Necessity of Lithium Power Battery Equalization and Characteristics of Passive Equalization Charging Circuit
1. The definition of equalization charging and the necessity of equalization
1. Definition of equalizing charge:
Equalizing charging is abbreviated as equalizing charging, which is the charging of equalizing battery characteristics. It refers to the voltage imbalance at the battery terminal due to individual differences in the battery, temperature differences and other reasons during the use of the battery. In order to avoid the deterioration of this imbalance trend, it is necessary to Increase the charging voltage of the battery pack, and charge the battery in a balanced manner, so as to balance the characteristics of each battery cell in the battery pack and prolong the service life of the battery.
Equalization charging is in the middle and late stages of the power battery charging process. When the power battery cell voltage reaches or exceeds the cut-off voltage, the balancing circuit starts to work to reduce the power battery cell current in order to limit the power battery cell voltage not to be higher than the charge cut-off voltage . The only function of equalizing charging is to prevent overcharging, and it will bring negative effects during discharging use.
When using equalization charging, the small-capacity power battery cell is not overcharged, and the amount of power that can be released is less than the power that can be released when the equalizer is not used for light overcharge, making the power battery cell discharge time shorter and possible overdischarge Sex is even greater.
2. Necessity of equalizing charging:
With the current level and technology of lithium power battery manufacturing, in the production process of lithium power battery cells, there will be subtle differences between each lithium power battery cell, which is the consistency problem. The inconsistency is mainly manifested in the lithium power battery cell. Capacity, internal resistance, self-discharge rate, charge-discharge efficiency, etc. The inconsistency of the lithium power battery cells is transmitted to the lithium power battery pack, which will inevitably bring about the loss of the lithium power battery pack's capacity, which in turn leads to a decrease in life.
In the process of using the assembled lithium power battery, the inconsistency of the monomers will also appear due to the degree of self-discharge and the temperature of the parts. The inconsistency of the lithium power battery monomers affects the charging and discharging of the lithium power battery pack. characteristic. Studies have shown that a 20% difference in the capacity of lithium power battery cells will bring about 40% of the capacity loss of lithium power battery packs.
The meaning of lithium power battery balance is to use power electronic technology to keep the voltage deviation of the lithium ion lithium power battery cell or the lithium power battery pack voltage within the expected range, so as to ensure that each single lithium power battery is maintained during normal use. The same state to avoid the occurrence of overcharge and overdischarge. If the balance control is not carried out, as the charge and discharge cycles increase, the voltage of each single lithium power battery will gradually differentiate, and the service life will be greatly reduced.
The inconsistency of lithium power battery cells will further deteriorate over time under the influence of random factors such as temperature. Under normal circumstances, when the operating environment temperature of the lithium power battery is 10°C higher than its optimal temperature, the life of the lithium power battery will be reduced by half. Due to the large number of vehicular lithium power battery systems in series, generally between 88 and 100 series, their capacity is generally 20 to 60kWh, and the location of each string of lithium power batteries is different, which will cause a temperature difference.
Even in the same power battery box, there will be a temperature difference due to the location and heating of the lithium power battery, and this temperature difference will have a major negative impact on the life of the lithium power battery, causing the lithium power battery to appear unbalanced, and the cruising range will decrease. , The cycle life is shortened. It is precisely because of these problems that the capacity of the entire battery system cannot be fully used, causing battery system losses, and mitigating such system losses will also greatly extend the service life of the battery system.
The consistency between the lithium power battery cells is the most direct and most important influence on the lithium power battery capacity, because the lithium power battery capacity is a parameter that cannot be directly measured in a short time, but the lithium power battery cell capacity is There is a one-to-one correspondence between open circuit voltages. The voltage of a lithium power battery cell can be measured online in real time, which makes it a favorable condition for measuring the consistency level of a lithium power battery cell. In the management strategy of the battery management system, there are discharge termination conditions, charging termination conditions, etc., where the voltage value of the lithium power battery cell is used as the trigger condition.
For a parameter in this position, the excessive difference in the voltage consistency of the lithium power battery cells directly limits the charge and discharge power of the lithium power battery pack. Based on this, using the lithium power battery equalization method to solve the problem of excessive voltage difference of the lithium power battery pack that is already in operation is an effective measure to increase the capacity of the lithium power battery pack and prolong the service life of the lithium power battery.
Second, the advantages and disadvantages of passive equilibrium
In the equalization management of lithium power battery packs, the current methods for voltage equalization of series-parallel lithium power battery packs are divided into passive equalization and active equalization. Generally, energy consumption type balance is defined as passive balance. Passive balance uses resistors to consume the energy of high-voltage or high-charge batteries to achieve the purpose of reducing the gap between different batteries. It is an energy-consuming type. balanced. At present, there are many battery management systems that adopt passive balance in the market. Because passive balance technology is applied in the lithium power battery market before active balance, the technology is relatively mature, and the passive balance structure is simpler and more widely used.
The balance management of lithium power battery packs includes voltage balance, current balance, and temperature balance. Among them, the voltage balance of lithium power battery packs is the most basic, that is, the voltage balance of lithium power battery cells in series lithium power battery packs. Similarly, current balance refers to the balance of the current of each lithium power battery cell in the lithium power battery pack in parallel.
In lithium power battery packs, the reason why the performance of lithium power battery cells decays too quickly is that the current is inconsistent, and individual cells work in overrate conditions, resulting in excessive performance decay. The temperature difference of the lithium battery cells is caused by inconsistent heat generation and inconsistent heat dissipation. At present, the temperature balance of lithium power battery packs is generally solved by physical methods such as natural air cooling, forced air cooling, and liquid cooling.
Because passive equalization uses resistors to consume energy, heat is generated, and the equalization current is small, which reduces the efficiency of the entire system. Based on the requirements of thermal management, passive equalization can only be equalized section by section. Lithium power batteries are very sensitive to heat, and it is necessary to absolutely avoid the increase in external temperature. Passive equalization will cause local heating of the lithium power battery pack, and high temperature will increase the failure rate of components. For this reason, in view of the heat generated by passive equilibrium, special requirements are put forward for the safety and structural design of lithium power batteries.
3. The working principle of passive equilibrium
Passive equalization generally discharges lithium power batteries with higher voltage through resistance discharge, and releases electricity in the form of heat, so as to gain more charging time for other lithium power batteries. During the charging process, the lithium power battery generally has a charging upper limit protection voltage value. If the voltage during charging exceeds this value, which is commonly known as "overcharge", the lithium power battery may burn or explode.
Therefore, the lithium power battery protection board generally has an overcharge protection function to prevent the lithium power battery from overcharging. That is, when a string of lithium power batteries reaches this voltage value, the lithium power battery protection board will cut off the charging circuit and stop charging.
Charge equalization is in the middle and late stages of the power battery charging process, when the power battery cell voltage reaches or exceeds the cut-off voltage, the equalization circuit starts to work to reduce the power battery cell current, in order to limit the power battery cell voltage not to be higher than the charge cut-off voltage . The only function of charge equalization is to prevent overcharging, and it will bring negative effects during discharging use. When using charge equalization, the small-capacity power battery cell is not overcharged, and the amount of power that can be released is less than the power that can be released when the equalizer is not used for light overcharge, making the power battery cell discharge time shorter and possible overdischarge Sex is even greater.
The schematic diagram of the capacity loss of the lithium power battery pack during charging is shown in Figure 1. In Figure 1, the terminal voltage of the 2# lithium power battery is first charged to the set protection voltage value, which triggers the protection mechanism of the lithium power battery protection circuit and stops the lithium The charging of the power battery pack directly causes the 1#, 3##, and 4 lithium power batteries to be unable to be fully charged. The full charge capacity of the entire lithium power battery pack is limited to the 2# lithium power battery, causing the lithium power battery pack to fail to be fully charged. In order to fully charge the lithium power battery pack, an equalizing charging circuit must be used when charging.
During the charging process of the lithium power battery, each lithium power battery is equipped with an equalization circuit as shown in Figure 2 (each lithium power battery is connected with a parallel voltage stabilization equalization circuit), and each lithium power battery is controlled by the equalization circuit during charging. The voltage of the lithium power battery keeps each string of lithium power batteries in the same state, ensuring the performance and life of the lithium power battery.
If the voltage set by the lithium power battery equalization circuit is 4.2V, when the lithium power battery does not reach 4.2V, the parallel voltage regulator circuit does not work, each lithium power battery continues to be charged, and the charging current continues to pass through the lithium power battery. As shown in Figure 3.
When the 2# lithium power battery terminal voltage reaches 4.2V, the equalization circuit starts to work, and it will stabilize the voltage to 4.2V, that is, the charging current will no longer pass through the 2# lithium power battery, as shown in Figure 4. In this way, the charging time of the 1#, 3#, and 4# lithium power batteries is correspondingly extended, thereby increasing the power of the entire lithium power battery pack. However, 100% of the discharged power of the No. 2 lithium power battery is converted into heat release, causing a lot of waste (the heat dissipation of the No. 2 lithium power battery is a loss of the system and a waste of power).
The working principle of the shunt regulator circuit shown in Figure 2 is: TL431 is the reference voltage, and the voltage is adjusted to 4.2V by adjusting the variable resistance. If the two ends of the lithium power battery are less than 4.2V, the TL431 does not absorb current, that is, Ib=0 below, so Ic=0, the transistor is cut off, and the charging current still passes through the lithium power battery. If both ends of the lithium power battery reach 4.2V, TL431 starts to absorb current, Ib>0, and the charging current (ie Ic) passes through the triode and does not pass through the lithium power battery, that is, the lithium power battery is no longer charged.
The three diodes IN4001 connected in series in the circuit act as a voltage divider, which can reduce the power dissipated on the transistor TIP42. If these three diodes IN4001 are not connected, the power dissipated on the transistor TIP42: P=4.2V×charging current, after adding the diode IN4001, P=(4.2V-3×0.7V)×charging current. The light-emitting diode on the far right has an indication function. The light is on, indicating that the voltage has reached 4.2V, that is, the battery corresponding to this equalization circuit is fully charged.
Fourth, the characteristics of equalizing charging circuit based on shunt resistance
The simplest balance circuit is load consumption balance, that is, a resistor is connected in parallel to each lithium power battery, and a switch is connected in series for control. When the voltage of a lithium power battery is too high, the switch is turned on and the charging current is shunted through the resistor. In this way, the high-voltage lithium power battery has a small charging current, and the low-voltage lithium power battery has a large charging current. In this way, the voltage of the lithium power battery can be balanced, but this method can only be applied to small-capacity lithium power batteries. It is unrealistic for the capacity lithium power battery.
Connect resistors in parallel at both ends of the lithium power battery cell to allow the resistance to consume part of the energy of the lithium power battery. There are two forms of parallel resistance. One is a fixed connection. The resistor is connected in parallel at both ends of the lithium power battery for a long time. The voltage of the lithium power battery cell When it is high, the current through the resistor is large and consumes more power. When the voltage of the lithium power battery is low, the resistor consumes less power. Through the pressure-sensitive characteristic of resistance, the voltage balance of the lithium power battery terminal is realized. This is a theoretically feasible method and is rarely used in practice.
Analyze the necessity of lithium power battery equalization and the characteristics of passive equalization charging circuit
Another way to connect resistors in parallel is to connect resistors in parallel at both ends of the cell through a switch loop. The switch is triggered by a signal from the management system. When the system determines which cell voltage or SOC is high, it connects its parallel resistance to consume its energy.
The principle of balanced charging based on shunt resistance is shown in Figure 5, that is, each lithium power battery cell is connected in parallel with a shunt resistance. From the circuit shown in Figure 5, it can be seen that the shunt current on the resistance must be much larger than that of the lithium power battery. The self-discharge current can achieve the effect of balanced charging. Generally, the self-discharge current of a lithium power battery is about C/20000, so C/200 is more appropriate for the current flowing through the shunt resistor. In addition, the deviation of each shunt resistance is also an important factor affecting the equalization effect. After a certain number of charge and discharge cycles, the deviation of the lithium power battery cell can be determined by the following formula:
Analyze the necessity of lithium power battery equalization and the characteristics of passive equalization charging circuit
Where: VC is the voltage deviation of the lithium power battery; R is the shunt resistance; I is the self-discharge current of the lithium power battery; VD is the voltage of the lithium power battery cell; K is the resistance deviation.
If the shunt resistance is 20Ω±0.05%, the voltage deviation of the lithium power battery can be controlled within the range of 50mV. The average power of each resistor is 0.72W, but the shunt resistor always consumes power regardless of the charging process or the discharging process of the lithium power battery.
The principle of balanced charging based on shunt resistance with the addition of an on-off switch is shown in Figure 6. The difference between on-off shunt resistor balanced charging and resistance shunt balanced charging is the addition of an on-off switch, which can be controlled by the control system software , Can also be realized by simple logic circuits. The equalization circuit adopting this control mode only works in the constant voltage charging section of lithium power battery charging, and the on-off switch is always off at other times, so that when the lithium power battery pack is discharged, the shunt resistor does not consume energy. But the main disadvantage of this circuit is that the failure rate of the on-off switch is relatively high, and redundant means are required.
