Auxiliary Electrometer Option – Experiments
66
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, because 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 V
and 33 V. This can also be described as 31 V 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 specification, we can calculate the DC error in the voltage
caused by 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 V cell voltage, so we can ignore it.
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/ 5000 = 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, because 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 Electrochemical Energy 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.
Table 8-1 shows the pin-out of these cables.
Table 8-1
AE Cable Connections
Pin Type
Name
Wire Color
Connector
Color
Normal Connection
1
Analog Input
Odd Ch −
Black
Yellow
Low side of odd channel
reference side
2
Analog Input
Odd Ch +
Black
Purple
High side of odd channel
working side
3
Ground
Floating Ground --
--
4
Analog Input
Even Ch −
Red
Yellow
Low side of even channel
reference side
The cables are interchangeable, but we do not recommend using a numbered cable in a
differently-numbered connector. The connections get much too confusing.
