Chapter 7 -- Auxiliary Electrometer Option--Experiments
7 - 2
CMR is especially important when the AE is used to measure the voltage of individual cells in a battery or fuel-
cell stack. Cells near the Working Electrode have relatively low DC voltages, since the working electrode
voltage is near ground. Cells near the Counter electrode have higher DC voltages. When an AC signal is
applied to the stack, the counter electrode end of the stack has higher AC voltages.
Let’s look at a hypothetical example. A battery stack has 22 cells, with an average DC cell voltage equal to 1.5
volts. The bottom cell in the battery stack is attached to the working electrode and the top cell is attached to
the counter electrode. A 100 kHz AC signal is applied, creating an average AC voltage of 10 mV
rms
per cell.
Assume the working electrode is at zero volts versus floating ground. The DC voltages on the top cell are 31.5
volts and 33 volts. This can also be described as 31 volts of common mode voltage plus a 1.5 volt differential
voltage. The AC voltages on the top cell are 210 mV of common mode voltage and 10 mV of differential
voltage.
Applying the 94 dB minimum low frequency CMRR spec, we can calculate the DC error in the voltage due to
the DC common mode voltage.
Maximum DC Error = V
cm
/ CMRR = 31 V / 94 dB = 31 V/ 50,000 = 620 µV.
This is quite small compared to the 1.5 volt cell voltage, so it can be ignored.
At 100 kHz, the CMRR is specified to be at least 74 dB.
Maximum AC error = V
cm
/ CMRR = 0.21 V / 74 dB = 0.21 V/ 5,000 = 42 µV.
If this AC error is 90° out of phase with the true AC voltage on the cell, it creates a phase error of 0.2° in the
measured AC voltage. This is not significant, since the AC accuracy specification at 100 kHz is ± 2°.
Experiments
The AE option is supported only by the Galvanostatic EIS, Galvanostatic Single Frequency EIS, and Hybrid EIS
Framework scripts, as well as the experiments performed through Gamry PWR800 Software.
Connections Using Standard Cables
The AE connects to an electrochemical cell using one, two, three or four cables. Other than labeling, all four
cables are identical. Each cable supports two AE channels.
NOTE
The cables are interchangeable, but we do not recommend using a numbered cable in a
differently numbered connector. The connections get much too confusing.
Table 7-1 shows the pin-out of these cables.
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