In addition to the central reduction and oxidation reaction which comprise a battery, secondary or side reactions may occur. In most cases, these side reactions give rise to unwanted or detrimental effects. In all cases, these secondary reactions reduce the coulombic efficiency of the battery. If the secondary reaction occurs during discharging, some of the charge (current that would normally flow to the load is used by the secondary reaction). Similarly, during charging, the secondary reactions use charge intended to drive the main battery reactions, thus reducing the couloumbic efficiency. The electrolysis of water described in the activation overpotential is an example of an unwanted secondary reaction. Secondary reactions give rise to several unwanted effects, such a gassing, self-discharge and corrosion of the electrodes.