Battery problem

[IDBoating]

...the diode will still pass voltage...

...if you test it in the correct direction! {grin} Otherwise the diode will act like a back-biased diode and block current, just like it's supposed to.

Presuming the DMM in question produces a positive voltage on its red lead when switched into resistance mode, the red lead should be on the alternator terminal and the black lead should be on the battery terminal under test. This will forward-bias the diode on that side.

Update: It occurred to me that some DMM's may not use enough voltage in their resistance mode to bias the diode, so I checked. My Fluke DMM here on my bench uses less than one volt (measured with an oscilloscope). I tried "measuring" a standard, small signal 1N4148 diode and was not able to properly forward bias the diode. Then I switched the Fluke into its "diode" mode - and the voltage it uses jumped to ~2.75V, more than enough to properly bias diodes (including those big high current monsters in isolators, which despite often being Schottky diodes still sometimes have higher Vf's). The moral of the story here is that if your DMM has a dedicated diode mode, use it. If not, you may get a false negative when testing diodes in normal resistance mode and need to conduct different tests to really know what is going on.

[chpthril]

...if you test it in the correct direction! {grin} Otherwise the diode will act like a back-biased diode and block current, just like it's supposed to.

Presuming the DMM in question produces a positive voltage on its red lead when switched into resistance mode, the red lead should be on the alternator terminal and the black lead should be on the battery terminal under test. This will forward-bias the diode on that side.

Update: It occurred to me that some DMM's may not use enough voltage in their resistance mode to bias the diode, so I checked. My Fluke DMM here on my bench uses less than one volt (measured with an oscilloscope). I tried "measuring" a standard, small signal 1N4148 diode and was not able to properly forward bias the diode. Then I switched the Fluke into its "diode" mode - and the voltage it uses jumped to ~2.75V, more than enough to properly bias diodes (including those big high current monsters in isolators, which despite often being Schottky diodes still sometimes have higher Vf's). The moral of the story here is that if your DMM has a dedicated diode mode, use it. If not, you may get a false negative when testing diodes in normal resistance mode and need to conduct different tests to really know what is going on.

yes, I understand how a diode, i.e. one-way electrical valve works and how to test one both directions. But, theres no need to go into all of that since the starting battery is receiving a charge from the alternator. IF the alt cable IS connected to the diode, its connected to the right post as evident by the later, agree? Since the iso is a dual diode and not serviceable, if one side passes current and the other side doesnt, who cares if its blocking voltage in reverse. The whole thing needs replaced so no need to test it further. if the diode is taken out of the loop all together, then still no need to test the reverse voltage blocking, since its a condition of the battery not receiving a charge, as opposed to the battery being drained by the cranking battery loads. This would point to direction issue.

Im a KISS kinda guy and like to work smart, not hard

Oh, what model Fluke U use? Ive got an 88 that ive had since 1995 or so. Replace the 9V about every 2 years, new leads about every 5 and thats it.

[IDBoating]

theres no need to go into all of that since the starting battery is receiving a charge from the alternator.

From what I've read, we know that ONE of the batteries is receiving a higher voltage when the engine is running. The other isn't, based on his comment that the battery voltage didn't change on one of the batteries with and without the engine running. So when you said:

Easy check though with a DVM. test center post then test each outer post.

...I thought you were pointing out that one side might be bad while the other side was good. I was just trying to be helpful in case he doesn't realize that polarity matters when using a DMM to test diodes, and the voltage put out by your DMM can affect the test too.

IF the alt cable IS connected to the diode, its connected to the right post as evident by the later, agree?

Definitely. But that still doesn't answer the question of why the other battery isn't seeing an increase in voltage. Could be a cabling problem (my vote at present), bad diode, etc.

He could also get this behavior if the battery isolator isn't in the circuit at all, and the alternator is wired directly to the battery that is seeing the increase in voltage. That's why I suggested he follow the wire from the alternator and see what is on the other end. We already suspect wiring problems, but we don't yet know what kind.... {grin}

Oh, what model Fluke U use?

The one sitting about four feet from me at the moment is a Fluke 16. I bought this meter because it has a capacitance mode and a dedicated temperature mode with an included temp sensor. I have no illusions that the cap mode is terribly accurate but it's useful for quick sanity checks once in a while. The 'scope I used is a Philips PM3214 dual trace delayed timebase which, again, just happened to be set up and therefore convenient. A nearby batch of 1N4148's provided the DUT. I'm working on a project that I look forward to talking with you (specifically) about, but that's a couple of months away yet {smile}. In the meantime, here's a shot of an early breadboard:

breadboard1.jpg

[chpthril]

I saw no reason to go into deep testing of the actual diode for its operation. The house battery is staying at its static level, so its not receiving alt voltage. If he now touches his meters B+ lead to the two outside Iso posts and both show alt voltage, when know its a cable issue between Iso and house battery. The chances of a bad diode are slim. For that reason, I want him to focus on the voltage trail all the way from the diodes center ALT post to the battery, specifically the house battery. This keeps him on the path to find a bad cable or left off cable. If one outer ISO post shows voltage and the other does not, BAM, bad ISO. If both outer posts show voltage, then we know its a cable left off or bad cable as we already know the house battery is not receiving that voltage.

Headless Horseman walks into the Optometrists office and tells the Dr....Doc, I cant see too well. Dr says....lets test your vision: If the ISO is out of the loop, then placing hands on it will determine it, so no need for a meter to test it .

Given that I know whos worked on this boat and that I tried to fix it prior to doing a sea trial for another buyer, my money is on a left off cable some where. Either accidental or intentional.

I went with the 88 because it did min/max like the 87, but also does RPM which is a good feature for a wrench-head like I used to be, as well as an amp prob.

[IDBoating]

I went with the 88 because it did min/max like the 87, but also does RPM which is a good feature for a wrench-head like I used to be, as well as an amp prob.

Yep, the 16 does min/max too which has proven handy a few times while doing overnight temp/voltage/current tests. I don't have RPM in this one but I consider that a reasonable tradeoff for the capacitance, and I have other tools for measuring RPM what that's necessary.

How does your 88 measure RPM? Optically? Cable clamp on a plug wire?

[smitty1994]

Well chp was right. The cranking battery had both of the leads from the alternator/diode connected to it. Switched one over to the house battery and everything is charging correctly now.
Thanks alot to all of yall for the help. I really appreciate it. I woulda felt like my stupid father in law for taking it in for this (he takes his $100k BMW into the dealership to get air put in his tires when the light comes on and no he does not have nitrogen in his BMW tires just his Porche).

Since I have two cranking batteries, is the general consensus to just use a cranking battery as the house until it dies and then replace it with a deep cycle?

Thanks again guys!

The best part of the whole ordeal was building a homemade fake-a-lake for $6.48

[chpthril]

Smitty,

Glad you found that misplaced cable and all is working correctly now. If you are going to be spending a considerable amount of time and on a regular basis, playing the system while anchored, then a deep-cycle would be better suited. Other wise, the cranking will work just fine.