A lead-acid battery is composed of key components such as positive and negative plates, separators, a container, terminals, and a safety valve. The nominal voltage of each individual cell is 2V; therefore, a 6V or 12V automotive lead-acid battery is typically composed of 3 or 6 individual cells connected in series. Several individual cells are connected in series to form the battery pack, meeting the requirements of the electrical equipment. The following sections detail the components of a lead-acid battery.

​​I. Plate Group​​

The plate group is the core part of the battery. Its function is to receive electrical energy during charging and release it during discharging. It is divided into two types: positive plates and negative plates. Plates consist of a grid framework and active material. Thin plates are beneficial for increasing the specific capacity of the battery and improving its starting performance. The charge-discharge process of the battery is achieved through the electrochemical reaction between the active material on the plates and the electrolyte.

The function of the ​​grid​​ is to hold the active material and give the plate its form. The grid is typically made from a lead-antimony alloy. The antimony content is generally between 5%-7% by weight. Adding antimony improves casting properties and mechanical strength, but it accelerates hydrogen evolution, leading to self-discharge, increased electrolyte consumption, and reduced battery life. The ​​active material​​ is the primary substance involved in the electrochemical reaction. After formation treatment (the process of converting the active material on the positive and negative plates is called formation), the active material on the positive plate is porous lead dioxide (PbO₂), which is reddish-brown; the active material on the negative plate is spongy pure lead (Pb), which is greyish-blue.

When one positive and one negative plate are immersed in the electrolyte, an electromotive force of 2V can be obtained. To increase the capacity of the battery, multiple positive and negative plates are often assembled into a plate group within a single cell. Because the positive plate has poorer mechanical strength, operating on one side would cause uneven volume changes on both sides of the active material, leading to plate warping and active material shedding. Therefore, the number of negative plates is always one more than the number of positive plates. This arrangement places each positive plate between negative plates, ensuring uniform discharge on both sides.

​​II. Separator​​

The function of the ​​separator​​ is to insulate the positive and negative plates immersed in the sulfuric acid solution. To minimize the battery volume, the plates should be as close as possible, but a necessary insulating layer must be ensured between them. Therefore, separators are made from insulating materials with microporous structures, such as rubber, plastic, glass, or fiber. Besides providing insulation between the plates, the separator must allow the positive and negative ions in the electrolyte to pass through easily, slow down the shedding of active material from the plates, and protect the positive plate from vibration damage. Thus, separators must have sufficient porosity (around 60%), small pore size, acid resistance, no leaching of harmful substances, certain strength, low electrical resistance in the electrolyte, and chemical stability.

Since the chemical reaction on the positive plate is more vigorous during charging and discharging, during installation, the ribbed side of the separator should face the positive plate, and the grooves must be vertical to the bottom of the container. These grooves allow the electrolyte and gases to move up and down and enable shedded active material to settle. In recent years, some manufacturers envelope the positive plate with the separator, which effectively prevents active material shedding.

​​III. Electrolyte​​

The ​​electrolyte​​, also known as the electrolyte solution, is commonly called battery acid. Its function is to facilitate the ionization of the active material on the plates, enabling the electrochemical reaction. The electrolyte is prepared by mixing specialized battery-grade sulfuric acid with distilled water in a specific ratio. Typically, the electrolyte used in automotive lead-acid batteries is dilute sulfuric acid with a density of (1.280 ± 0.010) g/cm³ at 25°C. The density of the electrolyte significantly impacts battery performance and service life. Increasing the density can enhance battery capacity and lower the freezing point. However, excessive density increases viscosity, which can instead reduce capacity. Given China's vast territory and complex climate conditions, the electrolyte density values must be specified according to different climatic conditions.

​​IV. Container (Case)​​

The battery ​​container​​ houses the plate group and the electrolyte. Its shape is generally a long rectangular prism, and the interior is typically divided into 3 or 6 separate cell compartments. There are special sealing grooves around the top edge for connecting with the cover. Convex ribs at the bottom of the container support the plate group.

Materials used for containers are generally hard rubber or polypropylene plastic. Hard rubber containers offer advantages such as acid resistance, heat resistance, cold resistance, vibration resistance, good insulation properties, and certain mechanical strength, but the wall thickness is relatively large, typically around 10mm. Containers made from polypropylene plastic are not only acid-resistant, heat-resistant, and vibration-resistant but also offer high strength, good toughness, and light weight. Their wall thickness is thinner, typically about 3.5mm, and they feature an attractive, transparent appearance. Plastic cases are easy to heat-seal, allowing high production efficiency. The development of polypropylene containers has become a trend.

Each cell cover has a small vent hole to allow the timely release of hydrogen and oxygen produced during charging (from water electrolysis), preventing gas buildup that could increase internal pressure and cause the container to swell or even explode. Additionally, an oxygen recombination filter can be installed on the cover to reduce water vapor escape, thereby minimizing water loss.

​​V. Safety Valve (Vent Valve)​​

The ​​safety valve​​ is a critical component of Valve-Regulated Lead-Acid (VRLA) batteries. The quality of the safety valve directly affects the battery's service life, uniformity, and safety. According to relevant standards and the usage conditions of VRLA batteries, the safety valve should meet the following technical requirements:

(1) Open in one direction only.

(2) Seal in one direction, preventing air from entering the battery.

(3) The opening/closing pressure difference between the safety valves of any battery in the same group should not exceed 20% of the average value.

(4) Service life should be no less than 15 years.

(5) Acid filtration, preventing acid and acid mist from being expelled through the valve vent.

(6) Flame arrestance; the battery interior should not ignite if exposed to an external flame.

(7) Vibration resistance; the valve should not become loose or fail due to vibration and repeated opening/closing during transport and use.

(8) Acid resistance.

(9) Resistance to high and low temperatures.