Lead-acid Battery

Lead-acid Battery

Lead-acid batteries are secondary (rechargeable) batteries that consist of a housing, two lead plates or groups of plates, one of them serving as a positive electrode and the other as a negative electrode, and a filling of 37% sulfuric acid (H₂SO₄) as electrolyte. The battery contains liquid electrolyte in an unsealed container, requiring it to be kept upright and the area well ventilated to ensure safe dispersal of the hydrogen gas it produces during overcharging. Lead acid batteries typically have coulombic efficiencies of 85% and energy efficiencies in the order of 70%.

Lead and lead dioxide, the active materials on the battery’s plates, react with sulfuric acid in the electrolyte to form lead sulfate. The lead sulfate first forms in a finely divided, amorphous state and easily reverts to lead, lead dioxide, and sulfuric acid when the battery recharges.

The lead–acid battery is relatively heavy for the amount of electrical energy it can supply. Its low manufacturing cost and its high surge current levels make it common where its capacity (over approximately 10 Ah) is more important than weight and handling issues. The disadvantage of this battery chemistry is that it is very sensitive to deep cycling compared to other battery systems, and due to the high density of lead, the specific energy of the batteries is quite low.

Most of the world’s lead–acid batteries are automobile starting, lighting, and ignition (SLI) batteries, with an estimated 320 million units shipped in 1999. In 1992 about 3 million tons of lead were used in the manufacture of batteries. Industrial fields of applications for lead acid batteries are as traction power for mining vehicles, forklifts and as stationary power sources such as emergency back up power storage (UPS) and signaling stations for railroads and telecommunication.

Composition of Lead-acid Battery

A lead-acid battery consists of a negative electrode made of spongy or porous lead. The lead is porous to facilitate the formation and dissolution of lead. The positive electrode consists of lead oxide. Both electrodes are immersed in an electrolytic solution of sulfuric acid and water. In case the electrodes come into contact with each other through the physical movement of the battery or changes in the thickness of the electrodes, an electrically insulating but chemically permeable membrane separates the two electrodes. This membrane also prevents electrical shorting through the electrolyte. The nominal voltage of a cell is about 2V, but the voltage varies between approximately 1.75 V and 2.4V depending on the SoC and the charging or discharging current. Lead acid batteries can deliver high currents for short periods and have a high power density. 

Lead-acid batteries can achieve quite a long lifetime of several years. However, voltage regulation is crucial here. Especially in cars as starter battery, this is often insufficiently accurate, so that only 2 to 4 years of use are typical. Drive batteries or storage batteries can achieve a lifetime of between 5 and 15 years, depending on quality and stress. But these batteries differ significantly from car batteries. When lead acid batteries are of the same capacity and size, but of different weights, the heavier battery usually lasts longer because the lead frames are stronger. The power rating when new is not directly affected by this, as a weaker lead structure can also be designed with a large active surface area. The aging of lead acid batteries is mainly caused by internal corrosion of the lead structure of the electrodes, the formation of fine short circuits, and by sulfating of the lead.

Types of Lead-acid Batteries

We can find 2 main groups of lead-acid batteries:

• VLA battery (vented lead-acid battery) is a flooded or ventilated electrolyte lead-acid battery, where the electrodes are submerged in excess of liquid electrolyte. In the vented lead-acid batteries (VLA), there are 3 groups:
  • Traction or deep cycle: These types of batteries are designed to produce a constant and small discharge for long periods of time. They are much less susceptible to degradation due to cycling and are required for applications where the batteries are regularly discharged, such as photovoltaic systems and electric vehicles.
  • SLI battery: The term SLI refers to starting, lighting, and ignition. SLI batteries are designed to deliver the maximum current in a short space of time, keeping the voltage constant, therefore, they have a very low internal resistance. These batteries have a good life under shallow-cycle conditions but have a very poor lifetime under deep cycling (around 12-15 cycles).
• VRLA battery (valve-regulated lead-acid battery) is sealed or regulated by a valve where the electrolyte is immobilized in an absorbent separator or in a gel. There are two primary types of VRLA batteries:
  • Absorbent glass mat (AGM). AGM lead-acid battery is a type of valve-regulated lead acid (VRLA) battery that has small gas channels in the electrolyte. Absorbed glass mat batteries lead acid battery is one of the lead acid technologies widely used for those applications because of its increased power and energy density and longer cycle life than regular flooded and maintenance free type lead acid batteries.
  • Gel cell (gel battery). A modern gel battery is a VRLA battery with a gelated electrolyte. Gel batteries reduce the electrolyte evaporation and spillage (and subsequent corrosion problems) common to the wet-cell battery and boast greater resistance to shock and vibration.

Chemistry of Lead-acid Batteries – How it works

The principle of operation of the lead-acid battery can be illustrated by the chemical processes that take place during charging and discharging. During discharge, the process

Pb + SO₄^2- → PbSO₄ + 2e^–

takes place at the anode. Lead is oxidized with the electrolyte to lead sulfate, releasing two electrons. Lead sulfate is also formed at the cathode by:

PbO₂ + SO₄^2- + 4H⁺ +2e^– → PbSO₄ + 2H₂O

But in this reaction, a reduction of lead oxide takes place. Formed lead sulfate deposits as a coating on the electrodes and, to some extent also on the bottom of the housing. Since sulfuric acid is utilized during the discharge process, the SoC can be determined by measuring the density of the electrolyte.

During charging, the processes take place in the opposite direction so that the lead sulfate formed during discharging is oxidized to lead and reduced lead oxide, respectively. If the lead sulfate is completely consumed and the changing process is not stopped, electrolysis of the electrolyte begins. Overcharging with high charging voltages generates oxygen and hydrogen gas by electrolysis of water, which bubbles out and is lost. Sealed batteries have catalysts (Pd, Pt) above the vent where oxyhydrogen gas can recombine to water.

The resulting cell voltage can be determined from the galvanic series.

The total voltage of the redox reaction is thus:

E⁰ = 1.68V – ( – 0.36V) = 2.04V.

Advantages and Disadvantages of Lead-acid Batteries

Characteristics of Lead-acid Batteries

To compare and understand the capability of each battery, some important parameters are characteristic of each battery, also within a type of battery. These parameters are a reference when a battery is needed, and specific qualities are required since batteries are used in all types of devices and for infinite purposes.

Other Types of Batteries

The following list summarizes notable electric battery types composed of one or more electrochemical cells. Four lists are provided in the table. The first list is a battery classification by size and format. Then, the primary (non-rechargeable) and secondary (rechargeable) cell lists are lists of battery chemistry. The third list is a list of battery applications. The final list is a list of different battery voltages.