A deep cycle battery is often misunderstood. An automotive starting battery is manufactured specifically to provide a quick burst of energy thousands of times in its lifetime, while only being able to deeply discharge less than 50 cycles during its life. A cycle refers to one battery discharge and recharge of any depth. A deep cycle battery has more dense plates and fewer pores than a starting battery. It is this nature that precludes a deep cycle from being used as a starting battery.
Several qualities sperate deep cycles from starting type batteries.
Thicker plates, Denser active chemical plate material, and a specially formulated grid alloy. These batteries are not intended for starting, and should not be used as such. Tying deep cycle batteries together in parallel to assist in starting an engine is not recommended. Use a starting battery for your engine, and a deep cycle to run your other small loads. The deep cycles will have a much longer life.