*3.4. Plates and Grids*

The failure mode *Plates and Grids* summarises ageing mechanisms that contribute to the increase of the internal resistance. Besides the main ageing mechanism corrosion in this category, this is to some extent also sulphation. Corrosion is favoured by a low acid concentration in the electrolyte, i.e., at low states of charge. However, high charging voltage and especially operation at high temperatures have a major influence. The general cycling of the battery also leads to corrosion over time. Corrosion mainly affects the positive electrode. In this case, corrosion leads to a reduction of the grid cross-section and thus to less active area for electron transport between the active mass of the electrode and the grid itself, and thus the grid resistance increases. With the decreasing grid cross-section, the mechanical stability also decreases. If the battery is very corroded, even parts of the active mass can lose contact with the grid and thereby no longer be reached and contribute to the reaction. In extreme cases, this can also reduce the available capacity. Primarily, however, corrosion leads to an increase in internal resistance. In addition to corrosion, sulphation can contribute to an increase in internal resistance. Sulphation occurs when the battery is frequently operated in states of partial charge and when the battery is rarely fully charged. This operation sometimes also leads to the occurrence of acid stratification. In addition to the operating conditions, high temperatures favour the formation of sulphation on the one hand but also lead to good dissolution of fine sulphate crystals if the charging voltage is sufficiently high. Low temperatures, on the other hand, have negative effects on the charging ability and can lead to the already mentioned rare full charges and frequent partial states of charge. In particular, the low states of charge lead to a higher solubility of Pb2<sup>+</sup> ions due to a lower acid concentration in the electrolyte, which form a sulphate layer by recrystallisation to PbSO4. The sulphate layer develops on both electrodes during discharge; as the pores of the negative electrode are larger, the sulphate crystals can become much larger than on the positive electrode. The larger the sulphate crystals are, the more difficult it is to dissolve them again by extensive full charging; during normal charging processes in the vehicle, reversible dissolution of the fine sulphate crystals hardly takes place. Due to the coarse sulphate crystal structure on the active electrode surface, this is reduced and loses conductivity. The lower conductivity increases the internal resistance. The binding of active material in the sulphate crystals reduces the available active mass and leads to a lower acid density of the electrolyte. The lower acid density means that very high maximum open-circuit voltages can no longer be achieved, thus limiting the capacity from above. This influence is assigned to the next category *Worn out and Abused* as well. Overall, the failure mode *Plates and Grids* with its ageing mechanisms corrosion and sulphation is to be assigned to the gradual faults, as they can only arise through extensive use over time.

#### *3.5. Worn out and Abused*

This failure mode covers the ageing mechanisms that lead to loss of capacity. The capacity of the battery determines to a large extent how much electrical energy it can store. If it loses capacity, it may no longer be able to supply enough energy for an application. This process thus has a decisive influence on the lifetime of a battery. The loss of capacity is primarily influenced by loss of active mass and due to cycling and sulphation. The sulphation has already been shortly explained in the previous Section 3.4, but decrease is mainly due to the processes described in the following. During the normal use of the battery, it is discharged and charged, that means cycled. This leads gradually to the wearing out of the active mass, mainly at the positive electrode, which is decisive for the capacity. This cycling stress is intensified by deep discharging or discharging with high currents, because the mechanical stress for the active material is correspondingly higher. When charging or discharging, parts of the electrode material are transformed and the volume changes. If large rapid volume changes take place, parts of the active material can become internally decontacted and thus can no longer contribute in the reaction. The porosity and conductivity decreases, which mainly reduces the available capacity but also slightly increases the internal resistance. In summary, cycling in particular contributes significantly to the loss of capacity, as does sulphation to some extent. These processes only become apparent when the battery is used or remains in a low state of charge and therefore *Worn out and Abused* is allocated to gradual faults.
