Car batteries and deep cycle batteries are both lead-acid batteries and they operate with exactly the same chemical processes.
The function of a car battery is to provide a large amount of current in a short period of time. This inrush current is needed to start the engine when the car is started. Once the engine has started, the alternator will provide all the power the car needs, so the car battery will not consume more than 20% of its total capacity over its lifetime. In this way, the car battery can last for several years. In order to obtain a large amount of current, car batteries use thin plates to expand the surface area.
The purpose of the deep cycle battery is to provide a more constant current over a longer period of time. If required, deep cycle batteries are also capable of providing inrush current, but not as powerful as that provided by car batteries. The deep cycle battery also has to be repeatedly deep discharged (which would quickly lead to the end of a car battery). For this reason, deep cycle batteries use thicker plates. Car batteries usually have two ratings.
CCA (cold start amperage) – the number of amps the battery can produce in 30 seconds at zero degrees Celsius
RC (reserve capacity) – the number of minutes it takes for the battery to deliver 25 amps of current while maintaining a voltage above 10.5 volts
Typically, a deep cycle battery has twice or three times the reserve capacity of a car battery but only delivers half or three quarters of the cold start amperage. In addition, deep cycle batteries can withstand hundreds of full discharge/recharge cycles, whereas car batteries cannot be fully discharged.