What is the difference between a 100 m sprinter and a marathon runner? That is the difference between a car starting battery and a deep cycle battery. The sprinter is capable of putting out intense energy for a short while but if he or she continues at the same rate for long some muscular or other damage may result. The marathon runner, on the other hand cannot sprint. He can expend energy at a slow rate but for a very long time till he is almost exhausted.
The deep cycle battery is the marathon runner of batteries while the vehicle starting battery is the sprinter. It can be drained down till virtually exhausted. The deep cycle battery should be able to discharge to 80% level without being damaged. However, different manufacturers specify discharge limits for their batteries differently which could be close to 45% to 50%. Batteries used in renewable energy storage systems should be deep cycle batteries. Find more here
Deep cycle batteries can be of different types. These can be the conventional flooded lead acid (FLA) type, or sealed or valve regulated lead acid (VRLA), more here. The latter are further subdivided into gel batteries and AGM (Absorbed Glass Mat) batteries, read more here. Llithium-ion batteries are also entering into this class. Obviously, deep cycle batteries made using each of these technologies are made differently.
The flooded lead acid deep cycle battery is quite similar to the ordinary lead acid flooded battery used for starting vehicles, but the plates are optimized for deep discharge, albeit at slower rates of charge and discharge. The plates are thicker which gives the plates the strength to withstand the deep discharge without sulfation. Among the conventional deep cycle batteries, the flooded battery is the most common type, and looks quite like it. The gel batteries do not have the electrolyte in liquid form. Rather fine silica and the electrolyte make a gel which fills the space between the positive and negative plates. That is the reason for the name gel. The AGM batteries are also valve-regulated type, but the electrolyte is now held absorbed in a glass mat which also acts as the separator.
Deep cycle battery ratings
Conventionally, batteries have been rated by specifying nominal volts, and ampere -hours. Ampere-hour rating expresses the capacity C of a battery to deliver. It is the product of amperes with the time in hours these amperes flow. A 100 AH battery should be able to deliver a constant current of 10 A for 10 hours or 20 A for 5 hours. Since the battery efficiency cannot be 100 %, charging at 10 A constant will take more than 10 hours. Faster discharges will give lesser products of ampere and hours. That means a fast charge or discharge will reduce the battery capacity. A charging rate of C/4 is considered a safe rate. For a 100 AH battery it means a charging current of 25 A.
Discharge Cycle rating
Every charge/discharge cycle reduces the battery performance. Therefore, every battery has a limited life. Battery life is often expressed as the number of charge discharge cycles it will allow during its useful life. This parameter is especially applicable to deep cycle batteries because they mean a lot of precious investment. An applicable benchmark is the IEC 896-2 which is based on a 100% discharge. 100% discharge of a battery is not recommended, and the IEC recommendation provides a baseline for testing and comparison. Of all the deep cycle batteries, those with AGM technology have the highest cycle rating.