Cell resistance

Measuring the DC resistance of a Li-ion cell

Most cell manufacturers' specs give cell AC impedance at 1 kHz (if anything at all).

But what matters to the user is DC resistance.

Note that DC resistance of a Li-ion cell is not constant, but varies with temoerature, SOC and current

DC resistance increases with cell age and usage.

You can get DC resistance:

• From the spec sheets of the few manufacturers that report it
• Graphically from the voltage vs SOC curves is the spec sheets (if you can trust them)
• Measure it directly

Here is one way to measure the DC resistance, roughly

Equipment:

• A high power resistor, 0.5 Ohm, 25 W
• DC Ammeter, 10 A full scale
• Voltmeter, 4-1/2 digits

1. Cell at about 50 % SOC, about 25 C temperature
2. Place the voltmeter across the cell
3. Measure V-ocv, the open circuit voltage of the cell
4. Place the resistor and the ammeter in series, then place bith across the cell
5. Wait 10 s
6. Measure I-loaded, the current through the resistor
7. Measure V-loaded, the loaded voltage of the cell
8. Disconnect the resistor and ammeter
9. Calculate the resistance as: R = (V-ocv - V-loaded) / I-loaded