Lead acid battery has the longest application history, is also the most mature battery with the lowest cost and price, and has realized mass production.
Lead-acid battery is widely used because of its simple structure, low price and small internal resistance. It can supply strong starting current to the starter in a short time.
- Classification of lead-acid batteries
According to the structure of battery plate, there are formative, paste coated and tubular batteries.
Classified by battery cover and structure: open type, exhaust type, acid proof and explosion-proof type and sealed valve regulated battery.
According to the battery maintenance mode, there are ordinary, less maintenance and maintenance free batteries.
According to relevant Chinese standards, the main battery series products are as follows:
Starting battery: mainly used for starting and lighting of automobiles, tractors, diesel engines and ships.
Fixed battery: mainly used for communication, power plant and computer system as backup power supply for protection and automatic control.
Traction battery: it is mainly used for power supply of various battery vehicles, vehicles and forklifts.
Railway storage battery: mainly used for the power of starting and lighting of railway diesel locomotives, electric locomotives and passenger cars.
Motorcycle battery: mainly used for starting and lighting of motorcycles of various specifications.
Coal Mine Battery: mainly used for traction power supply of electric locomotive.
Storage battery for energy storage: it is mainly used for electric energy storage of wind power and hydropower generation.
- Lead acid battery composition
The electromotive force generated by the positive and negative plates of the battery is about 2V. In order to obtain a higher electromotive force, multiple 2V battery units are usually connected in series. A single cell battery is composed of a positive plate, a negative plate, a separator, electrolyte, a battery cover plate, a filling hole plug and a battery shell. When manufacturing the battery shell, a whole shell is divided into several single cells, The battery is generally divided into 3 groups of 6V or 6 non interconnected battery shells.
Electrolyte is the main substance of chemical reaction in the battery, which is prepared by pure sulfuric acid and distilled water in a certain proportion. The two are mixed in different proportions to form electrolytes with different densities.
The density of battery electrolyte is generally 1.24g/cm3-1.30g/cm3. The medium density should be determined according to the region, climate conditions and the requirements of the manufacturer.
- Model, specification and selection of lead-acid battery
According to Chinese national standard GB / t5008 Starting lead-acid batteries (1 ~ 3-2005), the mechanical industry standard of the people’s Republic of China JB / t2599-1993 “preparation method of lead-acid batteries and product models”, stipulates that its model consists of three sections. The model of batteries is generally marked on the shell. The product model and meaning of batteries are as follows:
The first paragraph indicates that the battery assembly is composed of several single cells, which are represented by Arabic numerals.
The second paragraph indicates the type and characteristic code of battery, which is expressed in capital letters, such as “Q” for automobile battery, “m” for motorcycle battery, “JC” for marine lead battery and “HK” for aircraft lead battery. See table 2-3 for characteristic code of storage battery.
The third paragraph indicates the rated capacity of 20h discharge rate, which is expressed in Arabic numerals, and the unit is a · h. when necessary, other codes can be marked after the rated capacity to indicate special performance, which is expressed in capital letters (no words are general performance batteries). For example, the high starting rate battery with thin plate is represented by “g”.
For example, 3-q-90 refers to a starting battery composed of three single cells with rated voltage of 6V and rated capacity of 90a · H. As another example, 6-qaw-100 refers to a dry charge maintenance free battery for startup, which is composed of 6 single cells, with rated voltage of 12V and rated capacity of 100A · H.
- Basic working principle of lead-acid battery
The working process of battery is the mutual transformation of chemical energy and electric energy. When the battery converts chemical energy into electrical energy and supplies power to the outside, it is called discharge process; When the battery is connected with the external DC power supply and converts the electric energy into chemical energy for storage, it is called the charging process.
When the battery is fully charged, the active material on the positive plate is lead dioxide and the active material on the negative plate is pure lead.
In the discharge process, before discharge, the lead dioxide on the positive plate is ionized into tetravalent lead ion (PB4 +) and divalent oxygen ion (O2 -). The lead ion is attached to the positive plate, and the oxygen ion enters the electrolyte to make the positive plate have a positive potential of 2.0V. The pure lead on the negative plate is ionized into divalent lead ion (Pb2 +) and two electrons (2e). The lead ion enters the electrolyte and the electrons remain in the negative electrode, so that the negative plate has 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, the tetravalent lead ions on the positive plate combine with electrons to form divalent lead ions, enter the electrolyte and then combine with sulfate ions to form lead sulfate (attached to the positive electrode); On the negative plate, divalent lead ions also combine with sulfate ions to form lead sulfate (attached to the negative plate). The sulfuric acid in the electrolyte is consumed due to the migration of hydrogen ions and sulfate ions to form water. Therefore, the density of electrolyte decreases gradually after discharge.
During charging, if the discharged battery is connected to a DC power supply, the positive pole of the battery is connected to the positive pole of the DC power supply, and the negative pole of the battery is connected to the negative pole of the DC power supply. When the voltage of the external power supply is higher than the electromotive force of the battery, the current will flow through the battery in the opposite direction of the discharge current, causing the positive and negative electrodes of the battery to have a chemical reaction opposite to the discharge. During charging, the applied current at the positive plate transmits two electrons to the negative plate through the external circuit. The original divalent lead ion on the positive plate becomes tetravalent lead ion due to the loss of two electrons, and then reacts with water to form lead dioxide (attached to the positive plate). On the negative plate, pure lead is generated by the combination of two electrons with the original divalent lead ions (attached to the negative plate). At the same time, the sulfate ions ionized from the positive and negative electrodes are combined with hydrogen ions in water to generate sulfuric acid. Therefore, when charging, water is consumed, sulfuric acid increases and electrolyte density increases. During the charging process, the above chemical reactions continue. When the material and electrolyte on the electrode plate are completely restored to the state before discharge, the battery is charged.
To sum up, the chemical reaction in the charge and discharge process of the battery is reversible.
The total reaction formula is as follows:
PbO2+2H2SO4+Pb=2PbSO4+2H2O
- Capacity of storage battery and its influencing factors
The capacity of lead-acid battery refers to the external output of the battery within the allowable discharge range when the battery is fully charged. The unit is a · h. The battery capacity is used to represent the external power supply capacity of the battery. When the battery discharges at a constant current value, its capacity Q is equal to the product of discharge current I and discharge time t, i.e. q = it
Where: q is battery capacity (a · h); I is discharge current (a); T is the discharge time (H). The capacity of the battery is related to the discharge current and the temperature of the electrolyte. In order to accurately express the accurate capacity of the battery, the discharge conditions of the battery should be specified. Under certain discharge conditions, the capacity of the battery is divided into rated capacity and starting capacity.
(1) Rated capacity refers to the amount of electricity output by a fully charged battery when the average temperature of electrolyte is 25 ℃ and the current discharged at the rate of 20h (equivalent to 1 / 20 of rated capacity) is continuously discharged until the single grid voltage drops to 1.75v.
For example, for 3-q-90 battery, when the average temperature of electrolyte is 25 ℃, after continuous discharge with 4.5a discharge current for 20h, the single cell voltage drops to 1.75v, and its rated capacity Q = 4.5 × 20=90（A· h）。
(2) Starting capacity refers to the power supply capacity of the battery when connected to the starter. There are two starting capacities: normal temperature and low temperature. ① Normal temperature starting capacity: when the electrolyte temperature is 25 ℃, the discharge current (3 times the rated capacity) discharged at the rate of 5min continuously discharges to the specified termination voltage (4.5V for 6V battery and 9V for 12V battery), the discharge duration of the output power shall be more than 5min. For example, 3-q-90 battery is discharged at 270A current at 25 ℃ for 5min. The terminal voltage of the battery drops to 4.5V and its starting capacity is 270 × 5/60=22.5A·h。 ② Low temperature starting capacity, low temperature starting capacity, that is, when the electrolyte temperature is – 18 ℃, the amount of electricity discharged when the current of 3 times the rated capacity is continuously discharged to the specified termination voltage (6V for 12V battery and 3V for 6V battery). The discharge duration shall be more than 2.5min.
The influence of service conditions on the capacity of battery. The capacity of battery is related to factors such as discharge current, temperature of electrolyte, density of electrolyte and structure of electrode plate.
① The influence of discharge current on the capacity of the battery. When the discharge current is too large, the chemical reaction acts on the surface of the electrode plate. Before the electrolyte can penetrate into the interior of the electrode plate, it has been blocked by the lead sulfate generated on the surface, so that a large number of active substances in the electrode plate can not participate in the chemical reaction, so the capacity of the battery is reduced.
② The influence of electrolyte temperature on battery capacity: when the temperature is low, the electrolyte viscosity increases and the ion movement speed is slow; On the other hand, the shrinkage of the electrode plate reduces the pores on the surface of the electrode plate, and it is difficult for the electrolyte to penetrate into the inner layer of the pores of the electrode plate, so that the active substances in the pores of the electrode plate can not be fully utilized, which reduces the discharge capacity of the battery. When the starter is used to start the car in winter, the discharge current is large and the temperature is low, which greatly reduces the capacity of the battery, which is one of the main reasons for the lack of battery power when starting in winter. Because the temperature has a great influence on the discharge capacity and terminal voltage of the battery, we should pay attention to the heat preservation of the battery in northern winter.
③ The influence of the density of electrolyte on the capacity of battery. Within a certain range, appropriately increasing the density of electrolyte can improve the electromotive force of battery and the permeability of electrolyte active substances into the electrode plate, reduce the resistance of electrolyte, and increase the capacity of battery. However, if the density is too high, its viscosity will increase. If the density exceeds a certain value, it can reduce the permeability, increase the internal resistance, and reduce the terminal voltage and capacity. In addition, the electrolyte density is too high, the self discharge speed of the battery is accelerated, and the corrosion effect on the electrode grid and diaphragm is intensified, which shortens the service life of the battery. Generally, the electrolyte with low density is conducive to improve the discharge current and capacity, and prolong the service life of lead-acid battery. The density of the electrolyte of lead-acid battery shall vary according to the climatic conditions in the user’s area. For the electrolyte used in winter, the electrolyte with slightly lower density shall be used as far as possible under the condition that it will not freeze.
④ The purity of electrolyte has great influence on the capacity of battery. Therefore, the electrolyte shall be prepared with chemical pure sulfuric acid and distilled water. Some harmful impurities in the electrolyte corrode the grid frame, and the impurities deposited on the electrode plate form a partial battery to produce self discharge. If the electrolyte contains 1% iron, the battery will be discharged within 24 hours. Therefore, the use of electrolyte with poor purity can significantly reduce the capacity of the battery and shorten the service life of the battery.