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Topic: Bad Isolator? (Read 3276 times) previous topic - next topic

Bad Isolator?

Sorry for long question.

Working on my battery upgrade and electrical update.  New AGMs installed, cleaned up everything on the isolator panel, installed new circuit breakers.  After I hooked everything back up, went to checking voltages and noticed what I THINK is a problem with my isolator.  I have the Powerline PLI-190-3, with 4 posts.  Cables are installed correctly: coach batteries on one end, start batteries on the other end.  One of center posts (nearest start battery post) is jumpered to the coach battery post.  The other center post has the alternator lead attached.  Coach has been plugged in to 50 amp shore power overnight, new Powermax 4-stage converter/charger and Trik-L-Start seem to be working properly.

Problem:  Checked voltages on isolator posts this morning.  coach batt post = 13.20, start batt post = 13.01, ALT post = 12.90.  The 4th post, of course, is 13.20 same as coach batt.  The engine is off, alternator is not turning, and I have almost 13 volts on the alternator post.  I figure this is not good.  Pulled the alternator cable off the post, just to make sure it wasn't somehow coming from alternator - zero volts on loose cable, still 12.90 on post.

Tried the diode check function on my multimeter.  Held NEG lead on alternator post, touched POS lead to start batt post: reads zero.  Same with coach batt post.  Held POS lead on alternator post, touched NEG lead to start battery post: reads .390.  Checking coach batt post: reads .215  I'm not clear on how to interpret those results, but it IS pretty clear that voltage is somehow "leaking" from one or both end posts to the alternator post on the isolator.

SO, 2 questions:

1.  Do I need a new isolator?

2.  Can I use a 3 post isolator (of suitable amp rating)?  I don't see any purpose to the 4th post on the existing unit.

Re: Bad Isolator?

Reply #1 –
Don't know why there would be a jumper on those posts. Looking at the picture of the Isolator it indicates that three battery banks can be used with this one.  Sorry can't help further.
Gary B

Re: Bad Isolator?

Reply #2 –
Chuck,

Here are a couple of schematics of your coach with the four terminal isolator. I have a three terminal on mine as a replacement but either will work. The second schematic is just a blowup of the isolator part. This has all the wire codes.

The alternator needs voltage to excite it. Not like a generator.

Good idea to make these into PDFs and take to Kinko and have them blown up to working size. I can make these into a PDF from JPG if you can't and send it to you. Store in a mailing tube on the coach.

Pierce

Re: Bad Isolator?

Reply #3 –
Foretravel used a 4 terminal isolator designed for 3 batteries, and paralleled 2 diodes together for the house batteries - to get the extra amperage capacity for the house batteries.

I had the same original 3 battery isolator that you have and replaced it with this 2 battery 240 amp isolator when I upgraded my alternator to a 180 amp Delco 28si:

Sure Power 24023aIB Battery Isolator

This 240 amp isolator also has the small ignition exciter post which you can just leave unused (as I did). This isolator has larger 3/8" terminal posts to handle the current, and is a heavier capacity unit.

After pulling in off the road with headlights on, fridge running on the inverter (approx 70 amp draw from the alternator) the old isolator would be hot to the touch, the new isolator is just noticably warm.

Here are my before and after pics:

Re: Bad Isolator?

Reply #4 –
Gary:  I think Peter provided a answer (above) to "Why the Jumper?"  I'm like you - it was a mystery to me.  But since it clearly shows the jumper on my wiring diagram, I figured Foretravel must have had some purpose in mind.

Pierce:  Thanks for the wiring diagrams, but I already have those in my owner's manual.  The voltage I am seeing on my alternator post has nothing to do with "exciting" the alternator.

My alternator is "self-exciting" and does not require a separate "excite" connection.  My alternator does require a external "sense" connection.  On your wiring diagram, the wire labeled #59 - RD - ALT EXCITER, is actually (on my coach at least) a "sense wire", and it is correctly attached to the "Start Battery" post.  You can see it in my photo - it is the small wire wrapped in plastic protector.

If my understanding of how isolators work is correct, then the voltage I am asking about should not be present on the isolator alternator post.  The only way it could get there is to "leak" backwards through the diodes from the battery posts, which I think indicates a malfunctioning isolator.  I was only asking for verification of this conclusion from some of you who know more than me.

Peter:  Thanks for your answer, and for giving a logical explanation for the jumper cable.  I would think Foretravel could have sourced a suitable 3-post isolator and avoided the "Band-Aid" jumper cable, but perhaps they got a good deal on them from the manufacturer.

Did you replace your isolator only because you upgraded your alternator, or did you have other problems with it?  If problems, what were the symptoms?

And thanks for the link to the Sure Power unit.  I will check it out.  There are also other solutions to consider.  Brett Wolfe likes the idea of replacing the isolator with a manual switch - the KISS principal.  And there are newer technologies available, like "smart" switches that accomplish the same task without the "volts turned into heat" penalty of diodes.  However, I'll probably end up doing the same as you - I'll just replace the isolator.

Re: Bad Isolator?

Reply #5 –
Chuck,

Where you have 12.9 at your alternator post, I have 6.1 at mine.

Pierce

Re: Bad Isolator?

Reply #6 –
Sounds like you have the same "problem" I do, only not as severe.  I need to read up on diodes (I'm not a EE major).  Do they (if functioning properly) allow ONLY one-way current flow, or can they have some degree of allowable "back flow"?  I don't know...

Re: Bad Isolator?

Reply #7 –
Reading voltage on isolator's alternator post could indicate flow from house / start to that post , but more than likely indicates voltage is coming from alternator, which could be normal? Disconnect alternator wire from one end or other and check isolator again.

Then leave alternator wires attached and temporarily disconnect other wires one at a time and see which could be leaking.

Isolator is probably not bad.

Re: Bad Isolator?

Reply #8 –
Quote from: cajk –
Sounds like you have the same "problem" I do, only not as severe.  I need to read up on diodes (I'm not a EE major).  Do they (if functioning properly) allow ONLY one-way current flow, or can they have some degree of allowable "back flow"?  I don't know...
Guys,
Diodes, by design, have some leakage "back flow" of electrons.
One way to visualize it is to think of a diode as a check valve.  To make a big check valve 100% leak free would be very expensive -- right?  To make a big check valve very rugged and also reasonably leak free (a few drops per hour seeping by the seating surface, at maximum applied pressure differential, would be more realistic -- right?
The "few drops an hour leakage analogy" is what you are seeing. 
The impedance of your measuring equipment (how much meter current flow is required for the meter to make its measurement) also comes into play.  In general, the higher the quality of the meter, the higher the input impedance/lower current flow needed for an accurate measurement.
Big planar diodes (large current diodes) like our battery isolators are not perfect diodes.  They exhibit minor (fractions of milliamperes) backleakage.  A high impedance meter then shows voltage on the "wrong" side of the diode, just like a big check valve would show fluid on the wrong side of the check valve.
For a power diode, the important measurement is the voltage drop across the diode when it is forward biased (current flowing in the proper direction).  For our application, we should see 0.7 to 0.9 Vdc drop across the diode (e.g. - 14.1 Vdc battery side, 14.8 to 15.0 Vdc alternator side).  The reason for the range in value is that different diodes and different designs have different characteristics.
But if the diode is defective, it will show one of three things:
Open -- input side = alternator output and battery side equal to positive battery terminal -- therefore much more than 0.7 to 0.9 Vdc differential
Shorted -- input side = output side -- 0.0 Vdc differential
Partially shorted -- MOST OFTEN THE SCENARIO -- for one diode in your isolator, input and output sides nearly the same, therefore the output side for the partially shorted diode will be high in comparison to the second diode output side in your isolator ( e.g. - 0.7 drop on the good diode, only 0.2 Vdc drop on the partially shorted diode.  Ideally, they should be identical).
If you have a very good, high impedance meter, you may also be able to measure the pico or nano amps of diode back leakage of the reverse biased diode, but knowing whether or not that is good or bad would require knowing the manufacture's specifications for back leakage.  And that probably won't be easily obtained.
Neal

Re: Bad Isolator?

Reply #9 –
Very nice Neal!

Pierce

Re: Bad Isolator?

Reply #10 –
Quote from: cajk –
..............  I don't see any purpose to the 4th post on the existing unit...........
Chuck,
Three diodes (four posts on the isolator) may well be the more desirable isolator.
Going back to the check valve analogy, it may well be less expensive and can be a more reliable, less demanding design to use two check valves in parallel, rather than using one big check valve.
Think about the demands on the isolator:
The start battery has low current flow (check valve flow) requirements, because only the smaller batteries and engine starting current depletion must be replenished.
The house batteries, however, generally have much higher current requirements to replenish "last night's" battery draw down on much larger batteries.
So using a three diode (three check valve) design may well be the more desirable, intelligent and less demanding design.  Use one check valve to replenish the low demand load and use the other two check valves , in parallel, to replenish the higher demand load.
If I ever have to replace my two diode isolator, I will definitely look for a three diode replacement isolator that offers higher current flow at less expense, lower operating temperature, higher reliability operation.  Going to a two diode isolator may well be the wrong direction, even though it might possibly be the less costly direction.  You have to consider the design current flow capacity for each diode vs. desired capacity vs. cost vs. the quality of the isolator choices.
Neal

Re: Bad Isolator?

Reply #12 –
This looks like the isolator my 1988 Winnebago came with. Works very well most of the time. When the battery was completely dead, I needed to jumper across it so it had power to energize. I like the idea of eliminating the diode junction voltage drop, but in the real world that is no big deal.

Mine had a three position switch, one to disconnect the alternator from charging (overcharging) the house batteries, one for normal charging/isolation, and a momentary boost position.

It is really an automated boost switch.

Re: Bad Isolator?

Reply #13 –
Quote from: Barry & Cindy –
Reading voltage on isolator's alternator post could indicate flow from house / start to that post , but more than likely indicates voltage is coming from alternator, which could be normal? Disconnect alternator wire from one end or other and check isolator again.

Barry:  Did that (see my original post).  Did not try the 2nd part of your suggestion, due to time constraints.

Re: Bad Isolator?

Reply #14 –
Any wrong way leakage across a reverse biased diode should be very low current and not an issue.

Re: Bad Isolator?

Reply #15 –
Neal:  Thanks for the great tutorial on diodes!  (See, I knew there was somebody here who could explain this stuff)  So, if I've got this right, between your explanation and Tom Lang's comment, I gather I am probably getting worked up over a non-problem.  With the isolator "at rest" and no input from the alternator, just seeing some random "voltage" reading on the alternator post is no big deal.  What I really need to do is wait until the next time I have the engine running, then check the isolator post volt readings.  Using Neal's parameters I can then assess the condition of my isolator when it is actually "working".  That is what I will do.

AND, if I do decide to replace my 21-year-old isolator with a fresh unit, I will take Neal's advice and buy a 4-post model.

I learned something today - Thanks for all the good info!

Re: Bad Isolator?

Reply #16 –
OK, Tom and Neal. Going to put you guys on the spot. :D What is the advantage of the big aluminum finned isolator over Cole Hersee's 48530 isolator. They claim it will do the same thing and even more in a very small package. Are they missing something or is this a recent bright idea that makes the old stuff obsolete?

Pierce

Re: Bad Isolator?

Reply #17 –
Looking at the info on the 48530, what I find interesting is that "It allows bi-directional charging from the alternator or from shore power."  Sounds (to me) like it would allow charging of both coach and start batteries from either the alternator or the charger/converter (shore power) OR the generator?  If that is the case, it would eliminate the need for any type of Trik-L-Start device.  I'm also waiting to hear Tom and Neal's opinion...

Re: Bad Isolator?

Reply #18 –
Quote from: Pierce & Gaylie Stewart –
OK, Tom and Neal. Going to put you guys on the spot. :D What is the advantage of the big aluminum finned isolator over Cole Hersee's 48530 isolator. They claim it will do the same thing and even more in a very small package. Are they missing something or is this a recent bright idea that makes the old stuff obsolete?

Pierce
Limited time available to respond -- but,
KISS, may be one problem with it.  The American Heritage uses the Cole unit.  I helped an owner chase a problem with his for several weeks.  We put LEDs, digital voltmeters, and finally shunts with DVM PC INPUTS on them to finally prove that the replacement Cole (units) were being destroyed by momentary starting current surges.  American uses the 200 amp units and those aren't adequate for their application.
Troubleshooting was very difficult and not at all intuitive due to American's/Liberty Chassis' poor documentation and the many variables possible in the Cole unit.  American (Fleetwood) finally supplied an even more difficult to fathom replacement device from a different manufacturer.
Easiest to UNDERSTAND DESIGN is simple switches, as Brett suggests.  But then, the operator has to operate the switches correctly to avoid battery damage.
The next easiest to UNDERSTAND AND USE is the simple isolator. 
There is elegance in good KISS design.
Neal

Re: Bad Isolator?

Reply #19 –
I agree, the diode unit is simple, it just works.

The Cole unit is a boost solenoid, not much different than what we already have. It has contacts and coils to burn out. My Winnebago owners manual went to some detail to explain what to do when the house battery is too low to let the contacts close. I think simpler is better, and a 0.7 volt reduction in charging voltage from the alternator to the house batteries is really nothing.