Lead-Acid Battery

Image: ups battery center

Lead acid batteries have a longer lifespan and lower cost than other battery types despite having poor energy density, middling efficiency, and a high maintenance requirement. Because lead acid batteries are the most often used kind of battery for most rechargeable battery applications, including starting vehicle engines, they offer the distinct advantage of having a well-established, mature technological foundation.

Operation

A lead acid battery's negative electrode is made of porous or spongy lead. Lead may grow and dissolve more readily because it is permeable. The positive electrode is composed of lead oxide. Both electrodes are immersed in an electrolytic solution of sulfuric acid and water. If the two electrodes come into contact as a result of the physical movement of the battery or variations in electrode thickness, a membrane that is chemically permeable but electrically insulating keeps them apart. This membrane also protects the electrolyte from electrical shorts.

When a battery is discharged, lead sulphate crystals form at both the positive and negative terminals, and electrons are released as a result of the change in the valence charge of the lead. Sulphate from the electrolyte of sulfuric acid that surrounds the battery is used to create this lead sulphate. The electrolyte concentration falls as a result. Instead of sulfuric acid encircling the electrodes, a complete discharge would cover both electrodes in lead sulphate and water. At complete discharge, there is no chemical potential or voltage between the two electrodes since they are made of the same material. In reality, the discharge stops considerably earlier than the cut-off voltage. The battery should not be discharged below this voltage as a result.

The voltage of a lead acid battery will progressively drop between the fully charged and fully discharged states. A battery's charge level is frequently represented by its voltage level. Massive lead sulphate crystals may grow in a battery that has been kept in a low state of charge for an extended length of time, permanently lowering its capacity. These crystals are challenging to turn back into lead since they are different from the typical porous lead electrode structure. The battery's reliance on its charge level is seen in the image below.

Depth of Discharge

As the energy that can be taken from the battery is determined by multiplying the battery capacity by the depth of discharge, the depth of discharge along with the battery capacity is a critical parameter in the design of a battery bank for a PV system. Batteries are classified as either shallow-cycle batteries or deep-cycle batteries. A deep-cycle battery will drain to a depth of more than 50% and maybe even 80%. To reach the same useful capacity, a shallow-cycle battery bank has to be larger than a deep-cycle battery bank.

Lifetime

Due to the battery's sulfation and active material loss, batteries lose capacity with time. The battery function development for a shallow-cycle lead acid battery is depicted in the following graph as a function of cycle count and depth of discharge. Even with a DOD of over 50%, a deep-cycle lead acid battery is expected to be able to sustain a cycle life of over 1,000.

Battery Efficiency and Maintenance

Because the formation and release of hydrogen and oxygen gas from a battery causes water loss, lead acid batteries must be filled up with water on a regular basis. Water loss might be a serious issue because other battery system components do not require as much maintenance. Checking water loss might be costly if the system is located in a remote place. Maintenance-free batteries reduce the need for routine maintenance by avoiding or lowering the quantity of gas that exits the battery. Lead acid batteries have an energy efficiency of 70%.