Hi Chip,
I've done similar work in the past in the automotive and industrial
lead-acid battery world and can share a few points.
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!).
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.
[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.]
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.
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.
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.
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.
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.
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.
I hope this helps.
Regards,
Rick
On 3/5/2021 1:15 PM, Chip McClelland via TriEmbed wrote:
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.
How can I tell if it is time to replace a Single Cell LiPo Battery?
Here is the battery I currently use:
https://www.adafruit.com/product/2011
<https://www.adafruit.com/product/2011>
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.
I have tried both of these:
- Tenergy:
https://power.tenergy.com/tenergy-5-in-1-intelligent-cell-meter-capacity-checker-battery-balancer-battery-discharger-internal-resistance-tester-esc-servo-ppm-tester/
<https://power.tenergy.com/tenergy-5-in-1-intelligent-cell-meter-capacity-checker-battery-balancer-battery-discharger-internal-resistance-tester-esc-servo-ppm-tester/>
- HTRC:
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
<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>
Any suggestions?
Thanks,
Chip
____________________________________
Chip McClelland
[email protected] <mailto:[email protected]>
919-624-5562
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