Analyze the necessity of 18650 battery pack 12v balancing and the characteristics of passive balancing charging circuit

1. Definition of balanced charging and the necessity of balancing

1. Definition of balanced charging:

Balanced charging, referred to as equalizing charge, is charging that balances battery characteristics. It means that during the use of the battery, the voltage at the battery end is unbalanced due to individual differences, temperature differences, etc. of the battery. In order to avoid the worsening of this imbalance trend, it is necessary to Increase the charging voltage of the battery pack and charge the battery in a balanced manner to balance the characteristics of each battery cell in the battery pack and extend the service life of the battery.

Balanced 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 and reduces the power battery cell current in order to limit the power battery cell voltage to not be higher than the charging cut-off voltage. . The only function of balanced charging is to prevent overcharging, which will have a negative impact during discharge use.

When using equalizing charging, the small-capacity power battery cell is not overcharged, and the power it can release is less than the power that can be released by a slight overcharge without an equalizer, making the power battery cell's discharge time shorter and the possibility of over-discharging. Sex is even greater.

2. The necessity of balanced charging:

With the current lithium power battery manufacturing level and technology, during the production process of lithium power battery cells, there will be subtle differences in each lithium power battery cell, which is a consistency problem. The inconsistency is mainly manifested in the lithium power battery cells. Capacity, internal resistance, self-discharge rate, charge and discharge efficiency, etc. The inconsistency of the lithium power battery cells is transmitted to the lithium power battery pack, which inevitably leads to the loss of the lithium power battery pack capacity, thereby causing a decrease in life.

During the use of the assembled lithium-powered battery vehicle, the inconsistency of the lithium-powered battery cells will also occur due to reasons such as the degree of self-discharge and the temperature of the parts. The inconsistency of the lithium-powered battery cells will also affect the charge and discharge of the lithium-powered battery pack. characteristic. Studies have shown that a 20% capacity difference in a lithium power battery cell will cause a 40% capacity loss in the lithium power battery pack.

The meaning of lithium power battery balancing is to use power electronics technology to keep the lithium-ion lithium power battery cell voltage or lithium power battery pack voltage deviation within the expected range, thereby ensuring that each single lithium power battery maintains its normal performance during normal use. The same state to avoid overcharge and over-discharge. If 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 ambient 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. Since the number of series connections in the vehicle-mounted lithium power battery system is very large, generally between 88 and 100 series connections, and its capacity is generally between 20 and 60kWh, the different loading positions of each string of lithium power batteries will produce temperature differences.

Even in the same power battery box, there will be a temperature difference due to the different location and heating of the lithium power battery. This temperature difference will have a significant negative impact on the life of the lithium power battery, causing the lithium power battery to become unbalanced and reducing the cruising range. , 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. Slowing down such system losses will greatly extend the service life of the battery system.

The consistency between lithium power battery cells has the most direct and important impact on the capacity of lithium power battery, because the capacity of lithium power battery is a parameter that cannot be directly measured in a short time, but the capacity of lithium power battery cells is closely related to its 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, the voltage value of the lithium power battery cell as a trigger condition also includes discharge termination conditions, charging termination conditions, etc.

For a parameter in such a position, if the voltage consistency difference between lithium power battery cells is too large, it will directly limit the charging and discharging capacity of the lithium power battery pack. Based on this, using the lithium power battery equalization method to solve the problem of excessive voltage differences among the lithium power battery packs that are already in operation is an effective measure to increase the capacity of the lithium power battery pack and extend the service life of the lithium power battery.

2. Advantages and Disadvantages of Passive Equilibrium

In the balancing management of lithium power battery packs, the current voltage balancing methods for series and parallel lithium power battery packs are divided into passive balancing and active balancing. Energy consumption balancing is usually defined as passive balancing. Passive balancing uses resistors to consume the energy of high-voltage or high-charge cells in order to reduce the gap between different cells. It is an energy consumption type. balanced. Currently, there are many battery management systems using passive balancing on the market, because passive balancing technology has been applied in the lithium power battery market before active balancing, and the technology is relatively mature. The passive balancing structure is simpler and more widely used.

The balance management of lithium power battery packs includes voltage balance, current balance and temperature balance. The voltage balance of lithium power battery packs is the most basic, which refers to the voltage balance of lithium power battery cells in series lithium power battery packs. Similarly, current balancing refers to the balancing of the current of each lithium power battery cell in a parallel lithium power battery pack.

In lithium power battery packs, the reason why the performance of lithium power battery cells decays too quickly is because the current is inconsistent and individual cells work in over-rate conditions, causing the performance to decay too quickly. The temperature difference of lithium power battery cells is caused by inconsistent heating 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.

Since passive balancing uses resistors to consume energy, it will generate heat and the balancing current is small, thus reducing the efficiency of the entire system. Based on the requirements of thermal management, passive balancing can only balance section by section. Lithium-powered batteries are very sensitive to heat, and it is necessary to absolutely avoid an increase in external temperature. Passive equalization will cause local heating of the lithium power battery pack. In addition, high temperatures will increase the failure rate of components. For this reason, special requirements are put forward for the safety and structural design of lithium power batteries in view of the heat generated by passive equalization.

3. Working principle of passive equalization

Passive equalization generally discharges higher-voltage lithium power batteries through resistive discharge, releasing power in the form of heat to gain more charging time for other lithium power batteries. During the charging process, lithium-powered batteries generally have a charging upper limit protection voltage value. If the voltage during charging exceeds this value, which is commonly known as "overcharging", the lithium-powered battery may burn or explode.

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