[TriEmbed] Single Cell LiPO Battery Health

Fri Mar 5 15:46:12 CST 2021

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
> chip at mcclellands.org <mailto:chip at mcclellands.org>
> 919-624-5562
>
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