Research on the tap power supply problem of series battery group

1. Introduction

Emergency command communication vehicles are equipped with a variety of communication equipment, such as shortwave, ultra-shortwave, and cluster radios, most of which are powered by 12V power supplies. In order to meet the requirements of such equipment for on-the-go communication, designers sometimes directly use the car's battery to power the equipment. If the original car battery is a single 12V voltage, it can be directly powered; if the original car is powered by two 24V battery packs to provide the starting voltage, during the project implementation, the middle tap is generally used to take the last battery to provide power for the equipment.

2. Brief description of examples

A mobile command communication system is equipped with a conventional trunking radio. The motor vehicle engine starting battery is two batteries connected in series, with a voltage of DC 24V. In order to realize the trunking communication while the vehicle is moving, the designer taps the middle of the two battery packs and uses the rear battery to provide power for the trunking radio. The system power supply topology is shown in Figure 1.

After the motor vehicle had been used for a period of time, the car could not ignite and start. Upon inspection, the open-circuit voltage of the front battery was 12.6V, and the voltage of the rear battery was only 11V. During use, the car engine was always running, and the original car generator was always charging the two batteries. Why was the voltage of the rear battery too low? In order to find out the cause, the following is an in-depth analysis of the charging, discharging and polarization process of the lead-acid battery.

3. Brief introduction to the structure of lead-acid battery

Lead-acid batteries are generally composed of six cells, each of which 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 an acid-resistant, heat-resistant and shock-resistant hard rubber or plastic shell is used as the outer structure of the battery.

The plate is made of lead-antimony alloy and coated with a soft lead paste. After chemical treatment, the outer layer of the anode generates active material lead peroxide (PbO2), and the outer layer of the cathode generates active material lead (Pb). The separators include glass fiber separators, microporous rubber separators, and plastic fiber 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

The discharge of lead-acid batteries is a relatively complex electrochemical reaction process.

Cathode 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:

Because the divalent lead produced by the electrode plate during the reaction repels the hydrogen ions in the solution, although there are excess hydrogen ions near the cathode, they will not absorb electrons from the electrode plate and precipitate hydrogen. Since the reaction products cannot be removed from the reaction point, the reaction is prevented from continuing. Therefore, when the battery is open, the cathode reaction is a reversible reaction in dynamic equilibrium.

Anode 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 tetravalent lead at the anode, and the nearby solution contains hydroxide ions, while the cathode has excess free electrons. The two plates and the electrolyte form a double layer, generating a potential difference, as shown in Figure 2. The battery consists of six cells connected in series, thus forming the battery open circuit voltage, that is, the battery's power source electromotive force ES.

When the two plates are connected with wires and loads, under the action of the electric field, the excess free electrons at the cathode move toward the anode in a directional manner, forming an external current. The lead ions at the anode capture two free electrons, and after being reduced, they react with sulfuric acid to form insoluble lead sulfate. The dynamic balance of the reversible reaction formula-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, forming an internal current, and water is generated after interaction; because the cathode reaction product (excess free electrons) is consumed in the anode reaction, the dynamic balance of the reversible reaction formula-1 is also destroyed, and the reaction formula-1 will continue. The reaction equation is as follows:

From reaction equation 4, we can see that as the discharge reaction continues, the sulfuric acid molecules in the solution will gradually decrease. When the concentration of sulfuric acid drops to a certain level, the plates are covered with lead sulfate, the battery electromotive force decreases, 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 charge between the two plates will move in the opposite direction, that is, under the action of the charger, the electrons are forced to migrate from the anode to the cathode; at the same time, the hydrogen ions in the solution are pressed to the cathode under the action of the electric field force generated by the charger and participate in the cathode reaction, and 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, the concentration of sulfuric acid in the solution increases and the battery's capacity increases.

5.2 Polarization process

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

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 is directional movement of positive and negative ions in the solution. The ions in the solution need to overcome the resistance of the plates, electrolyte, 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, and the heat generated by the battery electrode during the charging process conforms to Joule's law:

Q=I2RΩt.

Electrochemical polarization: The speed at which the charger delivers charge to the plates is greater than the speed of the electrochemical reaction on the plates. Charges that are unable to react in time remain on the plates, causing the anode plate potential to deviate to the positive direction and the cathode plate potential to deviate to the negative direction. Theoretically, the electrochemical polarization voltage is: 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. It cannot diffuse quickly, and the reaction products cannot be removed in time, which inhibits the reaction speed. It is necessary to wait until the sulfuric acid molecules near the plates diffuse before the reaction speed can 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: when charging, the charger needs to overcome the open circuit voltage and polarization voltage of the battery plates, and the charging voltage U=ES+ΔU. Among them, ΔU is the sum of the ohmic polarization voltage, electrochemical polarization voltage and concentration difference polarization voltage.

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

When charging, the polarization voltage of the battery changes dynamically. For example, when a 14V constant voltage charger charges a single 11V battery, as shown in Figure 4, the charging voltage is U=ES+ΔU. At the initial moment of charging, the polarization voltage is 3V, and the concentration polarization voltage is dominant. 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 is quickly generated near the plate, and a high concentration polarization voltage; as charging continues, the battery's power increases, the battery's electromotive force ES increases, and the polarization voltage ΔU gradually decreases. When charging is completed, the battery's 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, so there is no concentration polarization and electrochemical polarization.

At this time, the electrochemical reaction of the plate is not an effective charging electrochemical reaction, 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 directional movement of ions in the solution. The electrochemical reaction equation is as follows:

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

When a constant voltage charger U=26V is used to charge two series-connected battery packs with severely uneven charge, it is ineffective to charge batteries with larger discharge amounts.

The experimental data are as follows: the first battery is discharged by 10%, and the open circuit voltage ES1=12.4V is measured; the second battery is discharged by 80%, and the open circuit voltage ES2=11.2V is measured. The experiment shows that the plate voltages of the two batteries are seriously unbalanced. The first battery receives 14.7V of the charger voltage, while the battery with 80% discharge 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, since the first battery plate is covered with only a small amount of lead sulfate, the charging electrochemical reaction speed is fast, the sulfuric acid concentration near the plate is high, and a higher concentration polarization voltage ΔU1 is generated; while the second battery plate is covered with a large amount of lead sulfate, the reaction speed is slow, and a lower concentration polarization voltage ΔU2 is generated; after multiple positive feedbacks, after reaching the equilibrium state, ΔU1≈U-(ES1+ES2), ΔU2≈0, the battery with a large discharge capacity does not undergo polarization, and its plate only produces an electrolytic 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 a two-cell battery pack in series, if the discharge of the two cells is seriously unbalanced, charging the battery with a larger discharge capacity will be ineffective.

7. Conclusion

In the mobile command and communication system, the designers tapped the middle of the two battery packs of the motor vehicle and used the 12V of the rear battery to provide power for the cluster radio. This design will cause a large discharge of the rear battery, causing the car engine generator to be unable to effectively charge the battery pack, ultimately leading to battery failure.

Correct design method: Cancel the middle tap design of the battery pack, add a 24V to 12V DC step-down converter, and then power the 12V load, as shown in Figure 6.