Meaning
Unintended electrical charge storage that occurs between conductive elements separated by an insulator in a test circuit. In battery testing, stray capacitance often exists between the leads, the cell casing and the ground plane of the laboratory bench. This effect bypasses the device under test, creating a parallel path for high-frequency signals.
It defines the point where the measurement environment begins to influence the reported electrochemical properties.
Signal Leakage
High-frequency excitation current can leak through the insulation of the cables to the surroundings. When stray capacitance is high, the measured current at the device under test is lower than the current sent by the source. This results in an impedance reading that is higher than the true value.
Shielding the cables and connecting the shields to a common guard point can significantly reduce this leakage.
Ground Loops
Multiple paths to the electrical ground can create loops that act as capacitors. These paths allow stray capacitance to introduce noise into the voltage sensing lines. High-precision measurements require the cell to be electrically isolated from the metal chassis of the environmental chamber.
This isolation prevents the chamber walls from acting as one plate of a large, unwanted capacitor.
Frequency Response
The impact of this phenomenon increases linearly with the frequency of the test signal. While negligible at low frequencies, stray capacitance can completely distort the high-frequency portion of an impedance spectrum. Engineers use open-circuit calibration to measure the baseline capacitance of the setup.
Subtracting this baseline from the final results improves the accuracy of the data for small-capacity cells. Without this correction, the high-frequency resistance would appear much lower than the physical reality of the electrolyte conductivity.
