*3.6. Short Circuit*

The failure mode *Short Circuit* implies an additional electrical path between positive and negative electrodes within a cell of the battery [25]. This additional connection has a resistance that varies depending on the characteristics of the short circuit and can discharge the cell at different rates. In addition to a changed open-circuit voltage, a short circuit can be recognised by an increased overall resistance of the battery. In principle, there are four possibilities for internal short circuits in lead batteries. Due to strong corrosion, the grid can change considerably and grow together above the active electrode surface in the area of the busbar. In addition, due to severe corrosion and accompanying change in volume of the electrodes, the separator can be damaged, causing a connection between the positive and negative electrodes. The mechanism of corrosion has already been explained in the previous Section 3.4. On the other hand, deep discharge of the battery and operation out of the specification can cause dendrites to grow, which can create a connection between the electrodes. Similarly to sulphation, at low states of charge and accompanying low acid density, the solubility of the lead increases and fine nonconductive dendrites of lead sulphate can form. Frequent use of the battery outside the specification at very low states of charge can cause the dendrites to grow and form fine filaments which can grow through the separator. At higher states of charge, the nonconductive lead sulphate is reduced to electrically conductive metallic lead, which can create an electrical connection. The fine short-circuit thus develops slowly during use, as do the other ageing mechanisms, and it can be avoided by appropriate operational management and monitoring. Only the hard short-circuit, by penetration with, e.g., a nail, could produce a sudden hard shortcircuit, which, however, can only be achieved by external action and is not possible by normal operation.
