Estimating the Capacity of Charged Used Li-ion Cells:

Question

I acquired 6 used li-ion cells 1 year ago. Since I wanted to know for certain how well the cells would hold a charge after 1 year, I conducted an experiment. I recently tested the cells. Below is some data related to my testing:

Charger info:
Voc = 5.25 (Open circuit Voltage--no cell in charger)
Imax = .68A (max charge current=680mA--no cell in charger)

Cell Info:

Manuf:  Panasonic
   Id:  CGR18650CE
    V:  3.6 (nominal)
  mAh:  2150 (typical capacity when fully charged)
+Chrg:  CC=1.43A (max), 4.2V (max)     // "+Chrg"=Charging
-Chrg:  CC=2.04A (max), cut-off @ 3V   // "-Chrg"=Discharging

Notes:
- All cells were charged at a CC=.68A to a CV=4.2V. Charging was cutoff when the charge current reached [email protected]
- The resting V of all cells after 24 hrs was at least 4.05V.

Here are the cell measurements after 1 year of resting:

Cell V___ A___ VA(W)  Est mAh (i.e. VA/3.6*1000)
1    4.16 3.29 13.67  3802
2    4.01 3.23 12.95  3598
3    3.95 3.18 12.56  3489
4    3.85 3.09 11.90  3305
5    3.95 3.17 12.52  3478
6    3.77 2.95 11.12  3089

The multimeter was verified to be operating properly using a bench power supply.

We can see from the data that some cells held their charge better than other cells. My question is, how can the cells have such a high capacity when the datasheet indicates their capacity is 2150mAh? I know the cells are at least 5 years old. I also realize that my estimated mAh capacity is not based on actual discharge tests, but is there some other way of estimating the capacity of such cells using a multimeter & a bit of math?

Answer 1

I don't get the point where you get from the cell voltage to the capacity. What you need to do is to discharge the cell with a constant current and measure the time until 2.7V is reached. This is done best with a electrical load or 4 quadrant source.

Answer 2

No, you cannot estimate battery capacity quickly and accurately with a multimeter. Measuring battery capacity is complex, and the most accurate methods require a full discharge cycle (and even these aren't perfect). Otherwise, you need techniques like coulomb counting during charge and discharge, or look at the battery's response in the frequency domain to AC pulses. There are specialized ICs built for these purposes.

For more info on estimating capacity and state-of-charge, see here: https://batteryuniversity.com/learn/article/how_to_measure_capacity

and for info on how one IC manufacturer does it, see here: https://www.maximintegrated.com/en/design/partners-and-technology/design-technology/modelgauge-battery-fuel-gauge-technology.html

As to why your back-of-the-envelope method didn't work, to measure power from the battery you need to multiply voltage provided and current being drawn at the same time. The open-circuit voltages you see will immediately drop when you apply a load, and in general, for most of the discharge cycle, the voltage will be about 3.6V, so multiplying current draw by the voltage you see now will be an overestimate. And multiplying that by a burst of short-circuit current won't tell you anything meaningful about capacity.

Answer 3

To measure cell capacity accurately you should charge a cell to a specified voltage, for example 4.1 or 4.2 Volts. Then DISCHARGE the cell with a constant current, for example 1 A, until the unloaded voltage (when no current flows) has reached 3.6 V (for example).

It is also not completely clear to me if you assumed that the charging current is constant. Because it should not be ! A proper Li-Ion charger will charge with a lower current when the cell's voltage is quite low and when it is nearly full. Only when the cell is around 40 % - 80 % full the charge current should be at it's maximum value.

You measured the charging of the cells which is not a 100% efficient process, some of the energy will be converterted into heat. This can explain why the measured capacity appears higher. The cell probably did not absorb all the mAhrs.