Research On Plate Polarization Of VRLA Batteries And Its Failure Modes

Nowadays, lead-acid battery is still the most widely-used battery. Due to the difference of the grid structure, ambient temperature and discharge current, the current distribution on the plates is no uniform. It will have a great impact on the lead-acid battery performance. The utilization of active mass will be affect by the polarization in different parts of the battery. The area which has higher positive active mass (PAM) utilization will fail earlier than the other part after a certain number of cycles. To improve the battery design, the failure modes are necessary to be understood. In this paper, the effects of temperature and grid structure on polarization of the plates and the performance of the batteries are studied. A single cell is assembled to investigate the potential distribution between the positive and negative plates in the charging-discharging processes. Also the effect of lug position on the current distribution is studied. The changes of active mass of the batteries in different cycles are studied. The following conclusions have been drawn:1. The ohmic polarization of the plates of 22 Ah and 22 Ah cells discharged at different temperatures and different rates are measured. The result shows that the ohmic polarization of the plates will decrease with the decline of temperature. Grid structures have great influence on the polarization of plates. There are more grid bars in the vertical direction of the upper part of the grid which is used in the 22 Ah battery. It can obviously reduce the polarization and the current distribution on the plates is more uniform. The experimental data show that the current on the plate mainly flow through vertical direction bars rather than the horizontal ones, and the factor should be took into account when the grid is designed. Moreover, the ohmic polarization in the vertical direction of the plate is proportional to discharge current. The ohmic polarization of upper half of positive plate of 22 Ah cell is 20 mV、6.8 mV、3.45 mV when it is discharged at 3,1 and 0.5 C rates at 25℃, respectively.2. It is found that the potential distribution on left and right sides of the plate is affected by the position of grid lug. Current distribution on the plate will be more uniform when the lug is located in the middle of the plate, and its discharge capacity will also be greater than that in side. The discharge capacity of cells with middle lug and side lug is 4.45 Ah and 3.98 Ah when discharge at 2.5 A, respectively. The results show that there existed electrolyte IR drop between positive and negative plates of the cell in the charging-discharge processes after measuring the potential change between positive and negative plates, and the value is proportional to the current. The potential difference between positive and negative plates reaches 0.131 V,0.197 V and 0.553 V when the cell is discharged at 2.5 A,5 A,15 A rates, respectively.3. Failure analysis was carried out on the battery, the results show that the battery failure in this paper is caused by the PAM soften and the grid corrosion of the upper part of positive plates. The conclusions are as followed:(1) The PAM crystallinity will increase during cycling and lost its hydrated structure. Also the a-PbO2 is gradually transformed intoβ-PbO2, and the size of β-PbO2 crystallites grows from 31.8 nm to 54.8 nm during cycling. The crystallite size of the active material without cohesion even reached 73.7 nm. Beyond a size threshold, some β-PbO2 crystallites will lose contact with the others and fall off from the plates. (2) Corrosion status of different parts of the grid is not the same. The corrosion layer in the upper part of the grid is obviously more serious than that of the middle part. It is found that nearly half of its grid bar is corroded when the battery is failed. There are lots of pores and cracks in the corrosion layer. The status of middle part of the grid is relatively better. (3) It indicates that the concentration of sulfuric acid in the middle part of separator is lower than that in the upper and lower parts after discharged. Acid concentration in the separator will increase with the cycle, and a little stratification of the electrolyte appears.