500

Figure 3 Weight change in the case of control method A
Figure 4 Weight change in the case of control method B

120 °C when the load was 100% , 110 °C when it was 75%, and 105 °C at 50%. Figure 5 shows the lifetime cycling characteristics at a 50% load both when the controlled temperature is maintained for each load (control method A), and when it is changed according to the load (control method B). Cycles 0 to 150 reflect data when control method A was used with a 100% load. During this time, the cell's voltage dropped at a rate of approximately O.lmV/cycle-cell. At a 50% load using control method A, it became fairly large at approximately 0.3mV/cycle-cell, and recovered to approximately O.lmV/cycle-cell at a 50% load using control method B. These results show that phosphoric acid concentration and volume changes must be controlled as far as possible in order to increase the lifetime of cells. In other words, the importance of controlling changes in the weight of the cell became clear.

1000 900 800 > 700 5 600 ■§ 500 § 400 j 300 200 100 0

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