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<p>Hi Chip,</p>
<p>I've done similar work in the past in the automotive and
industrial lead-acid battery world and can share a few points.</p>
<p>First, if you plan on measuring relative changes to internal
resistance, you need to have baseline measurements on cells built
at the same time, and preferably within the same batch. Otherwise
you are guessing. Not that guessing is bad, but you can cause
yourself (and others) needless headache and expense if you decide
a cell is prematurely aged out when it's built to different
specifications, at a different temperature than your reference, or
how you were holding the test leads (trust me, experience here!).<br>
</p>
<p>Second, measuring internal resistance is simply a measurement of
cell voltage during a constant current drain event. We typically
used a two-second test period between 1000-1500 amps for lead-acid
batteries (even U1 lawn mower batteries passed over 1000 amps in
this test). I used PWM-controlled silicon switches and measured
voltage and current every 100 mS during the discharge using an
isolation amplifier for voltage measurement and a current
transducer for the current measurement. I was using a Motorola
56DSP80x series processor with dual ADCs (two 8-channel muxed ADCs
... but kept things simple and didn't change the mux channel). I
also tossed the first measurement and then averaged the next 7-8
measurements and then passed the data along to the front-end
processor (data warehouse, system controller, and decision maker)
PC.</p>
<p>[Sidebar ... on multiplexed ADCs, the ADC input capacitor MAY
still hold a charge from the previous measurement ... thus you
read and discard the first measurement to ensure the subsequent
measurements are honest and true.]<br>
</p>
<p>There were (and may still be) devices that claim to measure
internal resistance, but at a very low current (Midtronics Biddle,
et all; I'm talking to you!). Their results were inconsistent and
did not relate to battery capability or life ... and really were
not based on any battery chemistry science.</p>
<p><br>
</p>
<p>You do NOT want to draw excessive current from your
Lithium-chemistry batteries ... in your case I would use
approximately 0.1C (10% of rated AH capacity) for no more than 1-2
seconds while monitoring battery temperature. Measure the
open-circuit voltage (no load cell voltage) and then apply the
load. With the automotive batteries, our typical open-circuit
voltage was 12.5 to 13.5 volts, depending on when the last charge
cycle occurred, and no less than 10 volts under load. With these
parameters defined, we were able to show the constant-current
voltage over the test period and observe the change (slope) of the
battery (cell in your case) voltage. Obviously the shallower the
slope, the better the battery performed.</p>
<p>You can produce a direct internal resistance measurement from
this test, however you MUST use Kelvin connections at the cell
terminals ... keeping the voltage measurement leads adjacent to,
but not sharing the current-carrying leads beyond the cell for
maximum accuracy and repeatability. You absolutely need to specify
the internal resistance as x Ohms @ y amps test current for this
measurement to be meaningful.<br>
</p>
<p><br>
</p>
<p>You can compare new vs old cells to get an approximate idea on
cell aging (i.e. internal resistance) ... and you can use this
method to more closely match internal resistance in series-string
connected cells. You really can't determine remaining cell life,
as you have no idea whether active material shedding, internal
current-carrying structure changes, or terminal connection
resistance changes have created the difference.</p>
<p>With the cost of Lithium-chemistry cells being at the commodity
level, it is probably more prudent to recycle your used cells, or
offer them to someone willing to risk cell failure for the
advantage of low cost (i.e. FREE) cells. While there are those
that are building huge battery banks from "broken" 18650 cell
based battery packs, they are gambling on individual cell life, as
physical age as well as cell/battery treatment is generally
unknown. I certainly wouldn't base any mission-critical project on
a hacked battery built this way.</p>
<p><br>
</p>
<p>This may be way more information than you need ... and it may not
answer your question, but I've seen questions like this come up
from time to time and now that the NDA is expired, I can talk
about some of my work.</p>
<p><br>
</p>
<p>I hope this helps.</p>
<p><br>
</p>
<p>Regards,</p>
<p><br>
</p>
<p>Rick</p>
<p><br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 3/5/2021 1:15 PM, Chip McClelland
via TriEmbed wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CABQO2+nVTKQc9yH=qN=+LA+P2E5BMxa9C8_neWVAguiudsPuxw@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<style>body{font-family:Helvetica,Arial;font-size:13px}</style>
<div style="font-family:Helvetica,Arial;font-size:13px">
<p
style="margin-top:0px;color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px">I
am currently upgrading my existing Electron-based counters
with newer Boron-based ones. As I do, I will rework / recycle
as much as I can from the older units to reduce waste. These
units have been operating on solar power for anywhere from 1
to 4 years.</p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px">How
can I tell if it is time to replace a Single Cell LiPo
Battery?</p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px">Here
is the battery I currently use: <a
href="https://www.adafruit.com/product/2011"
moz-do-not-send="true">https://www.adafruit.com/product/2011</a></p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px">I
thought it would be straight forward to test the internal
resistance of the batteries (at a consistent charge / temp) as
the resistance goes up as the battery ages. However, I am
struggling to find a battery tester / charger that will
measure a single LiPO cell’s internal resistance. </p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px">I
have tried both of these:</p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px"><span class="Apple-tab-span" style="white-space:pre"> </span>-
Tenergy: <a
href="https://power.tenergy.com/tenergy-5-in-1-intelligent-cell-meter-capacity-checker-battery-balancer-battery-discharger-internal-resistance-tester-esc-servo-ppm-tester/"
moz-do-not-send="true">https://power.tenergy.com/tenergy-5-in-1-intelligent-cell-meter-capacity-checker-battery-balancer-battery-discharger-internal-resistance-tester-esc-servo-ppm-tester/</a></p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px"><span class="Apple-tab-span" style="white-space:pre"> </span>-
HTRC: <a
href="https://www.amazon.com/HTRC-Charger-Battery-Balance-Discharger/dp/B07MWSW3TP/ref=sr_1_4?dchild=1&m=A193SNVHREJU7H&marketplaceID=ATVPDKIKX0DER&qid=1614968073&s=merchant-items&sr=1-4"
moz-do-not-send="true">https://www.amazon.com/HTRC-Charger-Battery-Balance-Discharger/dp/B07MWSW3TP/ref=sr_1_4?dchild=1&m=A193SNVHREJU7H&marketplaceID=ATVPDKIKX0DER&qid=1614968073&s=merchant-items&sr=1-4</a></p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px"><br>
</p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px">Any
suggestions?</p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px"><br>
</p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px">Thanks, </p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px"><br>
</p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px">Chip</p>
<p
style="color:rgb(34,34,34);font-family:Arial,sans-serif;font-size:15.008000373840332px"><span
style="font-family:Helvetica,Arial;font-size:13px">____________________________________</span></p>
</div>
<div class="gmail_signature">Chip McClelland
<div><a href="mailto:chip@mcclellands.org"
moz-do-not-send="true">chip@mcclellands.org</a></div>
<div>919-624-5562</div>
</div>
<br>
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