Lead acid batteries are typically used for deep cycle applications like a solar photovoltaic system intended for regular deep discharge of majority of its capacity. Deep cycle lead acid batteries can be divided into two distinct categories: flooded and sealed/valve regulated (SLA or VRLA). The two types are identical in their internal chemistry (shown in Figure 3). The most significant differences between the two types are the system level design considerations. Flooded lead acid batteries require three things that VRLA don’t:

Lead acid batteries are typically used for deep cycle applications like a solar photovoltaic system intended for regular deep discharge of majority of its capacity. Deep cycle lead acid batteries can be divided into two distinct categories: flooded and sealed/valve regulated (SLA or VRLA). The two types are identical in their internal chemistry (shown in Figure 3). The most significant differences between the two types are the system level design considerations. Flooded lead acid batteries require three things that VRLA don’t:

Due to these differences, the lower cost of flooded lead acid must be balanced against the added complexity and secondary costs. VRLA batteries are divided into two categories: Gel and Absorbed Glass Mat (AGM). The different names reflect different methods of containing the electrolyte. In Gel batteries, a thickening agent is added to turn the electrolyte from liquid to gel. In AGM cells, a glass matrix is used to contain the liquid electrolyte. “Deep cycle” and “shallow cycle” lead acid batteries can be found in both the VRLA and flooded classes. Shallow cycle VRLA batteries are commonly used for automotive start, light, ignition (“SLI”) batteries that must deliver high power pulses for short durations. The stationary power market uses deep cycle since the batteries will often discharge at a low rate over the course of multiple hours.

The concept of lithium-ion battery was initially conceived in the 1970’s and began to see widespread adoption by the 1990’s. The basic mechanism is that a charged lithium ion is shuttled back and forth between the cathode and the anode during charge and discharge. Over 90% of lithium -ion anodes are comprised of graphite; silicon and titanium based materials are occasionally used to get better life and power performance in exchange for significantly higher cost.

The electrolyte exists in liquid form, but for “lithium polymer” cells, the electrolyte is absorbed in a polymer membrane. This allows for cell manufacturers to use a pouch enclosure on the cell rather than the metal casing used when liquid electrolyte is present in cylindrical and prismatic shaped cells. Each of these variations influences the performance of a lithium-ion cell.

In spite of the various chemical variations, lithiumion batteries can generally be separated into two groups: lithium iron phosphate (LFP, LiFePO4) and metal oxides (NCM, NCA, Cobalt, Manganese). All lithium-ion cells are “deep cycle” meaning that they have the ability to be fully charged and discharged. The life of the battery will significantly increase if the depth of each discharge is limited to 80% of the rated capacity.

Lithium-ion has significantly higher cycle life than lead acid in deep discharge applications. The disparity is further increased as ambient temperatures increase. The cycle life of each chemistry can be increased by limiting the depth of discharge (DoD), discharge rate, and temperature, but lead acid is generally much more sensitive to each of these factors. It can be seen that the AGM pack must be limited to a 30% depth of discharge to get comparable life to a lithium-ion that is at 75% depth of discharge. This means that the AGM battery must be 2.5 times larger in capacity than the lithium-ion to get comparable life. In hot climates where the average temperature is 33°C, the disparity between lithium-ion and lead acid is further exacerbated. The cycle life for lead acid (flooded and VRLA) drops to 50% of its moderate climate rating while lithium-ion will remain stable until temperatures routinely exceed 48°C.

At the current price levels, Li-ion batteries might not be the best option for an Indian scenario considering that the batteries are not subjected to very high temperatures in most cases!