Research on tap power supply of series AAA rechargeable battery

　　1 Introduction

　　Emergency command communication vehicles are loaded with various types of communication equipment, such as shortwave, ultra-shortwave, and cluster vehicles. Most of these loads are powered by 12V power supplies. In order to meet the requirements of such equipment on the move, designers sometimes directly use the car's battery to power the equipment. If the original car battery has a single 12V voltage, it can be directly powered; if the original car is provided with starting voltage by two 24V AAA rechargeable battery, during the project implementation process, the middle tap is generally used to take the rear battery to provide equipment power.

　　2. Brief description of examples

　　A certain mobile command communication system is equipped with a conventional trunking radio station. The engine starting battery of the motor vehicle is two batteries connected in series with a voltage of 24V DC. In order to realize trunking communication while the vehicle is moving, the designer uses a rear tap on the middle tap of the two AAA rechargeable battery. A battery provides power to the cluster radio. The system power supply topology is shown in Figure 1.

　　After the motor vehicle has been used for a period of time, the car cannot be ignited and started. After inspection, the open circuit voltage of the front battery is 12.6V and the voltage of the rear battery is only 11V. During use, the car engine is running all the time, and the original car generator is powering the two batteries. The battery keeps charging, why is the rear battery voltage too low? In order to find out the reason, the following is an in-depth analysis of the charging, discharging and polarization processes of lead-acid batteries.

　　3. A brief introduction to the structure of lead-acid batteries

　　Lead-acid batteries are generally composed of six cells, each cell is composed of an anode plate, a cathode plate, a separator and a dilute sulfuric acid electrolyte; the cells are connected in series to output a voltage of 12.6V, and are made of acid-resistant, heat-resistant and shock-resistant hard rubber or plastic The casing serves as the outer structure of the battery.

　　The electrode plate uses lead-antimony alloy as the skeleton and is coated with a layer of soft lead paste. After chemical treatment, the active material lead peroxide (pbO2) is generated on the outer layer of the anode, and the active material lead (pb) is generated on the outer layer of the cathode. The separators include fiberglass separators, microporous rubber separators and plastic dimensional separators. Their function is to insulate the positive and negative plates, while the charged ions in the electrolyte can pass freely.

　　4. Discharge process of lead-acid battery

　　Lead-acid battery discharge is a relatively complex electrochemical reaction process.

　　Cathodic reaction: The active material lead on the outer layer of the cathode plate is oxidized in dilute sulfuric acid. The reaction equation is as follows:

　　Since the divalent lead produced by the plate during the reaction repels hydrogen ions in the solution, although there are excess hydrogen ions near the cathode, electrons will not be absorbed from the plate and hydrogen gas will be precipitated; because the reaction product cannot be removed from the reaction point , thus preventing the reaction from continuing, so when the battery is open circuit, the cathode reaction is a dynamically balanced reversible reaction.

　　Anodic reaction: In the absence of external charge, a small amount of lead oxide reacts with water, and the process is also a reversible reaction of dynamic equilibrium. The reaction equation is as follows:

　　When the battery is open circuit, there is only a small amount of positively charged 4-valent lead in the anode, and the nearby solution contains hydroxide ions, while the cathode has excess free electrons. The two plates and the electrolyte form an electric double layer, resulting in a potential difference, as shown in the figure 2 shown. The battery consists of six cells connected in series, thus forming the battery open circuit voltage, which is the battery's power electromotive force ES.

　　When the two plates are connected with a wire and a load, under the action of the electric field, the excess free electrons from the cathode move toward the anode, forming an external current. The anode lead ions capture 2 free electrons, and after being reduced, they react with sulfuric acid to form insoluble Lead sulfate, the dynamic equilibrium of the reversible reaction equation-2 is destroyed and continues to proceed in the forward direction. At the same time, the hydrogen ions near the cathode and the hydroxide ions near the anode attract each other to form an internal current, and water is generated after the interaction; as the cathode reaction products (excess free electrons) are consumed into the anode reaction, the reversible reaction equation-1 is dynamically balanced is also destroyed, reaction -1 will continue. The reaction equation is as follows:

　　It can be seen from Reaction Equation-4 that as the discharge reaction continues, the sulfuric acid molecules in the solution will gradually decrease. When the concentration of sulfuric acid reaches a certain level, the plates will be covered with lead sulfate, the electromotive force of the battery will decrease, and the battery needs to be charged.

　　5.Charging and polarization of lead-acid batteries

　　5.1 Charging process

　　When the voltage of the external charger is greater than the open circuit voltage of the battery, the charges between the two plates will move in the opposite direction, that is, under the action of the charger, electrons are forced to migrate from the anode to the cathode; at the same time, the hydrogen ions in the solution are Under the action of the electric field force generated by the charger, the cathode is pressed and participates in the cathode reaction. The reversible reaction equation will continue in the opposite direction, as shown in Figure 3.

　　Anode reaction equation:

　　The overall equation of the charging reaction shows that as the charging reaction continues to progress, the concentration of sulfuric acid in the solution increases, and the battery's power becomes larger.

　　5.2 Polarization process

　　During the charging process, three polarization processes occur on the plate:

　　Ohmic polarization, electrochemical polarization, concentration difference polarization.

　　Ohmic polarization: During the charging process, electrons move from the anode to the cathode through external wires; at the same time, there are also positive and negative ions moving directionally in the solution. The ions in the solution need to overcome the resistance of the plates, electrolytes, and battery separators. This resistance forms The ohmic polarization internal resistance of the battery. The ohmic polarization voltage conforms to Ohm's law: UΩ=I*RΩ. The heat generated by the battery electrode during the charging process conforms to Joule's law:

　　Electrochemical polarization: The charger transfers charge to the plate at a speed greater than the electrochemical reaction speed on the plate. The charge that has no time to participate in the reaction resides on the plate, causing the potential of the anode plate to deviate in the positive direction and the potential of the cathode plate to move in the negative direction. Deviation. The electrochemical polarization voltage is theoretically: U1=(RT/nF)*Ln(I/Io).

　　Concentration difference polarization: The charging reaction of both plates will produce sulfuric acid, which will cause the concentration of sulfuric acid near the plates to increase and cannot diffuse quickly. The reaction products will not be removed in time, which inhibits the speed of the reaction. You need to wait until near the plates. Only when the sulfuric acid molecules spread out can the reaction speed be restored. Therefore, during the charging process, the charger also needs to overcome the concentration difference polarization voltage: U2=(RT/nF)*Ln(Id/(Id-I)).

　　Based on the analysis of the charging and polarization process of the battery, the following conclusions can be drawn: During charging, the charger needs to overcome the open circuit voltage and polarization voltage of the battery plate, and the charging voltage U=ES+ΔU. Among them, ΔU is the ohmic polarization voltage, The sum of electrochemical polarization voltage and concentration difference polarization voltage.

　　6. Dynamic analysis of polarization voltage during lead-acid battery charging process

　　When charging, the polarization voltage of the battery changes dynamically. For example, if a 14V constant voltage charger charges a single 11V battery, as shown in Figure 4, the charging voltage U=ES+ΔU, at the initial moment of charging, the polarization voltage is 3V, and the concentration polarization voltage dominates. Because at the initial moment, the concentration of sulfuric acid in the solution is low and the reaction speed is fast, a high concentration of sulfuric acid and a high concentration polarization voltage are quickly generated near the electrode plates; as charging continues, the power of the battery becomes larger, and the electromotive force ES of the battery increases, the polarization voltage ΔU gradually decreases. When charging is completed, the battery electromotive force ES is 12.6V and the polarization voltage is 1.4V. At this time, the concentration of sulfuric acid no longer changes, and the charging reaction of the plate has been completed. Therefore, There is no concentration difference polarization and electrochemical polarization.

　　At this time, the electrochemical reaction of the plate is not an electrochemical reaction of effective charging, but an electrolysis reaction of water. O2 is precipitated at the anode and H2 is precipitated at the cathode. ΔU is the voltage caused by the ohmic resistance of the ions in the solution due to the directional movement. The electrochemical reaction equation is as follows:

　　The Nernst equation can prove that when the battery is charged, the potential of the charging reaction plate is higher than the potential of the gas evolution electrolysis reaction plate. It is precisely because of the effect of polarization that when a lead-acid battery is charged, due to the offset of the plate potential, the electrolysis reaction that should be gas evolution becomes a charging reaction with polarization. When the battery is fully charged, the diffusion of sulfuric acid is completed, the polarization disappears, and the charging reaction becomes an electrolysis reaction of gas evolution.

　　When a U=26V constant voltage charger is used to charge two series-connected AAA rechargeable battery with seriously uneven power, it will be ineffective to charge the battery with a larger discharge capacity.

　　The experimental data are as follows: the first battery was discharged 10%, and the open circuit voltage ES1=12.4V was measured; the second battery was discharged 80%, and the open circuit voltage ES2=11.2V was measured. The plate voltages of the two batteries were measured in the experiment There is a serious imbalance. The first battery receives a voltage of 14.7V from the charger, while the battery with 80% discharge capacity receives a charging voltage of only 11.3V; as shown in Figure 5.

　　This is because the two batteries are connected in series and the charging current is equal; at the initial moment of charging, because the first battery plate is only covered with a small amount of lead sulfate, the charging electrochemical reaction speed is fast, and the sulfuric acid concentration near the plate is high, resulting in a higher Concentration polarization voltage ΔU1; while the second battery plate is covered with a large amount of lead sulfate, the reaction speed is slow, resulting in a lower concentration polarization voltage ΔU2; after multiple positive feedbacks, after reaching the equilibrium state, ΔU1≈U -(ES1+ES2), ΔU2≈0, the battery with large discharge capacity does not undergo polarization, and its plates only produce an electrolysis reaction of gas evolution.

　　Through the dynamic analysis of polarization voltage during battery charging, the following conclusion can be drawn: When a constant voltage charger charges two series-connected AAA rechargeable battery, if the discharge capacity of the two batteries is seriously unbalanced, the battery with a large discharge capacity will not be charged.

　　7.Conclusion

　　In the mobile command and communication system, the designers used a 12V rear battery for the middle tap of the two AAA rechargeable battery of the motor vehicle to provide power for the cluster radio. Such a design will lead to a large discharge of the rear battery, causing the vehicle engine generator to fail. Ineffective charging of the battery pack will eventually lead to battery failure.

　　The correct design method: cancel the middle tap design of the battery pack, add a 24V to 12V DC buck converter, and then supply power to the 12V load, as shown in Figure 6.

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