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What are starting-lighting-ignition batteries used for?

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Cross-section diagram of an alkaline-manganese dioxide battery showing labeled internal components, including the steel-plated covers, cathode, anode gel, current collector, separator, steel can, and brass rivet. — battery Cutaway view of an alkaline–manganese dioxide power cell.

Starting-lighting-ignition (SLI) batteries are a type of lead-acid battery primarily used in automobiles to provide the necessary electrical power for three main functions:12

Starting: The battery delivers the high current required by the starting motor to crank and start the engine.12
Lighting: It provides power for the vehicle's lighting systems, including headlights, signal lamps, and interior lights, particularly when the engine is not running or the generator is not producing sufficient output.2
Ignition: The battery serves as a component of the ignition system, providing the low-voltage current that is converted into the high-voltage sparks needed to ignite the fuel-air mixture in the engine cylinders.32

As a reservoir for the vehicle's electrical system, the SLI battery also supplies energy to various other accessories and controls when the engine is off or when electrical demand exceeds the output of the vehicle's generator or alternator. These batteries are designed to provide high current for hundreds of shallow-depth discharges over several years.21