What makes Deep Cycle batteries behave differently?
Deep cycle batteries have a thicker plate design that consists of a higher density active material that will withstand the stress of repetitive cycling and vibration. The grid alloy is specifically formulated to increase the active material's adhesion to the grid. These features, as well as others, provide additional protection from the repeated stress created by deep cycling.
Automotive batteries have a completely different plate design and grid pattern. The thinner plates and standard paste design provide more initial cranking amps, but could never hold up to the rigors that a deep cycle battery is put through. If an automotive battery were cycled in a deep discharge application, it would fail considerably sooner than would a deep cycle battery.