White Paper - Resistance vs impedance
Cell impedance at 1 kHz is not the same as DC resistance
When cell manufacturers list resistance in their specs, they usually are talking about AC impedance at 1 kHz.
Yet, what the user needs to know is the DC series resistance, because DC is what flows through the cells.
Even though both are measured in mΩ, 1 kHz impedance and DC resistance are really different parameters.
Manufacturers specify impedance at 1 kHz because:
Tha value of the cell's AC impedance is useless to the cell user. Reporting this value is entirely misleading to the user, who assumes it;s the value of the cell's DC resistance.
The user need DC resistance because:
You can read more about this topic in section 1.2.7 of the book Battery Management Systems for Large Lithium-Ion Battery Packs .
Can you infer DC resistance from AC impedance?
No, not really.
However, if you look at data from many different cells, you can see some correlation.
Impedance at 1 kHz vs DC resistance, for a variety of Li-ion cells
It appears that, for most cells, the actual DC resistance value (at 50 % SOC, 25 °C) is within a range of 2:1 to 1:5 of the cell impedance impedance value reported by the manufacturer.
That is a nice correlation.
But, when you stop and think about it, it means that a cell that is specified to have a 1 mΩ AC impedance at 1 kHz, is likely (though not guaranteed) to have a DC resistance of anywhere between 0.5 and 5 mΩ.
That is a full order of magnitude, and not very helpful to determine if a cell is appropriate for an application.
Davide Andrea, Elithion, 1/23/13