Bad Isolator?

Topic: Bad Isolator? (Read 3040 times) previous topic - next topic

Bad Isolator?

March 11, 2014, 06:30:34 pm

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 – March 11, 2014, 07:17:27 pm

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 – March 11, 2014, 07:55:44 pm

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 – March 11, 2014, 09:10:32 pm

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 – March 12, 2014, 09:28:04 am

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 – March 12, 2014, 10:31:18 am

Chuck,

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

Pierce

Re: Bad Isolator?

Reply #6 – March 12, 2014, 10:52:20 am

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 – March 12, 2014, 11:27:17 am

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 – March 12, 2014, 01:16:18 pm

Quote from: cajk – March 12, 2014, 10:52:20 am

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 – March 12, 2014, 01:25:23 pm

Very nice Neal!

Pierce

Re: Bad Isolator?

Reply #10 – March 12, 2014, 02:14:32 pm

Quote from: cajk – March 11, 2014, 06:30:34 pm

.............. 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 #11 – March 12, 2014, 04:32:29 pm

Neal,

What do you think of the Cole-Hersee 48530 smart battery isolator? Battery-Related Products | Battery Isolators48530 | Cole Hersee - Littelfuse 200 amps with no efficiency loss from the diodes.

About the same price as a regular isolator.

Pierce

Re: Bad Isolator?

Reply #12 – March 12, 2014, 05:23:02 pm

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 – March 12, 2014, 05:27:04 pm

Quote from: Barry & Cindy – March 12, 2014, 11:27:17 am

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 – March 12, 2014, 05:40:13 pm

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

Re: Bad Isolator?

Reply #15 – March 12, 2014, 06:12:16 pm

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 – March 12, 2014, 07:38:05 pm

OK, Tom and Neal. Going to put you guys on the spot.

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 – March 12, 2014, 08:13:25 pm

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 – March 12, 2014, 08:50:45 pm

Quote from: Pierce & Gaylie Stewart – March 12, 2014, 07:38:05 pm

OK, Tom and Neal. Going to put you guys on the spot. 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 – March 12, 2014, 10:11:02 pm

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.

Re: Bad Isolator?

Reply #20 – March 15, 2014, 09:40:28 am

A few posts back Chuck asked why I replaced my original isolator. I had noticed after pulling in off the road it was hot to the touch. I know it's normal to produce some heat, but since I was going to replace my alternator I decided to put in a new isolator. The new isolator just feels warm now when I check it.

Here is the Sure Power 3 battery isolator for those that want to replicate the original parallel diode connection:

Sure Power 2403 Battery Isolator

This 240 amp unit will have 3 - 80 amp diodes, vs 2 - 120 amp diodes in the 2 battery unit. When high power diodes are paralleled, the current sharing will not be equal. When a diode heats up, the resistance decreases, as a result, one diode ends up taking on more current than the other diode, resulting in it heating up even more. As you can probably see this cycle will cause thermal run away, causing the diode taking the most current to eventually burn out if you give it enough current. This is why as a general practice, paralleling diodes is not recommended. This phenomenon can be mitigated if the diodes are heat-sinked together as they are in an isolator, and thermal runaway will probably not occur in our application, as high current conditions are short and infrequent. I don't want to cause concern to owners who have the 3 battery isolator setup - it will work just fine in our application.

Also, the thing to remember is the diode rating is the continuous current rating. The 120 amp diodes can probably handle 200 amps or more for short periods, which might occur for a short time when batteries are low, and this would exceed my alternator output anyway.

I considered the 3 battery isolator and decided one higher capacity 120 amp diode for each leg was optimal, and was a simpler setup without the extra jumper. Either approach will work fine, whichever one you feel will meet your needs.

Re: Bad Isolator?

Reply #21 – March 15, 2014, 12:00:17 pm

Peter: I was gonna let this thread die, but since YOU kept it going I will add another note about my continued investigation into my isolator behavior. I found a .pdf copy online of the "official" Hehr install sheet for Powerline isolators:

http://www.hehrintl.com/pdf/isoman.PDF

If you scroll down to page 5, you will find "test" instructions. I did the "ohmmeter" test, and got exactly what they say is the correct result. This would lead me to believe my isolator is OK. However, the next paragraph is "test information for electrician". The first step of that test says, "Engine not running: #1 terminal of isolator should read system battery voltage. #2 terminal should read auxiliary battery voltage. The "A" terminal should read 0 volts." My isolator fails this step (I have not yet performed the "engine running" steps 2 and 3). So this indicates (to me) that my isolator is malfunctioning.

At another website, I found reference to "stray voltage on the isolator alternator post". They suggested grounding the ALT post to bleed off "residual voltage", then checking again with multi-meter. So I tried that - I removed the alternator feed wire from ALT post, checked voltage on post: showed 13.9 volts. I then ran a jumper wire from ALT post to ground. There was no spark when I connected the wire. Then removed ground wire and checked voltage on ALT post: again showed 13.9 volts. I then tried connecting a 12 volt test light from Alt post to ground - the bulb remained off. This would seem to indicate that although I have voltage on the ALT post, there is no "current" or virtually zero amps? I tried the DC amp and milliamp position on my multimeter between ALT post and ground, and couldn't get any reading.

All seems kinda contradictory to me. I will wait until I get a chance to check the test steps 2 and 3 with the engine running, before I finally decide if I need a new isolator. Your points about "3-post" versus "4-post" seem logical, but so does Neal's basis for preferring the 4-post design. Price factor would seem to favor the 3-post model (at least in the Sure Power line).

I really like these kind of discussions! The more I read and listen to the excellent forum member input, the more I learn!

Re: Bad Isolator?

Reply #22 – March 15, 2014, 12:53:40 pm

Chuck:

As Neal has posted earlier, using voltage or resistance measurements to trouble-shoot high power diodes is problematic because of small reverse-bias leakage current that is normal. What I would recommend is trouble-shooting your isolator by measuring current flow. I purchased this clip-on DC current meter and find it extremely useful:

Extech DC400 400A DC Mini Clamp Meter - Amazon.com

With this meter you can do the following isolator checks (and at the same time check your alternator):

With meter on low current setting, shut engine off, turn on headlights and some coach lights (create a current draw on house batteries and chassis battery)
- Current flow on wire to alternator should be negligible to zero.
- Current flow on wire to coach batteries should be negligible to zero.
- Current flow on wire to chassis battery should be negligible to zero.
- If there is any measurable current flow on any of the wires, you have an isolator problem.

With meter on high current setting, start engine, turn on headlights and some coach lights (create a current draw on house batteries and chassis battery)
- Measure current on alternator wire (there should be measurable current, say 40 amps...)
- Measure current on house battery wire (there should be measurable current, say 20 amps...)
- Measure current on chassis battery wire (there should be measurable current, say 20 amps...)
- House and chassis currents should equal alternator current
- If there is proper charging voltage on the alternator wire (14.5 - 15 volts) and current on either of the battery wires is unusually low or zero, you have an isolator problem

If all measurements check out, isolator is fine and alternator is charging.

As an Amazon Associate Foretravel Owners' Forum earns from qualifying purchases.

Re: Bad Isolator?

Reply #23 – March 15, 2014, 04:53:12 pm

A voltmeter will also give you a good indication of isolator health.

Check the battery voltages with minimal loads and no charging. With the engine running, both batteries will see an increased voltage. With the ac charger charging, the house batteries will see an increased voltage, but not the chassis batteries.

Re: Bad Isolator?

Reply #24 – March 15, 2014, 11:04:22 pm

Peter: The Extech DC400 sounds like something that needs to be in my tool box. I'll add it to my Amazon "Wish List". Thanks for the Tool Tip!

Tom: I'll try your checks the next time I run the big engine, but I think the Trik-L-Start may negate the results, since it charges the start batteries anytime the coach batteries are being charged, regardless of charging source.

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