Electronic – Estimating the Capacity of Charged Used Li-ion Cells:

battery-charginglithium ion

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 .01A@4.2V
– 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?

Best Answer

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.

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