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3 Operating the Analyzer
after discharging. The peak of this current can be several orders of magnitude greater than the self-
discharge current alone.
Expected Self-Discharge Current and Temperature Dependency
It is normal for cells to exhibit a small amount of self-discharge. It is difficult to definitively state what a
cell's expected self-discharge current should be, as many factors exist regarding a cell's design and
makeup that can affect this value. Notwithstanding, for cells at a room temperature of typically 23 °C,
testing has shown it can typically range from just a few micro-amperes for sub 1 Ah cells to 50 to 100
micro-amperes for 10 to 20 Ah cells.
All other things being equal, a cell's self-discharge current can approximately double for a 10 °C
increase in temperature. This applies to good cells not exhibiting excessive self-discharge leakage
current caused by internal defects or other problems.
Temperature Impact on Current Measurement
A cell's voltage temperature dependency can be expressed as a temperature coefficient of voltage
(TCV), and can be quantified in microvolts per degree C (μV/ °C). Since the BT2152A/B's method to
measure a cell's self-discharge current relies on matching its internal voltage source to the cell's
voltage and then connecting them together, it is easy to see how the self-discharge current
measurement is impacted by any change of the cell's voltage afterward. Several things can be done to
reduce the effect of temperature on self-discharge current measurements:
It turns out a cell's TCV is dependent on its % SOC. It can vary from as low as zero at certain points to
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as much as hundreds of μV/ °C, typically at very low % SOC levels. Generally having the cell charged
to 70% SOC or better helps reduce the cell's TCV. Still, you may have to contend with a TCV in the
range of 20 to 100 μV/ °C
An indoor test environment can experience a few degrees temperature cycle each day. The series res-
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istance between the BT2152A/B and the cell affects the measurement's sensitivity to the cell's TCV
Increasing this series resistance reduces the sensitivity to a cell's TCV but increases the time it
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takes for the measurement to settle to the final self-discharge current value.
The BT2152A/B has a programmable series resistance for this purpose.
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Temperature sensitivity is a significant factor for larger cells, as they usually require a longer meas-
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urement settling time due to their proportionally larger capacity.
Depending on how long the self-discharge measurement takes, it may be necessary to take further
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steps to hold the cell's temperature steady. Possibilities include:
Surrounding the cell with thermal insulation.
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Incorporating a substantial thermal mass (metal plate) that the cell is held against, to dampen tem-
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perature change. The thermal mass can be passive or actively temperature regulated.
Using a thermal chamber having a regulated temperature.
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46
Keysight BT2152A/B Operating and Service Guide
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