I am using 2 RC Equivalent circuit model (Time domain) to fit discharge current pulses of Lithium ion battery (full cell). So basically the sum of R0 (ohmic) + R1 (charge transfer) + R2 (diffusion) resistance calculated by the battery model should be equal to total resistance (R) measured across the battery during the current pulse (Ohm's law principle). But the model value I get is little higher. For eg: 0.6 ohm is the model value and 0.4 is the battery resistance. Can anyone tell me the reason and how to solve the problem ? Any literature or methods will be helpful
I'm just guessing here, because I don't know much about impedance experiments, but I can think of three options: (1) are you using the exact same electric contacts? These can change the ohmic resistance. (2) Another possibility is the battery's state of charge. If you make the separate measurements at different states of charge you may also get different results. (3) What is the current density? at different current densities you will get different voltages that are not linear with the current because of the reaction overpotential according to Tafel's law. Perhaps there is a specific current density that you should use for this experiment.
The internal resistance of a battery should be viewed[1], ONLY, as a very general (imperfect), little one guideline; ... not as a drop out precise[2] value (even if, it is a "sum of some adaptable" lumped element model : R values), when applying it to the expected voltage drop, in a specific application.
1. TECHNICAL BULLEETIN (Energizer, Dec.2005) : Battery Internal Resistance http://data.energizer.com/pdfs/batteryir.pdf
2. see, also, a related dialogue : https://www.researchgate.net/post/How_does_the_IR_drop_of_charge-discharge_curve_for_supercapacitor_electrode_change_with_applied_current_density_and_why
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