[TriEmbed] Single Cell LiPO Battery Health

Shane Trent shanedtrent at gmail.com
Sat Mar 6 11:11:14 CST 2021


That is great information from Rick, (I would love to hear more
sometime Rick, maybe over a beverage when things settle down). I think you
can get a reasonable estimate for the health of your battery packs using
some inexpensive tools.

It has been my experience that the best way to estimate a battery's
capacity (and extrapolate to expected operating life) is to measure its
power delivered under actual use conditions by using an amp-hour meter
while loading the battery with your actual load (or slightly higher load to
give you a little operating margin) and allowing the system to operate
until the battery drops below your desired cutoff voltage under load.  This
is often a challenge. The post linked below touches on a Xantrex Link 10
battery monitor that I used in a system to allow me to periodically (every
6 months or so) measure the amp-hour capacity (I think Watt-hour is a
better measure because it automatically accounts for differences in battery
terminal voltages but amp-hour is still often used) of a lead-acid battery
bank in a mobile system.

The next best thing to measuring energy capacity under load, and usually
easier, is to allow your system to operate until it fails due to a low
battery charge and then measure the amount of energy required to fully
recharge the battery. I used this approach to sort through battery packs
for LEGO EV3 and NXT based robots in FIRST LEGO League to identify packs
that have high capacity for competitive use and determine when packs need
to be recycled and replaced.

With the robots, I would set the robot up to operate under peak load
(driving back and forth or in circles while operating the actuators) until
the robot shut down due to a low battery. Then I would use a USB power
meter to measure power delivered to a USB based battery charger while the
battery recharged. Once the battery was charged, it was easy to look at the
mAh (or Wh) to see how much energy was delivered and write this on the
battery pack. Note this is not the actual capacity of the battery but
should correlate with energy capacity in most cases. One of the Exceptions
is when a battery pack is failing with high internal current leakage so
that it never finishes charging but the energy delivered keeps ticking up
on the power monitor.

The USB power monitor and USB based charger also allowed students to
measure the charging current during robot testing to get an idea of the
battery's state of charge (the batteries had a nice charge current profile
so the charging current was roughly inversely proportional to the charge
level, so once the charge current crossed an amp or so, they knew the
battery needed to be swapped for a fully charged one). This setup also let
the students charge the robot from a USB power bank between runs at an
event while they carried around the robot.

You might consider either writing firmware to repeatedly cycle through your
highest power functions, or just load the cell at the recommended
maximum continuous current for your battery cell to speed testing, and let
the system operate until it either reaches your desired minimum terminal
voltage or shuts down. Then charge the battery back up and mark the energy
delivered for a full charge on the battery.

USB Power Meter (I have units on hand if you want to get your hands on one

USB Battery Charger for single cell (I expect you already have suitable
versions, I should have one or two somewhere if needed)

USB to 12V converter that I ended up using to generate the voltage needed
to charge the robot batteries.

And a story of my adventure custom building my own USB to voltage DC
converter before I thought to search for such a creature on the internet.

Good luck with your project!


On Fri, Mar 5, 2021 at 4:46 PM Rick via TriEmbed <triembed at triembed.org>

> 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
> 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/
> - 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
> Any suggestions?
> Thanks,
> Chip
> ____________________________________
> Chip McClelland
> chip at mcclellands.org
> 919-624-5562
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[image: photo]
*Shane D Trent*
Patent Agent

919-348-0061 | shanedtrent at gmail.com

ShaneTrent.com | Skype: skype:shane.trent1
Raleigh, North Carolina
<http://www.linkedin.com/in/shanetrent> <http://twitter.com/sdtrent>
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