Lead Acid, Deep Cycle batteries are used for off grid or grid tie renewable energy systems to store the generated power so that you'll always have a continuous 24 hour supply of electricity. When these batteries are used for grid tie systems (grid intertie or grid interactive), they'll only be called upon during a grid failure as an emergency power backup and only for a desired amount of run time.
The difference between a Deep Cycle battery and a car battery is that the plates within a Deep Cycle battery are thicker and contain specific lead oxides and additives within the active material (paste) on the plate grid. This design enables it to withstand the constant charging and discharging (cycling). A car battery contains more plates which are thinner and have different additives such as calcium to prevent the electrolyte from evaporating. Car batteries are designed to only provide quick and high amperage starts called cranking amps. Car batteries are primarily designed to stay fully charged at all times.
Using a car battery for deep cycling will eventually cause the thin plates inside to heat up, bend, warp and buckle. Which in turn will cause the active material (paste) that accepts the charge to fall off the plate grids. The electrolyte (diluted sulphuric acid) will also excessively heat up and transform into highly explosive hydrogen gas. An internal short can cause an explosion as a result of this type of unseen plate damage. NEVER use a car battery for any type of renewable energy system or for constant cycling. Sulphation occurs when a battery is not properly charged or left in a discharged state for a long period of time. Sulphation can occur in sealed batteries as well.
The common Deep Cycle battery types used in renewable energy systems are either Wet, flooded (conventional) or Sealed Lead Acid AGM (Absorbed Glass Mat) & Gel. The AGM & Gel batteries cost more but have better design features for certain installations. Deep Cycle Batteries are rated by the amount of current they can supply over a period of hours i.e. in amp hours (A/h). E.g. a 300 A/h battery will be able to supply 15 amps for 20 hours or 30 amps for 10 hours.