i saw a flash of "Alternator Failure" come and go on my dash while driving. I also saw the dash voltage indicator showing 10.7VDC

I had my generator running so i was ok until i got home

Back of the engine compartment shows, Alternator (Leece Neville 200A) connected to a Cole-Hersee 200A battery isolator (3 posts)

AT the ALT post it read 28.5V while engine was running and 12.7V at the ENGINE post and 12.6V at the HOUSE post.

After shutting down the engine, I decided to short the ALT and ENGINE post of the battery isolator and the dash showed 14.4V immediately. My conclusion was the Battery isolator had somehow failed. This was strange since a battery isolator is nothing but two DIODES with a total capacity of 200A (in this case) and the alternator can't provide more than 200A. So the question is how did the isolator fail?

In the newer coaches, Monaco doesn't put an isolator from cole-hersee
I am curious, how do they isolate the batteries (house and engine) and what do they use for it?
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Old 09-07-2010, 08:26 PM   #2
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Addressing your last question first, my '04 Diplomat came without any manner to charge both banks of batteries by the engine alternator (the alternator is connected to only the chassis battery bank).

Your measurements of 12.x VDC at each battery bank with an running alternator at 28.5 shows that (a) both diodes in the battery isolator are open (bad) or (b) that the ALT post has (in some manner) become separated from the internal connection to the two diodes.

I'd do a sanity check next:
Jumper ALT and CHASSIS posts of the isolator. This jumper needs to be a hefty wire (car jumper cables, for example). Make the connections first, then start the engine. Now measure voltage at this jumpering point and then also at the chassis battery battery-post. Hopefully you'll see 14.x VDC at both locations indicating that (1) the charge-path wiring is OK end-to-end, (2) the alternator is still OK, (3) that the chassis battery (or battery pair) is presenting a "load" to the alternator and (4) is being charged. Step (5) is to declare your isolator "toast".

About that 14.4 VDC reading you report at the dash meter (when you performed the shorting test): You said the engine was off, so where was 14.something Volts DC coming from? Assuming the dash meter is accurate, I can only think the Inverter (being plugged into shore power) was charging the house batts....and that's what the dash meter was indicating somehow. Stated differently, a battery static-state (fully charged) can't be above 13.2. Or maybe the dash meter is in error by reporting 1 volt too high.
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Old 09-07-2010, 08:55 PM   #3
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You hit it right
The isolator is toast. Although i am not sure how is that possible.
It is just two heavy duty diodes.
This was a Cole-Hersee 200A isolator
I ordered a Noco 200A instead as it can handle 200A in each leg
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Old 09-08-2010, 06:55 AM   #4
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Cayman,
My 2008 Sig has had 2 "Big Boy" battery isolator solenoids go bad in the last 4000 miles. Can't find a reason for it happening. Mine is rated for 200amp also.
Let me know if you have your new one go bad again or if you find anything that might be causing the problem.
Thanks
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Old 09-08-2010, 07:35 AM   #5
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An 04 Diplomat that was factory wired to charge both banks from the engine alternator seems very unusual.

You may want to make sure the isolator is installed such that the engine starter can not draw power from the battery on the other side of the isolator. If it can, the reason for failure is likley that the more capably, high available capacity, battery bank for starting is on the other side of the isolator.
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Old 09-08-2010, 12:22 PM   #6
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An 04 Diplomat that was factory wired to charge both banks from the engine alternator seems very unusual.

You may want to make sure the isolator is installed such that the engine starter can not draw power from the battery on the other side of the isolator. If it can, the reason for failure is likley that the more capably, high available capacity, battery bank for starting is on the other side of the isolator.

I am not sure i understand this....

The isolator flows current only in one direction...from the alternator to the two sets of batteries. The starter motor is directly connected to the engine battery on the load side of the isolator.
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Old 09-08-2010, 01:10 PM   #7
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........
Let me know if you have your new one go bad again or if you find anything that might be causing the problem.
Thanks
The only reason for failure (that i can think of) would be current draw exceeding the relay capacity. And i mean current draw for extended periods of time. This would cause a lot of heat and damage the materials.

This can happen if either of the batteries are in a low state of charge
In such a situation, i suggest using generator + inverter/charger to charge the house batteries before giving the alternator+isolator relays the stage to charge it

I wish there was a relay that would look at the battery voltage to be too low to charge with an alternator and immediately open up so no damage to the charging system can happen
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Old 09-08-2010, 01:22 PM   #8
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Depending on how the isolator is wired, there's a possibility that the starter can draw current from the house batteries through the isolator. These may now be rare. Isolators were placed between the house and start battery. The charge voltage came from the alternator or battery charger and fed the house battery. The isolator allowed voltage/current to pass to the starter battery but not from the starter battery to the house battery. This would allow the starter to draw from both banks.

If you have a diode based charger with an input and two outputs, then this should not happen.

If you have a diode based isolator that needs to be replaced, you should consider a solenoid based isolator. These don't suffer from the voltage drop that is characteristic of all diode based isolators. Many of the solenoid based ones also have on/off thresholds that can be set.

Gil
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Old 09-08-2010, 01:33 PM   #9
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Originally Posted by caymann View Post

I wish there was a relay that would look at the battery voltage to be too low to charge with an alternator and immediately open up so no damage to the charging system can happen
Here you go:

http://www.pplmotorhomes.com/parts/r...lator-inst.pdf

This one is limited to 100A continuous: http://www.ase-supply.com/v/vspfiles.../1314-1315.pdf
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Old 09-08-2010, 01:38 PM   #10
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Here's an even better smart isolator/combiner: ML-Series Heavy Duty Automatic Charging Relays (Magnetic Latch) - Blue Sea Systems
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Old 09-08-2010, 02:57 PM   #11
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An alternator can go over its rated output. Isolator’s go bad - most common failure is the solenoid type as they create heat & depending on it's signal can chatter, creating even more heat. When replacing an isolator (diode or solenoid) I try to go at least 25% more then the charging source, might be a few dollars more, but cheaper to have it last.
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Old 09-08-2010, 07:09 PM   #12
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I have diode based isolators (and that is what i am replacing the current one with)
This one lasted 10 years and at $50 for a 200Amp, i won't complain

If i had to replace the solenoid kind, at $200 a piece (and you need two), it gets mucho expensive

But i guess both has its advantages and disadvantages
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Old 09-09-2010, 11:18 AM   #13
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FWIW, you only need one solenoid, not two. On one side of the solenoid you have one battery bank and the alternator. The other side, just the other battery bank.

You make a great point on cost. I haven't priced the better diode based units. They are the ones that have very little voltage drop. They use a different type of diode.

At 10 years between replacements, you'll likely not replace the one in that coach again. That's hard to argue against.

Gil
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