Your car battery is a rechargeable source of electrical energy. It supplies power to all the electrical components and is charged by the alternator while the car is running. Sometimes known as the SLI (starting, lights, ignition), the battery’s primary role is to provide the energy needed for those three essential functions; it also powers the luxury electricals such as the stereo, sat nav and air conditioning. Sometimes known as the SLI (starting, lights, ignition), the battery’s primary role is to provide the energy needed for those three essential functions; it also powers the luxury electricals such as the stereo, sat nav and air conditioning. Battery life varies but a rule of thumb is around three to five years. However, this depends on usage, maintenance and climate. Making lots of short journeys in your car will reduce battery life as it can only be fully recharged during long runs. Hot weather also weakens the battery.