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Unread 11-19-2018, 02:20 PM   #1
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Default Can I test for Amp\Hr using a MM and diode?

I've been doing some reading of "how to" measure a battery for amp\hr and watt\hr.

Now, as most people know, I'm not an engineer and have a "fuzzy" amount of knowledge when it comes to this.

However, if I have one Lith battery that is charged to 7.9V, and I have a Multimeter, and a 1N4007 1A 1000V Through Hole Axial Rectifier Blocking Diode, is it possible to determine the amp\hr of this battery or the watt\hr, or both? (Or none?)

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Unread 11-19-2018, 10:09 PM   #2
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Default Re: Can I test for Amp\Hr using a MM and diode?

Why don't you contact the battery mfg. and ask that question?
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Unread 11-19-2018, 10:27 PM   #3
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Default Re: Can I test for Amp\Hr using a MM and diode?

Perhaps I'm missing something, but I don't see how you can use a diode to measure capacity of a battery.

If you have a fully charged battery, you can connect a certain load (resistive) to it and measure amount of time it takes to reach nominal voltage (50% charge). Knowing the resistance and voltage you can determine the current (approx, since voltage will be dropping non-linearly, especially in Lithium Ion). Knowing the current over time, or your Amp-hours to half charge, you will know your approx full Amp-hours by doubling that. Watt-hours would be Amp-hours times nominal voltage.
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Unread 11-20-2018, 07:08 AM   #4
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Default Re: Can I test for Amp\Hr using a MM and diode?

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Originally Posted by cricketo View Post
Perhaps I'm missing something, but I don't see how you can use a diode to measure capacity of a battery.
Yeah. I was attempting to use a given load that was\is easy to measure. Maybe the right terminology would be a resistor. But I don't think that would work.

What I think I need to do is just read more.
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Unread 11-20-2018, 08:03 AM   #5
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Default Re: Can I test for Amp\Hr using a MM and diode?

Just use the AH meter you already have installed.
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Unread 11-20-2018, 09:19 AM   #6
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Default Re: Can I test for Amp\Hr using a MM and diode?

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Just use the AH meter you already have installed.
Ok, My Lithium pack: Each "cell" is rated at 32.5ah. So, each Leaf Module...because of the internal Series and Parallel, has a "rated" AH of 64AH. (more or less)

In order to "test" the rated AH of a module, or even a pack, I believe I need to have a specific Load (a load that I know what the amp draw is) connected to either a given module, or a given pack, and then knowing what the amp draw is, I can "run" the load and measure the time it takes to reduce the Module down to....say 6V (per module....which would be 3.0V per cell) Then I "know" what my AH of the module is. Yes?

So, I happen to have one module that is currently "hanging out" on a shelf with nothing to do. Now, if I know the amp of a specific load, I should be able to have that load draw the one module down to a "given voltage" (not going below the knee) to determine it's "true" amp hours.

And why do I want to do this? Well, because I can. (Maybe).

And one other matter or discussion point concerning the ZEVA,....I know that I must manually enter the packs "Rated capacity" in AH. I believe that number is then used by the ZEVA to determine the remaining SOC. So, if I enter the "Rated capacity" at say 40AH, the meter will show a faster reduction of SOC than if I were to enter 50AH.

So....the "Rated AH" is kinda like a SWAG. Now, if I know the actual AH of a module, then I would know the "Real AH" as opposed to the "Rated AH."

Right?

Or maybe I'm chasing rainbows?
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Unread 11-20-2018, 09:43 AM   #7
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Default Re: Can I test for Amp\Hr using a MM and diode?

I use one of these to monitor the health of my 12 volt marine deep cycle batteries as applied to my boat application. The high beam draws 4.93 amps as measured my my multi-meter. Once a year I measure how many hours it takes to pull them down to 50% SOC.

https://www.amazon.com/SYLVANIA-Halo.../dp/B000BLGPEI

Perhaps you can accomplish the same with a proper voltage bulb.
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Unread 11-20-2018, 09:48 AM   #8
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Default Re: Can I test for Amp\Hr using a MM and diode?

You are chasing rainbows....

Since the current is the ratio between Voltage divided by resistance, the current would change as the voltage drops and that can result in a large margin of error if it takes 50 hours to discharge the module.

While for purposes of SOC percentage the AH entered is subjective, the Zeva AH reading is not.

The Zeva is measuring the actual AH used by the pack, not guessing it.

Change the LV cut off to be a little higher (safety margin), Set Your pack capacity to 100AH, change the SOC display to show AH remaining and just drive around.

Once the Zeva LV shuts down the cart, write down the AH used, change the LV to the original lower value and drive home to charge.
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Unread 11-20-2018, 12:09 PM   #9
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Default Re: Can I test for Amp\Hr using a MM and diode?

Ahhhh......"live testing". I didn't think I could do this.

Way easier than what I was thinking.

Thanks Sergio!!
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Unread 11-20-2018, 02:31 PM   #10
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Default Re: Can I test for Amp\Hr using a MM and diode?

Quote:
Originally Posted by Sergio View Post
You are chasing rainbows....

Since the current is the ratio between Voltage divided by resistance, the current would change as the voltage drops and that can result in a large margin of error if it takes 50 hours to discharge the module.
All true, but this can be done by taking multiple voltage samples during discharge and plotting the curve, it then should be a pretty good approximation. Doesn't have to be 50 hours either, as the load can be adjusted for whatever max current draw at the beginning of the cycle.
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