Car Battery

Car Battery – Automotive Battery

A car or automotive battery is a rechargeable battery used to start a motor vehicle. It is also known as the SLI battery. The term SLI refers to starting, lighting, and ignition. Its main purpose is to provide an electric current to the electric-powered starting motor, which in turn starts the chemically-powered internal combustion engine that actually propels the vehicle. Once the engine is running, power for the car’s electrical systems is still supplied by the battery, with the alternator charging the battery as demands increase or decrease.

Car 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). Current discharges might be with strong temperature changes, which is why the weight, design, and shape are characteristic. These types of batteries are usually frequent for starting cars and all types of diesel and gasoline vehicles.

Most car batteries are lead-acid 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 Car 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.

Chemistry of Car 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 Car Batteries

Characteristics of Car 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.