The continual charge action after fully charged with certain charge approach is overcharge. Under this situation,the battery internal pressure may rise.

For battery while overcharged the following reactions are generated:
Positive:4OH- - 4e → 2H2O + O2↑；   
Procreant oxygen would composite with negative through separator.

The procreant oxygen can’t be wasted on time will cause battery internal pressure rising, if the negative’s reaction speed lower then the positive.Because the positive capacity is higher than the negative on design, the generated oxygen in positive can composite, through separator, with Cd generated in negative. In general condition the internal pressure will not rise obviously, but if the charge current is so heavy or the charge time is so long, and the generated oxygen can not be used up in time, there will be some quality defect risen such as internal pressure rising, battery distortion, leakage and so on. Meanwhile its performance will decline.