Deep cycle means using the battery in an application that will discharge 60 to 70 percent or more of the battery capacity. An auto battery is an SLI (starting, lighting, ignition) battery. Its plates are designed to deliver maximum power for a short duration. Starting a car typically discharges an SLI battery 1 to 3 percent. When an SLI battery is used in a deep cycle application, or in a vehicle with heavy accessory loads, the battery life will be shortened.
Anytime you need a battery to supply all the operating power for a vehicle. Deep cycle batteries should be used in vehicles with heavy accessory loads where the alternator cannot maintain the battery in a fully charged condition. Some examples include:
An OPTIMA deep cycle battery uses a different chemical for the active paste material on the plates, and a slightly stronger acid. This chemistry change allows for a much longer life in cycling applications with only a slight reduction in starting power.
Most gel batteries have a much higher internal resistance, meaning they will not be able to deliver and receive a current as efficiently. This is especially apparent at higher amperage levels. For example, most gel batteries will not work successfully in engine starting applications.