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Capacity Measurements
Amp-hour capacity - the Agilent MCCD determines amp-hour cell capacity by making calculations
based on continuous current measurements.
During charge, every time the Agilent MCCD makes a measurement, it calculates the actual incremental
amp-hours put into the cell during each measurement interval by multiplying the measured current times
the measurement interval. It then adds this incremental amount to the accumulated amp-hour value to
determine the total amp-hours delivered into the cell. Amp-hour capacity will be positive during charge.
Thus, accurate amp-hour capacity measurements can be made even when charge current is not constant,
such as during constant voltage charging.
During discharge, every time the Agilent MCCD makes a measurement, it calculates the actual
incremental amp-hours taken out of the cell by multiplying the measured current times the measurement
interval. It then adds this incremental amount to the accumulated amp-hour value to determine the total
amp-hours removed from the cell. Amp-hour capacity will be negative during discharge. Thus, accurate
amp-hour capacity measurements can be made even when discharge current is not constant.
Watt-hour capacity - the Agilent MCCD determines watt-hour cell capacity by making calculations
based on continuous current and voltage measurements.
During charge, every time the Agilent MCCD makes a measurement, it calculates the actual incremental
watt-hours put into the cell during each measurement interval by multiplying the measured current times
the measured voltage times the measurement interval. It then adds this incremental amount to the
accumulated watt-hour value to determine the total watt-hours delivered into the cell. Watt-hour capacity
will be positive during charge. Thus, accurate watt-hour capacity measurements can be made even when
charge current and voltage is varying.
During discharge, every time the Agilent MCCD makes a measurement, it calculates the actual
incremental watt-hours taken from the cell during each measurement interval by multiplying the
measured current times the measured voltage times the measurement interval. It then adds this
incremental amount to the accumulated watt-hour value to determine the total watt-hours taken from the
cell. Watt-hour capacity will be negative during discharge. Thus, accurate watt-hour capacity
measurements can be made even when discharge current and voltage is varying.
Cell Resistance
In addition to continuous voltage, current, and capacity measurements, the Agilent MCCD can also
measure ac and dc cell resistance. This measurement is available on command when a sequence is not
running, or as its own step in the forming sequence.
The Agilent MCCD measures the ac cell resistance by first disconnecting the charge/discharge circuits
from all cells. An ac waveform generator in the Agilent MCCD mainframe is connected sequentially to
each cell. The ac waveform generator momentarily passes a small excitation current through each cell
while the measurement system measures the cell’s output voltage and current. By using a narrow band
tuned filter and computing the magnitude and phase angle of voltage relative to current, an ac resistance
measurement of the cell can be made. This method is very similar to the method used by LCR meters.
Since this measurement happens sequentially for each channel, the other channels stay at rest during this
test.