improvised voltage reference

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bob prohaska

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Aug 23, 2022, 10:24:09 PMAug 23
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Does anybody have a weblink for the temperature coefficient
of voltage for commercial (Energizer) size 357 silver oxide-zinc
button cells? Nominal is 1.55 volts, but knowing the temp effect
is (maybe) important if I'm stacking up nine or ten of them.

I'd like to check a trio of inexpensive Radio Shack DVMs for use
in charging relatively-expensive lithium iron phosphate batteries.
On the same battery the three meters report 14.41, 14.51 and 14.58
volts, so they certainly disgree among themselves. I suspect but
can't yet prove that the middle one is about 2% higher than actual.

Yes, I probably should buy a good DVM, but the Radio Shack units
are on-hand and except for charging batteries absolute accuracy
isn't usually of much importance.

Ten millivolts is plenty good for present purposes. If there's a
better/cheaper improvised reference in this range please post!
CR2032 cells are readily available, but voltage details are scarce.

Thanks for reading,

bob prohaska







Peter W.

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Aug 24, 2022, 7:29:59 AMAug 24
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Eveready states in their literature that the "open" voltage (unloaded) of such a cell is 1.5 to 1.6 volts - as a matter of the specific device in question. Suggesting that the loaded voltage will be lower. They state, typically, at 1.55V for a 1.6V battery. There is also a temperature chart in their literature, but it is applied only to storage and longevity-in-storage.

9 x 1.6 = 14.4, so you are definitely within the range even with the lowest reading given.
Typical accuracy (DC) on an RS meter (today) is +/- 0.5% - as advertised. Meaning some may be better, but should not be worse. As yours are likely older models, +/- 0.5% is likely optimistic.
0.5% x 14.4 = 0.072
Meaning that your range-of-readings should be somewhere between 14.472 and 14.328 for that particular group of nine (9) button cells.

Oh, RIGHT! You are comparing apples (liFe) cells to oranges (ZnAgO). Simple enough: Go to the Manufacturer and get the nominal unloaded and loaded values. Measure with the meters-in-hand. That which is the closest is the benchmark. If the others are consistent, there is your margin-of-error.

Peter Wieck
Melrose Park, PA

legg

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Aug 24, 2022, 9:39:23 AMAug 24
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10mV on a single 3.5V cell is 0.3% - so you're going to have to
calibrate SOMETHING, if you intend to provide that accuracy.
One calibrated reference is enough - you can set up cheap meters
to mimic it and calibrate microcontrollers to store correction
factors using it.

Using batteries as a reference is no longer a viable method for
most labs to maintain.

Get one good 6digit calibrated meter. If it's built into a data
logger, even better.

RL

ehsjr

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Aug 24, 2022, 6:45:02 PMAug 24
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You want a precision that is doomed by the technique you
intend to use - stacking cells. Instead, build a voltage
reference using a single IC: ADR01 available from Digikey
https://www.digikey.com/en/products/detail/analog-devices-inc/ADR01AUJZ-REEL7/996117
It provides 10V output at +/- point one percent (.1%)

Calibrate your meters to that.

Ed

amdx

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Aug 25, 2022, 6:29:48 AMAug 25
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Jeff Liebermann

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Aug 25, 2022, 2:49:03 PMAug 25
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On Wed, 24 Aug 2022 02:24:05 -0000 (UTC), bob prohaska
<b...@www.zefox.net> wrote:

>Does anybody have a weblink for the temperature coefficient
>of voltage for commercial (Energizer) size 357 silver oxide-zinc
>button cells? Nominal is 1.55 volts, but knowing the temp effect
>is (maybe) important if I'm stacking up nine or ten of them.

Bad idea. If a silver oxide battery goes below 30% charge, there will
be about a 100mv step increase in terminal voltage:
<http://members.iinet.net.au/~fotoplot/accbatcc.htm>
<http://members.iinet.net.au/~fotoplot/dis.jpg>

I've seen the step in my cheap Chinese digital calipers, where I need
to re-zero the indicator when the cell voltage crosses the transition.
Of course, the alkaline batteries are somewhat worse, where I need to
re-zero the calipers nearly every time I use them.
--
Jeff Liebermann je...@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann

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Aug 25, 2022, 3:02:22 PMAug 25
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On Thu, 25 Aug 2022 11:48:56 -0700, Jeff Liebermann <je...@cruzio.com>
wrote:

>If a silver oxide battery goes below 30% charge, there will
>be about a 100mv step increase in terminal voltage:
><http://members.iinet.net.au/~fotoplot/accbatcc.htm>
><http://members.iinet.net.au/~fotoplot/dis.jpg>

Oops. That should be "goes below 70% charge". I'm accustomed to
capacity being state of charge (SoC), not state of discharge.

Jeff Liebermann

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Aug 25, 2022, 3:17:09 PMAug 25
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On Wed, 24 Aug 2022 02:24:05 -0000 (UTC), bob prohaska
<b...@www.zefox.net> wrote:

>Ten millivolts is plenty good for present purposes. If there's a
>better/cheaper improvised reference in this range please post!
>CR2032 cells are readily available, but voltage details are scarce.

There are ready to use voltage reference PCB's available.
<https://www.adafruit.com/product/2200>
<https://www.amazon.com/Voltage-Reference-Source-Output-Precision/dp/B07WMYN32C>
<https://www.amazon.com/Reference-Precision-Resistance-Calibrating-Multimeter/dp/B0B6FK8Z1D>
<https://www.ebay.com/itm/393398250363>
<https://www.aliexpress.com/wholesale?SearchText=precision+voltage+reference>
<https://www.aliexpress.com/item/2251832677587870.html>
etc...
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