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performance; developing approximately 45% round-trip coulombic efficiency. Two successive cycles gave very similar results — substandard cells pulling the sub- module down to a round-trip usable coulombic efficiency of 59% average and a usable energy efficiency of 45.3% average. At this point a special test was conducted to determine whether the coulombic losses in cell 9 were occurring during charge or discharge. The submoduel was cycled to the normal full-charge condition. The bat tery was then flushed and unit cells 8, 9, and 10 disassembled. The zinc plating was carefully peeled from the 23 electrode substrates of combs from unit cell 8, a good performer, and unit cell 9, the lowest performer. The zinc plates were then each weighed, tabulated, and the totals and distributions analyzed. The results of this work are shown in Table 24-4. The coulombic efficiency on charge for cell 8 calculates to 85% while it is only 55% for cell 9. This is strong evidence that the excessive coulombic losses during cycling of the submodule are occurring during the charge portion of the cycle. The 5% deviation in weight among all zinc plates in an individual unit cell shows very satisfactory uniformity throughout the cell. The causes for a 30% deviation in zinc weight from one unit cell to another are not fully mderstood. Such a difference could be caused by dendritic short circuits, by intercell parasitic leakage currents, or by some bus or chlorine electrode related phenomenon. Visual inspection of the stack structure before and during disassembly revealed no den drites that could have shorted-out any cells. A later test during cycle #13 eli minated further consideration of parasitic leakage losses. The comb assembly from cell 9 was interchanged with the comb from the zinc terminal end, cell 10. During the next cycle test, it was found that the same previous loss of coulombic efficiency was now transferred to cell 10. This would seem to be strong evidence that these losses are related to the comb structure of the electrode array, but at present the mechanism of such a loss is not understood. During rehabilitation of submodule #3, the presumably faulty comb assembly from cell 9 was saved for future evaluation. DISCUSSION The special submodule test stand constructed for this program has proven to be an effective tool for meeting the first objective — evaluating and comparing the 8.3kWh building blocks of the 45/50kWh EPRI load-leveling battery stack. It incorporates capabilities and features which allow thorough diagnostics of individual unit cells and individual submodule assemblies. It affords a means to study the effects of 24-21PDF Image | Development of the Zinc-Chlorine Battery for Utility
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