This is a very interesting question and raises a fundamental issue. The question that is implied is “what do you (or for that matter, anyone) mean by a state of charge?”

As a practical matter, it means the stored charge relative to the maximum stored charge as determined under a predefined charging condition. The point is that a cell can accept different amounts of charge depending on the charging rate. Further, the maximum charge depends on the utilization of the active material that is the limiting reagent (i.e., the active material contained in the limiting electrode). Theoretically, 100% utilization of the active material would require that the cell be charged at infinitely low rate for infinite time. If that condition was actually met, then the cell would then achieve the true 100% state of charge. It is always interesting to compare the utilization of the active material relative to the charge that is actually stored. In a lead acid battery, when the cell is “fully charged,” the active material is about 50% utilized. Indeed, most of the active material merely occupies space and adds weight.

And to avoid oversimplifying the whole state of charge concept, there is the temperature dependence on the state of charge. That is, by varying the temperature, the active material utilization changes. Then there is the influence of current density but that implies an electrode design issue. There are also a number of other issues that can be rolled up into what is conveniently termed as an “aging effect.”

Tapper charge have certain advantage over Galvnostatic charging at high rate . if the system has higher impedance CV will take care of full charge (100% SOC),2nd utilization material at higher rate.

Voltage is illusive for determination of SOC of cell for battery material which involves phase transformation. But for solid solution you can roughly predict the cell SOC by cell voltage . SOC of cell changes with cycle but if your electrode's are not slipping relative each other then it's pretty easy to predict the cell SOC by knowing the voltage of a cell. In general SOC of cell varies 1. Electrode slippage 2. Impedance of each electrode 3. A/C ratio of cell 4. Cycling voltage window 5. How good your electrodes are so that local SOC of the electrode does not change many others..

Hope it helps.

Thanks,

Nupur

Yes,a constant voltage phase is important, as Mr. Sinha explained. If at a certain voltage a netto current flows than your system is not in electrochemical equilibirum and therfor your voltage is the sum of cell voltage and overvoltage due to system properties.

See attached a picture taken from [1]. Here you can see for different cathode materials the SOC and voltage dependency is different. For example in this graph the SOC of the material LiFePO4 could not be evaluated by using the voltage. Maybe you can use the voltage development of the anode material to get a guess, but it will never be the exact SOC.

Greetings,

S. Wolter

(1) Doeff, M. M. In Batteries for Sustainability: Selected Entries from the Encyclopedia of Sustainability Science and Technology; Brodd, R. J., Ed.; Springer Science+Business Media: New York, 2013.

This is a very interesting question and raises a fundamental issue. The question that is implied is “what do you (or for that matter, anyone) mean by a state of charge?”

As a practical matter, it means the stored charge relative to the maximum stored charge as determined under a predefined charging condition. The point is that a cell can accept different amounts of charge depending on the charging rate. Further, the maximum charge depends on the utilization of the active material that is the limiting reagent (i.e., the active material contained in the limiting electrode). Theoretically, 100% utilization of the active material would require that the cell be charged at infinitely low rate for infinite time. If that condition was actually met, then the cell would then achieve the true 100% state of charge. It is always interesting to compare the utilization of the active material relative to the charge that is actually stored. In a lead acid battery, when the cell is “fully charged,” the active material is about 50% utilized. Indeed, most of the active material merely occupies space and adds weight.

And to avoid oversimplifying the whole state of charge concept, there is the temperature dependence on the state of charge. That is, by varying the temperature, the active material utilization changes. Then there is the influence of current density but that implies an electrode design issue. There are also a number of other issues that can be rolled up into what is conveniently termed as an “aging effect.”