Basic knowledge of lead-acid batteries

1) Type
The active material of the positive plate is lead dioxide, the active material of the negative plate is lead, and the storage battery with an acid solution as the electrolyte is called a lead-acid storage battery.
According to the battery usage environment, lead-acid batteries are divided into mobile and stationary types. Fixed lead-acid batteries are generally only used in places with uninterrupted power supplies and relatively fixed locations due to their large size and quality; the batteries used in electric vehicles are all mobile lead-acid batteries.
According to the function of the battery, lead-acid batteries can be divided into two types: starter type and traction type. Start-up lead-acid batteries are generally used as auxiliary low-voltage power supplies for automobiles; traction-type lead-acid batteries have relatively large capacity, can be charged/discharged deeply, and have high specific energy, making them suitable as the main power source of automobiles.
According to the structural principle, lead-acid batteries have open type (ordinary type), valve-controlled sealed type (VRLA), colloidal type, two-stage sealed type, horizontal type, winding type and super battery.

2) Model
The power lead-acid battery model used in electric vehicles is V-Ah. For example, 12V-120Ah, the part before the hyphen indicates the nominal DC voltage of the lead-acid battery, and the part after the hyphen indicates the nominal capacity of the lead-acid battery.

3) Working principle
The chemical reaction that occurs in the battery is reversible. The active material on the positive plate of the lead-acid battery is lead dioxide (PbO₂), the negative plate is spongy pure lead (Pb), and the electrolyte is an aqueous sulfuric acid solution (H₂SO₄). When the battery and the load are connected and discharged, the positive plate Both the upper PbO₂ and the Pb on the negative plate will be converted into PbSO₄, the concentration of sulfuric acid in the electrolyte will decrease, and the relative density will decrease. When the battery is charged with a DC power supply, the PbSO4 on the positive and negative plates will be restored to the original PbO₂ and Pb, the concentration of sulfuric acid in the electrolyte will increase, and the relative density will increase.
The working process of the battery is shown in Figure 1.

(1) Discharge process
The process of converting the chemical energy of the battery into electrical energy is called the discharge process, as shown in Figure1. Before discharge, PbO₂ on the positive plate ionizes into positive tetravalent lead ions (Pb⁴⁺) and negative divalent oxygen ions (O^2-), Pb4+ is attached to the positive plate, O^2- enters the electrolyte, so that the positive plate has a positive potential of 2.0V . The Pb on the negative plate is ionized into positive divalent lead ions (Pb) and two electrons (2e-), Pb²⁺ enters the electrolyte, and e- stays on the negative plate, making the negative plate have a negative potential of -0.1V. In this way, there is a potential difference between the positive and negative plates, which is 2.1V. When discharging, under the action of a potential difference of 2.1V, the current flows from the positive pole and flows back to the negative pole through the load.
Theoretically, the discharge process will proceed until all the active materials on the negative plate are converted into PbSO₄. But in fact, since the electrolyte cannot penetrate into the innermost layer of the active material, the so-called fully discharged battery actually only has 20% to 30% of the active material converted into PbSO₄. To increase the utilization rate of the active material, it is necessary to increase the reaction area between the active material and the electrolyte. At present, the goal is often achieved through the use of thin plates and measures to increase the porosity of the active material.

(2) Charging process
The process of converting electrical energy into chemical energy is called the charging process. When charging, the battery is connected to a DC power supply, as shown in Figure 2. Under the action of the electric field, current flows in from the positive electrode of the battery and flows out from the negative electrode.
At the negative plate, a small amount of PbSO₄ enters the electrolyte and dissociates into Pb²⁺ and SO₄^2-. Under the action of the power supply, Pb²⁺ obtains two e- into metal Pb, which is deposited on the negative plate. And SO₄^2- combines with H⁺ in the electrolyte to generate H₂SO₄.
At the positive plate, a small amount of PbSO4 enters the electrolyte and dissociates into Pb²⁺ and SO₄^2-. Pb²⁺ loses two e and becomes Pb⁴⁺ under the action of the power source. It combines with the OH- dissociated from the water in the electrolyte to form Pb(OH) ₄, Pb(OH)₄ is decomposed into PbO₂ and H₂O, and SO₄^2- combines with H+ in the electrolyte to generate H₂SO₄.
It can be seen that during the charging process, the PbSO₄ on the positive and negative plates will gradually return to PbO₂ and Pb, the H₂SO₄ components in the electrolyte will gradually increase, and the water will gradually decrease.

4) Commonly used power lead-acid batteries
Power lead-acid batteries used in electric vehicles mainly include valve-regulated maintenance-free lead-acid batteries (VRLA), colloidal acid-acid batteries, horizontal lead-acid batteries and bipolar lead-acid batteries.

(1) Valve-controlled maintenance-free lead-acid battery
Valve-regulated maintenance-free lead-acid batteries refer to lead-acid batteries that do not require other maintenance except to keep the surface clean during their service life. This is closely related to its own structural characteristics.
① Use low antimony alloy or lead-calcium alloy as the pole plate grid. Because the grid contains little or no antimony, it increases the low potential of hydrogen precipitation at the negative electrode of the battery and oxygen at the positive electrode, thereby effectively preserving the moisture in the battery, and effectively reducing the self-discharge of the battery, which makes the battery in There is no need to add distilled water during use.
②Using a sealed partition. In this way, the active material on the positive plate can be effectively prevented from falling off, and the service life of the battery can be prolonged.
③Using built-in density meter. From the different colors indicated by the indicator of the densitometer, the storage state of the battery and the height of the liquid level can be judged.
④Use safety ventilation device (valve control device). This allows the battery to avoid direct contact between the internal sulfuric acid gas and external sparks to prevent explosion. In addition, the vent plug is also equipped with a catalyst (palladium), which can catalyze the hydrogen and oxygen to synthesize water, and then flow back to the battery, thereby retaining moisture.
⑤Using wall-through connecting strips. This connection method can reduce the internal resistance of the battery and increase the capacity of the battery.
The structure of the valve-controlled maintenance-free lead-acid battery for vehicles is shown in Figure 3.

(2) Gel-type lead-acid battery
The colloidal lead-acid battery refers to the alumina acid battery whose electrolyte is mixed into a colloidal substance from a dilute sodium sulfate solution and a silicic acid solution. This kind of storage battery is relatively safe during storage, storage, transportation and use because of the low fluidity of its electrolyte, but its capacity is somewhat lower than that of ordinary lead-acid storage batteries.

(3) Horizontal lead-acid battery and bipolar lead-acid battery
The horizontal lead-acid battery is a battery with the plates arranged horizontally.
For bipolar lead-acid batteries, the separator of the original battery is removed, and the positive and negative plates are combined, one side is coated with the active material of the positive plate, and the other side is coated with the active material of the negative plate. According to reports, a lead-acid battery made by a British company using titanium compounds as electrodes has a specific energy of 60W·h/kg, which is almost close to the specific energy of a nickel-hydrogen battery. If there is a technological breakthrough, low-cost lead-acid batteries will vigorously promote the promotion and popularization of electric vehicles.

5) Features and applications
(1) Features
The advantages of lead-acid batteries are as follows.
①The voltage is high, and the cell voltage is 2.0V. Among the commonly used batteries, the cell voltage is second only to lithium batteries.
②The price is low.
③It can be made into batteries of various sizes and structures ranging from 1A·h to several thousand amperes·hour
④High-rate discharge performance is good, and it can be used for engine starting.
⑤Good high and low temperature performance, can work under -40~60℃.
⑥High power efficiency, up to 60%.
⑦Easy to use floating charge, no memory effect.
⑧Easy to identify the state of charge.
The disadvantages of lead-acid batteries are as follows.
①The specific energy is low, the mass and volume occupied by electric vehicles are relatively large, and the mileage on a single charge is short.
②The service life is short and the use cost is high.
③Long charging time.
④ Lead pollution exists.

(2) Application
As the power source of electric vehicles, lead-acid batteries have many shortcomings, but due to their mature technology, they can be discharged at large currents, have the advantages of wide applicable temperature range and no memory effect, and the raw materials are easy to obtain.

The price is much lower than that of high-energy batteries such as nickel-metal hydride batteries and lithium batteries, so there are still more applications in electric vehicles, mainly for low-speed, low-cost electric vehicles. In addition to electric bicycles, lead-acid batteries are widely used in some low-speed short-distance electric vehicles, such as electric mini-cars, electric golf carts, and electric forklifts. The main application is valve-controlled maintenance-free batteries.
Most of the electric vehicles produced in the early days used lead-acid batteries, such as Isuzu ELF Resort, Daihatsu Hijet Van, Suzuki Alto, Fuji Samber EV and so on. Low-speed pure electric vehicles mostly use VRLA. For example, Shandong Shifeng electric vehicles use a battery pack consisting of 10 GD04B lead-acid power batteries in series, with a rated voltage of 60V. The representative of a typical pure electric passenger vehicle that uses lead-acid power batteries is the all-electric vehicle EV-1 of General Motors of the United States, which is all the rage. The TEG6120EV-2 electric bus developed by China’s Zhuzhou Times Group uses horizontal lead-acid batteries as its power source, and its working voltage is 384V.