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3 Operating the Analyzer
Median calculation assumes that majority of the cells in a measurement are good quality cells. If the
bad cells approach 50% of the total, the median may start to fluctuate. Note that for median stability,
it is important that a minimum number of at least eight channels are available for recording.
Optimizing the Initial Current and Resistance Settings
Finding the optimal values for initial current and output resistance can save measurement time
without using excessively low output resistance (which is more susceptible to noise). This section
provides a characterization procedure to help you choose the best values.
Use the following procedure whenever you change to a new cell type that may have different
characteristics (chemistry, construction, capacity, or package form-factor), or if something changes in
the manufacturing process that changes the nominal self-discharge current. You don't necessarily
have to repeat all steps for small changes in the above characteristics.
To minimize external temperature effects, perform this procedure using a stable thermal environment.
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It is critical to use and record the temperature as measured by the temperature sensor.
For the best representative results, always run the procedure with a statistically significant and rep-
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resentative sample from the population of cells to be tested.
The characterization procedure is an iterative process. For the most accurate results, you need to run
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all three tests - a 24-hour test, a 12-hour test, and a final 6-hour test. Additional time may be required
for the cells to settle between tests. The additional time produces more accurate starting values.
Taking the time to complete this iterative process reduces the time you will need to spend on many
successive tests, which can now be done using accurate (optimized) initial current and resistance
values. This iterative process also minimizes the effects of all errors, including fluctuations caused by
temperature variation.
24-hour test
Step 1. Set the initial current value for all cells to 0 A.
Step 2. Set the output resistance for all cells to a relatively high value. Low values are faster, but more
susceptible to temperature variations and noise.
For the most accurate results, use an output resistance of 5 Ω or even higher. Using too low of a res-
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istance makes your cells more sensitive to temperature variations and lead to wider variations in meas-
urement. Using a higher resistance especially with large-capacity cells may require days for the test
results to settle. The trade-off for a longer settling is that your results are less susceptible to noise or
temperature variations. You will get very stable baseline measurements to determine the best Initial
current value.
If you lack the time for a very long settling period, choose an output resistance appropriate for the cell
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size. You might start off with 5 Ω for a 2 Ah cell for example, or 0.5 Ω for a 20 Ah cell. The trade-off will
be a less accurate initial current value. Keep the start current at 0 A.
Step 3. Run the test for at least 24 hours or until the cells have obviously settled. The response plot
should resemble an asymptotic curve and the cells should settle at about the same rate. Allow
enough time to get a good average current.
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Keysight BT2152A/B Operating and Service Guide
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