Deep cycle batteries are not typically used as the primary starting batteries in cars. Instead, they are designed for applications where the battery needs to provide a steady, deep discharge over a longer period, such as in RVs, boats, golf carts, solar power systems, and electric forklifts. Deep cycle batteries have thicker plates that are better suited for these deep cycling applications.

Starting batteries, on the other hand, are designed to deliver high bursts of energy to crank the engine and then quickly recharge. These batteries have thinner plates and are optimized for providing the high current needed to start the vehicle's engine.

Using a deep cycle battery as the primary starting battery in a car is not recommended because it may not provide the necessary cranking power and could lead to difficulties in starting the engine. However, some vehicles, especially hybrids and electric cars, use a combination of a starting battery and an auxiliary deep cycle battery for accessories and electric power. In these cases, the deep cycle battery is used for different purposes and not as the primary starting source.

If you're experiencing issues with your car's battery, it's best to consult with a professional mechanic and choose the appropriate battery type for your vehicle's specific needs, which is typically a dedicated starting battery.