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Cell Testing Tips and Best Practices
A Cell's Percent State of Charge
Charge Redistribution and Equilibrium
Expected Self-Discharge Current and Temperature Dependency
Temperature Impact on Current Measurement
Impact of Stress and Vibration on Current Measurement
Noise Removal Functions
Optimizing Initial Current and Resistance Values
The voltage of most Lithium Ion cells is very sensitive to vibration and mechanical stress
or deflection. Once a self-discharge test has started, the cell must not be moved or
disturbed, and must not subjected to vibration. The temperature sensor must be
mounted on or attached to the cell before the test and not be touched or have its
connection adjusted during the test. Cells are microphonic – any touching, tapping,
vibration, or shock will affect the cell's voltage and thus the measured current value.
A Cell's Percent State of Charge
Self-discharge current is somewhat less for lower levels of percent state of charge (% SOC), falling off
at a greater rate below ~45% SOC. The cell's voltage tends to be less stable and more temperature
sensitive at lower levels of % SOC. For these reasons, it is generally recommended that the cells
should ideally be at 60% SOC or higher for achieving good self-discharge current measurements.
Charge Redistribution and Equilibrium
Right after a cell is charged or discharged, it takes a considerable amount of time for the electron
charge to uniformly distribute itself within the cell and be at equilibrium. During this time, the cell's
voltage is dropping or rising at an exponential rate, depending on whether it had been charged or
discharged. In comparison, when a cell's charge is fully distributed and at equilibrium, its voltage falls
off at a much lower and linear rate, due to the internal self-discharge current. Depending on the cell's
makeup, it can take from several days to two weeks for a cell to reach charge equilibrium.
When the cell is connected to the Keysight BT2152A/B, the system holds its output voltage constant
after the cell voltage matching process. If the cell is at charge equilibrium, the only influence on the
cell's voltage is due to self-discharge. As the BT2152A/B holds its voltage constant, the net result is
the BT2152A/B needs to only replenish the self-discharge current to maintain constant voltage.
However, when a cell has recently been charged or discharged and then connected to the
BT2152A/B, additional charging current will be drawn from the BT2191A/B after charging, or
discharging current in addition to the self-discharge current will be sourced back into the BT2152A/B
Keysight BT2152A/B Operating and Service Guide
3 Operating the Analyzer
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