Starting, Lighting, Ignition (SLI) – These batteries deliver a large burst of power for a short time as needed for normal engine starting. The battery is then recharged by the alternator. Unlike a deep-cycle battery, starting batteries are not designed to withstand multiple discharge/recharge cycles, and draining it can significantly shorten its life.
Deep-Cycle – These batteries are designed to provide a steady amount of current over a long period of time. Deep-cycle batteries can be repeatedly discharged and recharged without causing damage or shortening their life. They are well-suited to power numerous electronics and plug-in accessories, or other applications that place high demands on them.
Some deep-cycle batteries can be used for engine starting as well (these are sometimes referred to as dual-purpose), but be sure to check the CCA rating to ensure the battery has sufficient starting power.