Agilent Technologies E4371A, E4370A, E4374A manual Capacity Measurements, Cell Resistance

1 - General Information

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.

16