“Two methods are used to read the internal resistance of a battery: Direct current (DC) by measuring the voltage drop at a given current, and alternating current (AC), which takes reactance into account. When measuring a reactive device such as a battery, the resistance values vary greatly between the DC and AC test methods, but neither reading is right or wrong. The DC reading looks at pure resistance (R) and provides true results for a DC load such as a heating element. The AC reading includes reactive components and provides impedance (Z). Impedance provides realistic results on a digital load such as a mobile phone or an inductive motor. ( See BU-902: How to Measure Internal Resistance )” Source
Nécessaire d'être aussi précis ? Comparer avec testeur
Test on a 35E cell at 3.976 V :
ZKE EBD-A20H : 29 mR at 1A, 32.5 mR at 2A, 33 mR at 1A, 29 mR at 1A. Seems to vary a bit in a 29~33 mR range, which is sufficient in general
YR1035+ : 20.5~21.5 mR
⇒ Not the same value !
In the Samsung 30Q datasheet :
Comment from Luke Workman (liveforphysics) in 2014:
“AC Impedance meter's have there place in taking relative measurements of the same type of cell, but the value they give is not indicative of the DC load internal resistance, it's mostly a measurement of the ESR of the capacitance a cell inherently has.
Something like a 1-2Ohm resistor across the cell while measuring voltage sag (with a real DMM, at least 4.5digits) after 5 seconds is a much more meaningful value with respect to selecting them and matching them for building a pack.”
Only, I would take the measurement at mid-SOC, not low
They jump from 0.28A to 1.4A for IR measurement. Not starting from 0A
IR evolution during battery life
IR dependency on SOC
Measured IR for a single cells depends on its current SOC and on its temperature
Informations can be found on paper "Charge and discharge characteristics of a commercial LiCoO2-based 18650 Li-ion battery”, page 7
Resistance seems to go up as the cell is near-empty
This variation can and should be measured for each batch we receive. This way we could compute a comparable IR value for each tested cell, independent of its voltage / SOC
The sorting machine might not have this possibility
Ignore / compute cell IR on low SOC cells ?
IR dependency on temperature
Measured IR for a single cell depends on its temperature
We should ensure that the facility’s ambient temperature stays within a constrained range, e.g 20~25 °C to avoid variations in IR data due to this effect
For more precise data we could measure IR at different cell temperatures (can vary between cell models / chemistries) and possibly against different SOCs
IR dependency on battery aging
IR does not seem to be related to capacity fading, here for a Li-ion cell:
