Improving Measurement Accuracy with Compensation

When performing electrochemical impedance spectroscopy (EIS) on battery cells, compensation techniques improve measurement accuracy. Compensation techniques address various sources of error or impedance contributions that are not inherent to the cell. For repeatable measurements, perform compensation for each frequency at which you perform a test.

Obtain compensation values for each setup. If any of the following items change, you must obtain compensation values again:

  • DUT capacity
  • DUT form factor
  • Cabling
  • Fixture and probing
  • Cable position
  • Temperature
  • Frequency
  • Test current amplitude
Figure 3. Compensation Block Diagram

Empirically determine the type of compensation to use. Observe how the system and DUT combination respond at the target frequency and target excitation current. The following parameters help identify the correct type of compensation mechanism:

  • Measured voltage
  • Current phase angle
  • Linearity
  • Impedance linearity across the target frequency and current ranges

For EIS measurements up to 2 KHz, NI recommends short compensation for most scenarios.

Golden DUT Compensation

Golden DUT compensation in impedance measurements is a technique that is used to account for the effects of the load on the measurement system.

The golden DUT compensation device impedance must match as closely as possible the intended DUT impedance for the target range of frequencies.

Golden DUT compensation reduces the effects of load-induced uncertainties. Therefore, golden DUT compensation enhances the accuracy and the repeatability of impedance measurements from the cell.

Mitigating System Interaction

Golden DUT compensation mitigates system interaction. Golden DUT compensation addresses interactions between the cell and the measurement system that might affect the accuracy of impedance readings.

Potential interactions include signal attenuation, signal reflections, and loading effects.

Short Compensation

NI recommends using short compensation to measure the impedance of the cables, connectors, and other components in the measurement path.

For short compensation, NI uses a PCB with a resistor tested at 200 µΩ, 500 µΩ, and 1 mΩ.

The PCB resistor measures the impedance when the impedance of the cell does not influence the components in the measurement path. When performing cell measurements, the impedance of the cell is in series with the actual DUT impedance.

Mitigating Cable Resistance and Lead Resistance

Short compensation mitigates the effects of cable resistance, connector resistance, and lead resistance. Lead resistance is any resistance in the leads that might affect impedance measurements.

Short compensation removes the effects of resistance from the impedance data. Short compensation does this by measuring resistance directly when the cell is connected to a PCB resistor.