Sketch of cell 2, composed of two plates and two AGM separators. All around the plates is sealed by epoxy resin. 

Contexts in source publication

Context 1

... partial pressure in the gas space increase so that the horizontal oxygen transport becomes faster. In the oxygen cycle, oxygen can directly pass through the AGM separator from the positive to negative plates and/or it can also go into the gas space and then reach the surface of the negative plate vertically. So according to the charge current and the rate of the horizontal transport in Fig. 10A in a steady oxygen cycle, we can obtain the ratio of the horizontal to vertical oxygen transport at different saturation values; they are shown in Fig. 10B. It is interesting to find that the horizontal oxygen transport is dominant when the saturation is less than 93%, while the vertical oxygen transport becomes dominant when it is higher than 93%. At the same time, the rate of the vertical oxygen transport becomes quicker with the increase of the charge current. Therefore, two ways of oxygen transport exist in the oxygen cycle in an AGM VRLA cell and their transport resistance depends on the level of saturation in the AGM separator. Since cell 2 in Fig. 2 is sealed all around, there is almost no gas space in the cell and then no vertical oxygen transport occurs. Oxygen evolving from the positive plate only transports horizontally through the AGM separator. Figure 11 shows the changes in the overpressure in cell 2. It is similar to Fig. 5. But the overpressure grows very rapidly. This is because little gas space exists in the cell and only a little hydrogen evolution can make the overpressure rise quickly. Furthermore, the initial composition in the void space, es- pecially hydrogen, can also influence the change of the overpressure. In the case of cell 2, it is very difficult for the gas in the micropores to exchange with the air outside. So the change in the overpressure in Fig. 11 is not completely proportional to the charge current. When the charge is interrupted at point d, the overpressure drops quickly and the oxygen partial pressure can be calculated as in Fig. 7. Figure 12 shows the changes in the oxygen partial pressure at 93% saturation and different currents. For the purpose of compari- son, the data of cell 1 in Fig. 7 at 91.5 and 94% saturation are also shown. It is interesting to find that the oxygen partial pressure in cell 2 almost rises linearly with the increase of the charge current as found already for cell 1. Even if the level of the saturation for cell 2 lies between 91.5 and 94% for cell 1, the oxygen partial pressure in cell 2 is higher than that of cell 1. Similar results can also be obtained at other saturation values of cell 2. This indicates that if the vertical transport way for oxygen is prevented, a higher oxygen partial pressure is needed to accelerate the oxygen transport in the horizontal direction in order to achieve the steady state ...

Context 2

... measured by an HP 34970A data acquisition/switch unit connected with a PC computer at 10, 20, 50, and 100 mA overcharge currents, respectively. It is assumed that oxygen on the small pure lead electrode from the positive plate can be completely reduced at Ϫ 1 V, controlled by potentiostat ͑ HDV-7 ͒ . Therefore, the rate of oxygen transport through the AGM separator in the horizontal direction was obtained by measuring the reduction current. All experiments were conducted at 25 Ϯ 2°C. Figure 2 shows a sketch of cell 2, which comprises one positive plate, one negative plate, and a two-layer AGM separator between the two plates. The cell was fixed between two Plexiglas plates. And all sides of the cell were sealed by epoxy resin, except for two vents, one of which was linked to the pressure sensor while the other was used for acid filling. In this case, there is almost no gas space in the cell. The installation shown in Fig. 3 was used to measure the rate of the horizontal oxygen transport through the AGM separator. De- pending on the experiment series, a one- or two-layer AGM separator was fixed between two Plexiglas plates. The AGM separator was sealed around with epoxy resin. The oxygen transport area was a circle with 1 cm diam. First, the separator was flooded with sulfuric acid. Then the liquid level was lowered and set at different distances under the circular part of the AGM separator. Since the liquid can be absorbed in the micropores of the AGM separator, the liquid level might control the saturation of the circular AGM separator. The rate of oxygen transport at 8.4 kPa between the opposite sides of the AGM separator was measured by a self-made foam flowmeter, which can determine the volume of the oxygen flow per second ͑ mL s Ϫ 1 ͒ . The installation in Fig. 4 was similar to that in a cell, but a positive plate replaced the negative plate. The back side and edges of the positive plates were sealed by epoxy resin so that oxygen only passed through the AGM separator vertically. After the vertical oxygen transport in the separator without the electrolyte was measured, the plates and the separator were flooded with H 2 SO 4 solution ͑ sp gr 1.30 g cm Ϫ 3 ͒ for 40 h, to reach 100 % saturation. Then the expected saturation was obtained by charging both positive plates for a short time. During the period, one plate was discharged while the other plate was charged and the evolving oxygen removed a small amount of electrolyte from the separator. After this, the voltage between the two plates was less than 0.2 V so that there was no lead on the plate that could react with the oxygen. The rate of oxygen transport was measured by a foam flowmeter at different pressures. At the end of charging in a VRLA battery, the main reactions at the positive plate are the oxidation of PbSO 4 to PbO 2 , oxygen evolution, and the grid corrosion, and those at the negative plate are the reduction of PbSO 4 to Pb, hydrogen evolution, and oxygen reduction. During overcharging, the reactions on the active mass become very slow. The corrosion rate of the positive grid is less than 2% of the charging current. 11 And under the balanced conditions, the ...