A battery converts energy stored in the chemical bonds of a material into electrical energy via a set of oxidation/reduction (commonly abbreviated to redox) reactions. Redox reactions are chemical reactions in which an electron is either required or produced by the chemical reaction. For primary batteries, this is a one-way process – the chemical energy is converted to electrical energy, but the process is not reversible and electrical energy cannot be converted to chemical energy. This means that a primary battery cannot be recharged. Examples of primary batteries are alkaline consumer batteries used in flashlights, etc. In a secondary battery, the conversion process between electrical and chemical energy is reversible, – chemical energy is converted to electrical energy, and electrical energy can be converted to chemical energy, allowing the battery to be recharged. For photovoltaic systems, all batteries used must be rechargeable or secondary batteries. Common examples of secondary batteries are lead acid batteries and lithium-ion batteries used in higher power consumer electronic equipment such as computer laptops, camcorders, mobile phones, and some digital cameras.