The behavior of oxygen transport in valve-regulated lead-acid batteries with absorptive glass mat separator

In the oxygen cycle of valve-regulated lead-acid (VRLA) batteries, there ar e two ways in which oxygen can move from the positive to the negative plate s, namely, either horizontally to penetrate the absorptive glass mat (AGM) separator, and/or transport vertically via the gas space. It is found that the oxygen transport depends on the passageway with big void space in the A GM separator and its rate is proportional to the oxygen partial pressure. T he rate constant of vertical transport is about three orders higher than th at of horizontal transport because of the large void space between the AGM separator and plates. However, in the horizontal direction, the area is ver y large and the transport path is very short. So the way and the rate of ox ygen transport actually depend on the level of saturation in VRLA batteries . The horizontal transport is dominant when the saturation is less than 93% , while the vertical transport becomes dominant when it is higher than 93%. The experiments also indicate that with decreasing saturation, the recombi nation of more oxygen at the negative plate may oxidize more active H-ad at oms and therefore, the overpotential of hydrogen evolution increases obviou sly. (C) 2001 The Electrochemical Society.