What are lead-acid batteries for solar LED systems?

What are lead-acid batteries for solar LED systems?

1. The development of lead-acid batteries
The lead-acid battery was invented by Plante in 1859 and has a history of more than 100 years. Since its invention, lead-acid batteries have always had an absolute advantage in chemical power sources. This is because of its low price, easy availability of raw materials, sufficient reliability in use, suitable for high current discharge and a wide range of ambient temperature and other advantages.

By the early 20th century, lead-acid batteries had undergone many major improvements, improving performance such as energy density, cycle life, and high-rate discharge. However, open-type lead-acid batteries have two main disadvantages: 1. At the end of charging, water will be decomposed into hydrogen and oxygen gas, which requires frequent addition of acid and water, and maintenance work is heavy; Pollute the environment and limit the application of batteries. In order to solve the above two problems, it is hoped to realize the sealing of the battery and obtain clean green energy.

Thomas Edison published a patent in 1912, proposing to use platinum wire in the upper space of the single cell. When there is current passing through, platinum is heated and becomes a catalyst for hydrogen and oxygen combination, so that the precipitated H2 and O2 are recombined and returned to the electrolyte. .

In the 1960s, the invention of lead-calcium alloys by Gates Company in the United States caused the development of sealed lead-acid batteries, and the world’s major battery companies invested a lot of manpower and material resources in the development. In 1969, the U.S. moon landing plan was implemented. Valve-regulated sealed lead-acid batteries and Haojin batteries were included in the power supply for lunar rover. Finally, nickel-cadmium batteries were used, but the technology of sealed lead-acid batteries has been developed since then. From 1969 to 1970, the American EC company manufactured about 350,000 small sealed lead-acid batteries. The batteries used glass fiber cotton separators and a lean liquid system. This was the earliest commercial valve-regulated lead-acid battery, but it was not known at that time. to its oxygen recombination principle.

In 1975, Gates Rutter obtained a patent for the invention of a D-type sealed lead-acid dry battery after many years of hard work and high cost, which became the prototype of today’s VRLA (valve-regulated sealed lead-acid battery). In 1979, GNB Company invented the MFX positive grid patent alloy after purchasing the patent of Gates Company, and began to publicize and produce large-capacity liquid-absorbing sealed maintenance-free lead-acid batteries on a large scale.

In 1991, the British Telecom Department inspected and tested the VRLA batteries in use, and found that the VRLA batteries were not as advertised by the manufacturers. Extensive discussions and questions were expressed about the development prospects, capacity monitoring technology, thermal runaway and reliability of VRLA batteries. At this time, the market share of VRLA batteries was less than 50% of flooded batteries. The name “Maintenance lead-acid battery” was officially replaced by “VRLA battery” because VRLA battery is a battery that needs to be managed, and the use of “maintenance-free” is easy to cause misunderstanding. In 1992, the amount of VRLA batteries in the world increased significantly in Europe and America, and the telecom departments in Asian countries advocated the use of all VRLA batteries; in 1996, VRLA batteries basically replaced traditional flooded batteries, and VRLA batteries have been recognized by the majority of users. .

2. Principle of charge and discharge of lead-acid battery
Mastering and applying the charging and discharging characteristics of batteries is the basis for the application of photovoltaic power generation systems. Due to the common application of lead-acid batteries in photovoltaic power generation systems.

In the discharge process of lead-acid batteries, the negative electrode of the lead-acid battery is spongy lead, the positive electrode is porous lead dioxide, and the two electrode products are lead sulfate and water. Under ideal conditions, the lead sulfate and water on the two electrodes during the charging process can be restored to their original substances, respectively. According to the double sulfation theory lead-acid battery balance electrode reaction, as follows.
Negative electrode: Pb+HSO4-⇌PbSO4+H++2e
Positive electrode: PbO2+3H++HSO4+2e⇌PbSO4+2H2O

Therefore, the chemical reaction in the equilibrium state of the cell can be expressed as:

When the battery is in the open circuit state, the electrons released by Pb on the negative electrode become Pb2+ ions, which react in the electrolyte to generate PbSO4. At the same time, the Pb2+ ions in the generated PbSO4 will adsorb electrons on the surface of the electrode, and the reaction rates of the two are equal. PbO2 on the positive electrode absorbs electrons and reacts with the electrolyte to generate PbSO4 and H2O. At the same time, PbSO4 has the tendency to release electrons and oxidize into PbO2 and HSO4 ions. The reaction rates of the two are also equal, so they are balanced inside the battery. When the battery undergoes a discharge or charge reaction, current will flow through the battery, the electrodes will be out of balance and energy conversion will occur.

(1) Discharge of lead-acid batteries
When the lead-acid battery is not connected to the load, the electrode potential of the positive and negative poles forms an electromotive force inside the battery and is in a balanced state. A load is connected to both ends of the battery. Under the action of electromotive force, electrons continuously flow out from the negative electrode, and flow back to the positive electrode after passing through the load to form a current. Inside the battery, the electrolyte undergoes a dissociation reaction:

The positive ion H+ generated by the reaction migrates to the positive electrode through the solution, while the negative ion HSO4 migrates to the negative electrode. Outside the battery, under the action of the battery electromotive force, the negative charge on the negative electrode continuously flows to the positive electrode through the load. The whole system forms a loop. Oxidation reaction occurs at the negative electrode of the battery:

At the same time, a reduction reaction occurs on the positive electrode of the battery:

Due to the reduction reaction on the positive electrode, the electrode potential of the positive plate gradually decreases, while the oxidation reaction on the negative plate promotes the increase of the electrode potential. The whole process will cause the battery electromotive force to drop. In the oxidation-reduction reaction, the active materials Pb and PbO2 on the positive and negative plates are constantly transformed into PbSO4, which are deposited on the surface of the plates respectively. At the same time, the electrolyte solution H2SO4 gradually turns into water, causing the density of the electrolyte solution to decrease and the capacity of the battery to decrease. Therefore, the discharge process is a process in which the load consumes electrical energy, and the active material and electrolyte are continuously lost to generate PbSO4.

(2) Charging process
The charging process of the battery is the reverse process of discharging, and an external power supply is required to input energy. Under the action of an external power supply, the transport of current is realized by the migration of positive and negative ions to the two poles, and a chemical reaction occurs at the interface of the electrode solution. This process is electrolysis. During electrolysis, under the action of charging and current, the water molecules in the electrolyte will undergo electrolysis.

H+ migrates to the negative plate under the action of the external electric field, and OH migrates to the positive plate. At the same time, because the negative electrode absorbs external electrons at a great speed, the PbSO4 on the surface of the electrode plate is decomposed.

PbSO4 on the positive plate is decomposed

Also occurs in the electrolyte

As the charging progresses, the PbSO4 on the positive plate gradually becomes PbO2, and the PbSO4 on the negative plate gradually becomes Pb. At the same time, the sulfuric acid molecules in the electrolyte solution gradually increase, and the water molecules gradually decrease, so the density of the electrolyte solution is increasing, the terminal voltage of the battery is increasing, and the energy of the battery is also increasing.

3. Current law during charging and discharging
Knowing the charging current of the battery, the amount of charge Q stored in the battery after the charging time t can be obtained by integrating from time 0 to time t:

In the formula, I0 is the initial charging current; α is the current acceptance ratio. Charge amount at the end of charging

Therefore, for any given battery power, the higher the current acceptance ratio α, the greater the initial current and the faster the charging speed. If the charging current always changes according to an acceptable value, then the value of α can remain unchanged, and the charging will always be in the best state that its actual accepted charging current matches its inherent characteristics.

However, the charging process of lead-acid batteries is governed and influenced by three basic laws.

(1) The first law
For any given discharge current, the battery current acceptance ratio α is inversely proportional to the square root of the discharge power c:
Therefore, the acceptable initial charging current I0 of the battery is related to the power of the battery, and the greater the power, the greater the acceptable initial charging current of the battery.

(2) The second law
For any given amount of discharge, the battery charge current acceptance ratio α is proportional to the logarithm of the discharge current:
In the formula, Idis is the discharge current; K, k are constants. The ability of the battery to accept the charging current is related to the discharging current of the battery. The higher the discharge current, the stronger the ability of the battery to accept the charging current.

(3) The third law
After the battery is discharged at different discharge rates, the acceptable charging current is the sum of the acceptable charging currents for each discharge rate, i.e.
So the total current acceptance ratio is

These three basic laws lay an important theoretical foundation for the charging and discharging of lead-acid batteries, characterize the internal relationship between the acceptable charging current and the discharge capacity of the battery, and point out that a certain depth of discharge on the battery during the charging process is the best way to improve charging. The current acceptance ratio is an effective way to speed up the charging process.

4. Polarization process in charge and discharge
In the mid-1960s, American scientist Maas did a lot of experimental research on the charging process of open-ended batteries, and proposed an acceptable charging current curve for the battery on the premise of the minimum gas output rate, as shown in Figure 1. Experiments show that if the charging current changes according to this curve, the charging time can be greatly shortened, and the capacity and life of the battery will not be affected. In principle, this curve is called the optimal charging curve, which is the ideal charging curve for lead-acid batteries.

What are lead-acid batteries for solar LED systems?
Figure 1 Schematic diagram of the acceptable charging current curve of the battery

It can be seen from Figure 1 that the initial charging current is very large, but the decay is very fast, and the time to maintain high current charging is not long. It is difficult to directly use this curve to charge. In order to quickly charge the battery, can the initial charging current be maintained as long as possible to shorten the total charging time? The main reason is that the polarization phenomenon occurs during the charging process, which affects the charging speed and charging current of the battery. During the charging process of the sealed battery, oxygen and hydrogen are generated inside. When the oxygen cannot be absorbed in time, it accumulates on the positive plate (the positive plate produces oxygen), which increases the internal pressure of the battery, increases the battery temperature, and reduces the positive plate at the same time. The area of ​​​​is manifested as an increase in internal resistance, and the so-called polarization phenomenon occurs.

Lead-acid batteries are reversible batteries. When a DC voltage is applied to the battery, an electrolysis reaction occurs, and the battery is in a state of charge. When the battery is connected to an external load, it is in a state of discharge. The chemical reaction formula of charge and discharge is:
2PbSO4+2H2O ⇌Pb+PbO2+2H2SO4

The charging process and the discharging process are inverse reactions to each other. The reversible process is a thermodynamic equilibrium process. In order to ensure that the battery can always be charged in a balanced state, the current through the battery must be as small as possible. The ideal condition is that the applied voltage is equal to the electromotive force of the battery itself. However, practice has shown that when the battery is charged, the applied voltage must be increased to a certain value, and this value exceeds the equilibrium electromotive force value of the battery to varying degrees due to the differences in various factors such as electrode materials and solution concentration. In a chemical reaction, the phenomenon that the electromotive force exceeds the thermodynamic equilibrium value is the phenomenon of polarization. There are three main reasons for the polarization phenomenon.

(1) Ohmic polarization
During the charging process, positive and negative ions migrate to the poles. In the process of ion migration, it is inevitably subjected to a certain resistance, which is called ohmic internal resistance. In order to overcome this internal resistance, an additional voltage must be applied to overcome the resistance to promote ion migration. This voltage is thermally transferred to the environment, so-called ohmic polarization occurs. With the sharp increase of the charging current, the ohmic polarization will cause the high temperature of the battery during charging.

(2) Concentration polarization
When current flows through the battery, in order to maintain a normal reaction, the ideal situation is that the reactants on the electrode surface can be replenished in time, and the products can leave in time. In fact, the diffusion rate of the products and reactants is far less than the chemical reaction rate, which causes the concentration of the electrolyte solution near the plate to change. That is, the electrolyte concentration distribution is not uniform from the electrode surface to the middle solution. This phenomenon is called concentration polarization.

(3) Electrochemical polarization
This polarization is caused by the speed of the electrochemical reactions taking place at the electrodes, which lag behind the speed of electron movement at the electrodes. For example, before the negative electrode of the battery is discharged, the surface of the electrode has a negative charge, and the nearby solution has a positive charge, and the two are in a state of equilibrium. When discharging, electrons are immediately released to the external circuit. The negative charge on the electrode surface decreases, and the oxidation reaction of metal dissolution proceeds slowly, and the decrease in electrons on the electrode surface cannot be replenished in time, and the charged state of the electrode surface changes. This state of reduced surface negative charge promotes the electrons in the metal to leave the electrode, and the metal ions are transferred into the solution, which accelerates the oxidation reaction. After a certain period of time, a new dynamic equilibrium is reached, but compared with before discharge, the number of negative charges on the electrode surface decreases, and the electrochemical polarization voltage becomes higher, which seriously hinders the normal charging current. Similarly, when the positive electrode of the battery is discharged, the number of positive charges on the surface of the electrode decreases, and the electrode potential becomes negative. Due to the precipitation of hydrogen and oxygen, the reversibility of the electrode reaction is destroyed, causing the electrode plate to be damaged due to the falling off and falling off of the active material.

The above three polarization phenomena tend to be serious with the increase of charging current, and they are important factors hindering the electrolysis reaction. Due to the existence of these polarization phenomena, the characteristic curve of the acceptable charging current inherent in the battery has two limitations: the initial current I0 has a certain limit and the initial current I0 maintains a short time and decays at a certain rate. The polarization phenomenon of the battery is an important factor in determining the life of the battery and the charging and discharging mode in the photovoltaic power generation system. The charge and discharge control of the battery must follow the polarization law of the battery.