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Current loss across battery isolator

EMSJEEP

EMSJEEP

NAXJA Forum User
Location
Long Island
Current loss across battery isolator...or bad alternator?

Due to my lack of knowledge about the component parts of a battery isolator I have to ask...

Is it possible/likely to see a profound current loss (I'm not talking about the normal voltage drop) across an isolator (Vanner 51-140 250amp Schottky Diode isolator), specifically due to heat load and/or poor grounding of the isolator itself? Say a loss of nearly 60-80 amps...

If you have any other theories about where my 80 amps went I'm all ears. Replacing the alternator resulted in the persistance of stalling secondary to current loss.

Theories are, this is possibly a heat load issue (heat soak) with electrical symptoms...at some point we did measure 30 amps at 1500rpm when it should have been ~120...this happened again, once, following an alternator replacement too...
 
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If you are running a diode, there is resistance there. Measure that. That resistance will make it not charge at full capacity. Also, measure across it and see if there is a voltage drop. If there is, then I would say it is bad. I assume by your avatar, you are running lights and other equipment off a second battery? What batteries are you using?

Former Firefighter/EMT
 
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There will always be a couple tenths of a volt drop across a diode based battery isolator... germanium diodes (only used in small signal applications) have a forward voltage of around 0.3v, silicon are 0.5-0.7 generally.

I'm a bit confused though, you can't "lose" current across something (Kirchhoff's current law), what goes in comes out. What can happen is that the increased voltage drop between the alternator and batteries due to the isolator will make the batteries charge slower since they see a lower charging voltage. Depending on how you wire it, the PCM may not turn the field coil voltage up enough to compensate for the voltage drop through the diodes.
 
Your voltage drop across a Schottky diode should be in the order of .3v. That is why they are used instead of standard silicon diodes which have about a .6v drop. There is no such thing as current loss. If you drop the voltage, you will pull less current, but .3v is a very small percentage difference.
 
Current lack would have been a better choice of words. I'm exploring elements of the electrical system attempting to figure out how I was able to stall out the Jeep by rolling down the windows at 50mph. A new alternator saw the same problem. Since I have gone to a third alternator (a better one), directly grounded the isolator, cleaned up a number of grounds, etc. Its either alternator, isolator, breaker or battery....measured 30 amps post isolator to the main battery during an episode at idle (120amp stock (2001 XJ) alternator)...
 
Awright - I gotta ask The Question (since no-one else has...)

"Since before you noted the problem and after, what changed?"

Troubleshooting 101. Even if you didn't change something, something did indeed change, and that "what" needs to be found and corrected.

Identifying that "what" is the critical first step.

Yes, there is some voltage loss across a diode. Yes, that means that there will some current consumption involved. But no, the fact of it being 60-80A is pretty much impossible with everything in workin' order. Have you done a resistance test across the isolator, and between the isolator terminals (particularly the central feed from the alternator) and ground? Could find the problem that way...
 
I think I would like to ask a couple of questions here. What exactly are you trying to do? What batteries are you running? What is the capacity of the diode? 30 amps maybe? How do you have it wired?
 
Just because you have a 120 amp alternator does not mean it is always going to charge at 120 amps. If it did, your battery would explode. How were you measuring the current anyway. I have a clamp on DC ammeter, but they are definately not cheap.
 
4.3LXJ said:
I think I would like to ask a couple of questions here. What exactly are you trying to do? What batteries are you running? What is the capacity of the diode? 30 amps maybe? How do you have it wired?
Click to expand...

X2,depending on what your trying to do,you may not even want to run an isolator(and theres still better ways to acomplish the same thing without one).
 
EMSJEEP said:
at some point we did measure 30 amps at 1500rpm when it should have been ~120...this happened again, once, following an alternator replacement too...
Click to expand...
Under what conditions?See Below
old_man said:
Just because you have a 120 amp alternator does not mean it is always going to charge at 120 amps. If it did, your battery would explode. How were you measuring the current anyway. I have a clamp on DC ammeter, but they are definately not cheap.
Click to expand...

Exactly,if your battery(s) are fully charged you might not see more than 25-30 amps after cranking(starting) the vehicle.A "stock" rig with "everything" on only "pulls" about 30 maybe 40 amps.
 
The ammeter was at a shop...this is going to be a post battery issue anyway, I replicated the problem with the vehicle OFF.

As far as what has changed, I had a failure of the left headlight bulb connector which caused backfeed into the highbeam circuit during times of insufficient contact or shorting. After replacing the connector the backfeed, which manifests as a dimly lit high beam indicator, is constant. Under load from the windows I experiance a loss of electrical power with no corresponding decrease in voltage (ie. Its more than just a load issue). While driving this causes engine knocking and eventually a stall. In park, the windows will move a few inches, kill all power (to the interior light and door beeper for example) and then come back and alternate rolling up/down and killing the power. This happens after short runs, so its probly some form of heat soak, new battery and alternator did not help. Also, the rear lighting gets screwey, I get the fast turn signal (bulb out indicator) no or inappropriate reverse lights, etc.
 
Here are instructions on how to install a 3 post isolator: http://www.newmarpower.com/pdf/Manual-Battery Isolators.pdf

If I read the FSM right, the alternator connects to the PDC Main Bus

So the lead from the alternator should have been moved to the ALT terminal of the ISO, and the BAT1 Term should then be connected to the PDC with something like #6 wire. Durring normal operation, you should have your full 13.8V as the monitoring of the battery voltage is done on the PDC side of the ISO.

The Second battery should be on the BAT2 side.

Is that what you did? If not, please supply drawing.

Ron
 
Zuki-Ron said:
Here are instructions on how to install a 3 post isolator: http://www.newmarpower.com/pdf/Manual-Battery Isolators.pdf

If I read the FSM right, the alternator connects to the PDC Main Bus

So the lead from the alternator should have been moved to the ALT terminal of the ISO, and the BAT1 Term should then be connected to the PDC with something like #6 wire. Durring normal operation, you should have your full 13.8V as the monitoring of the battery voltage is done on the PDC side of the ISO.

The Second battery should be on the BAT2 side.

Is that what you did? If not, please supply drawing.

Ron
Click to expand...

This is how it was run with #2 welding cable. No way this is the isolator, the vehicle isn't driveable anymore. Was going to Best Buy tonight, hit the window switch, vechicle stalled, rough restart then stall, stall, stall until I could get up some speed, stalled when I stopped, stalled at the first three traffic lights, fast left turn signal and whacky running lights, then I restarted and gunned it off the third light and it started running perfect again....
 
Something is not right here. You need to ditch the isolator. I run a continuous duty 250 amp solenoid with dual optimas and have not problems at all. A diode will limit how much current you can pass through it. The solenoid has no current loss and is very effective in isolating the batteries.
 
4.3LXJ said:
A diode will limit how much current you can pass through it.
Click to expand...

That is a new one to me. If it limits, it better be extremely well heat sinked because it will get REALLY hot and that normally means self destruction.
 
I would try disconnecting the diode isolator entirely for a while, leave the accessory battery unpowered and wire the alternator feed to the main battery in OEM configuration. If the problem does not go away at least it will make it simpler to diagnose.
 
Diodes work off resistance. Remember the old tubes in radios and TVs? They were full of diodes. Once they heat up, they limit the current and then the amount of heat that is released. If he is trying to run something off it, which is what I take him to say, and if his symptoms are accurate, then it is doing what it is supposed to do and he is exceeding the design of it. Most isolators are supposed to isolate a battery from the charging circuit so that you control the battery voltage for each battery. If he is only getting 30A out of it it will not be enough to run the vehicle, which is why it is stalling.

As stated above, I am making some assumptions about how he is running it. That is why I asked for a detailed explanation of how it is wired and what type of batteries etc.
 
kastein said:
I would try disconnecting the diode isolator entirely for a while, leave the accessory battery unpowered and wire the alternator feed to the main battery in OEM configuration. If the problem does not go away at least it will make it simpler to diagnose.
Click to expand...


I'll make it simpler, lol, it happens when the vehicle is OFF. I went to go roll my windows up yesterday and they moved about 3 inches, power lost, 3 inches, power lost etc etc while continually holding down the switch. I don't think this is anything pre battery, in fact, replacing the battery didn't help either, and a problem with the alternator wouldn't account for the rear lighting issues (odd lights lit, bulb out indicator)...
 
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