R. Leek

The impedance of electrical storage cells

A brief review of the theory pertaining to the impedance of electrolytic cells is presented. Differences between the operational representations of cell impedances in the fields of electrochemistry and electrical engineering are noted. The experimental data on the impedance of complete batteries, available from the literature, is critically reviewed. The results are discussed in terms of the generation of a nondestructive,in situ, state-of-charge test.

The impedance of the Ledanch cell. I. The treatment of experimental data and the behaviour of a typical undischarged cell

The impedance spectrum of an undischarged commercial Leclanché cell (Ever Ready type SP11) is presented in the forms of the Sluyters plot and the modified Randies plot. The decomposition of the experimental cell impedances into the component parts has been achieved using a computer. The decomposition process and the component processes representing the overall cell behaviour are described.

The impedance of the alkaline zinc-mercuric oxide cell. I. Cell behaviour and interpretation of impedance spectra

The impedances of small (2400 mA h) alkaline Zn-HgO cells have been measured in the range 10 kHz-0.001 Hz at various states of charge from fully charged to fully discharged. The behaviour of the cell conforms to that expected for rate control by charge transfer at the zinc electrode and diffusion in solution. At low frequencies there is a relaxation in the diffusive circuit elements which ultimately results in a complete suppression of the capacitative component of the impedance at zero frequency. The low-frequency behaviour is analogous to convective diffusion and is due to the effective distance between the electrodes being small compared with the characteristic length (D/ω)1/2. The magnitude of the charge transfer resistance is the best measure of the state of charge.

The impedance of the Leclanché cell. III. The impedance of the cell at different stages of discharge and state-of-charge indication by the impedance method

The prediction of the residual capacity of some primary electrochemical storage cells has been investigated using the impedance technique over an extensive frequency range. The frequency responses of Leclanché cells in various states of charge are presented and it is shown that the gross changes in impedance which result from discharging provide an adequate parameter for state-of-charge prediction. The component factors of the whole-cell impedance have been investigated and the contributions of the various cell components to the resultant frequency-response (resistive and reactive) spectrum identified. A knowledge of these contributions is important for the development of a suitable device for the rapid testing of cell quality since in principle the examination of each mode of failure of each component demands a separate test. In practice the understanding of the frequency response of the cell has enabled us to construct a relatively simple instrument which gives an adequate estimate of cell condition for ordnance applications. A block schematic diagram is presented and the operation of the device is described in principle.

The impedance of the alkaline zinc-manganese dioxide cell. I. Variation with state of charge

The impedance spectra of undischarged commercial alkaline zinc-manganese dioxide cells (Mallory, type MN 1500) have been measured. Newly produced cells have different impedance characteristics from ones stored for periods of time (up to one year). The difference introduced by the ageing process is interpreted as the effect of Hg on the zinc particle electrode. The impedance behaviour of the cells is dominated by the zinc negative electrode which behaves as a relatively simple charge transfer and diffusion process. The porous nature of the particulate electrode has relatively little effect on the behaviour of the zinc which behaves as quasiplanar.On discharging, the frequency spectrum changes as does the open-circuit potential, the latter due to the progressive change in composition of the positive electrode. The former changes as a consequence of the negative electrode reaction.

The prediction of the state-of-charge of some commercial primary cells

Research carried out at Loughborough during the last five years on the impedance of Leclanché, alkaline Zn-MnO2, alkaline Zn-HgO, Li-CuO, Li-SO2, Li-SOCl2 primary cells is briefly reviewed. The use of alternating current methods for the estimation of the state of charge of the cells is discussed. A search was made for properties of the impedance of each cell system which change in a marked and reliable manner when a prescribed amount of charge is withdrawn from the cell. It is concluded that simple tests based upon an assessment of these properties are adequate for the estimation of the state of charge of some of the systems studied. However, other systems are not so ‘well-behaved’. The impedance of each cell changed significantly during discharge and, although making measurements of these changes would enable the state of charge to be estimated, the test techniques required would not necessarily be simple. Using the results obtained on the impedance of the cells, test sets have been constructed which provide a digital presentation of the open-circuit voltage and the state of charge of Leclanché cells (Ever Ready type SP11), mercury cells (Mallory, type RM502R) and lithium cells (Mallory, type L032S and SAFT, type LC01). The electronic techniques employed in these test sets are described in outline with reference in block-schematic diagrams. A proposal is made for the construction of a general-purpose tester for primary cells which would incorporate microprocessors and provide an indication of state of charge based upon data relating to impedance, temperature and discharge history. The tester might also be made self-calibrating for field use.

The impedance of 23 Ah Ni-Cd cells with sintered electrodes: measurements in the range 10 kHz-0.001 Hz as an indication of residual capacity

Impedance measurements on 23 Ah Ni-Cd cells at various residual capacity levels are described. These have been made using non-inductive connections coupled directly to a potentiostat and a frequency response analyser.The capacitive reactance at 0.39 Hz provided the best residual capacity indicator.

The partially discharged PbO2 electrode

Abstract The impedance of PbO2 electrodes in 5 M H2SO4 at potentials on the negative side of the reversible value was measured over a wide frequency range. The electrode interphase was matched to a model which includes the adsorption of electroactive species which undergo a reversible redox reaction. The exchange current for the electrode system was determined. The mode of passivation is by a spreading rather than a thickening process.

A state-of-charge test for the Li-CuO cell

The electrochemistry of the Li-CuO cell causes difficulties in the identification of suitable state-of-charge tests for commercial units. The available data has been analysed and a ‘best available’ test is noted.

The impedance of the Leclanché cell. II. The impedance of the individual cell components

The impedances of the various component parts of the Leclanché cell have been measured in experiments in which the complete cell is progressively built up. The results confirm previous conclusions that the zinc ‘can’ provides the impedance characteristic of the assembled cell. The contributions of the other parts of the cell are discussed.