Overcharge, gassing

Although water decomposition and elemental-lead corrosion reactions are strongly inhibited by the low hydrogen exchange current on lead, overcharge without proper controls may result in severe damage due to the excessive gassing (see below). 

Severe gassing during overcharge considerably accelerates shedding. 

FM3. Acid stratification. Acid concentration gradients can be removed through agitation of the acid by setting the battery to deliberate gassing during extended overcharge. 

It is possible to overcharge batteries and this is wasteful on electricity, causes gassing and can cause internal damage if the current is too high. 

Figure 3.16 shows the changes in acid concentration in the top and bottom electroljrte layers on cell cycling, when there is no overcharge with gassing [54]. The data in the figure evidence that, after six cycles, the acid concentration in the bottom layer has reached 1.35 g cm, while in the upper layer it is only 1.22 g cm... 

Acid stratification in flooded batteries used in cycling application, when there is no overcharging with gassing and no electrolyte circulation [54]. 

Cold cranking amps Deep cycling Electrolyte Gassing Hydrogen Hydrometer Lead peroxide Lead sulfate Maintenance free Overcharging... 

When the cell is charged galvanostatically, the potentials of the two electrodes increase. When the electrode potentials reach the potentials of H2 and O2 evolution, gassing starts. During overcharge, the basic electrochemical reaction that occurs is that of water decomposition and evolution of H2 and O2. When 1 Ah of electricity passes through the electrode, 0.3661 g of H2O is decomposed to 0.0367 g of H2 and 0.2985 g of O2. 

Therefore, the voltage evolution during the charge process is represented by Equation 9.38, up to the start of gassing Ug J and by Equation 9.42 for overcharging > Uga) until the end of charge voltage is reached. 

Thele, M., Karden, E., Surewaard, E., and Sauer, D. U. (2006). Impedance-based overcharging and gassing model for VRLA/AGM batteries. 

---