Equalization function of lithium battery protection board
Commonly used equalization charging techniques for the equalization principle of lithium battery protection boards include constant shunt resistance equalization charging, on-off shunt resistance equalization charging, average battery voltage equalization charging, switched capacitor equalization charging, step-down converter equalization charging, inductance equalization charging, etc. When charging a group of lithium batteries in series, ensure that each battery is charged in a balanced manner, otherwise the performance and life of the entire group of batteries will be affected during use. However, the existing single-cell lithium battery protection chips do not have a balanced charge control function. The balanced charge control function of a multi-cell lithium battery protection chip requires an external CPU; it is realized through serial communication with the protection chip (such as I2C bus). The complexity and design difficulty of the protection circuit are reduced, the efficiency and reliability of the system are reduced, and the power consumption is increased.
According to the needs of the application, the balancing principle of the lithium battery protection board can realize the protection and equalization of the power lithium battery pack of any structure and voltage level after changing the protection chip model and the number of series, the power level of the switching devices and the power consumption components in the circuit .
As a battery with relatively large electric storage, lithium batteries are used in various computer rooms, communication base stations, data centers and other fields. The lithium battery protection board has the function of protecting the battery and avoiding battery overcharging.
Lithium battery protection board passive balance and active balance:
The battery itself still has available capacity, but because of the imbalance between the batteries and the safety voltage limit set to protect the battery, the battery system cannot continue to perform its due performance. In addition, the service life of the battery in the vehicle is shorter than that of the vehicle itself. Even if the vehicle has not reached the end of life, the battery must be replaced in order to meet the power performance. However, the cost of replacing the battery is quite high, so this restricts the development of electric vehicles to a large extent.
The main cause of battery imbalance is temperature. In general, when the ambient temperature of the lithium-ion battery is 10°C higher than its optimal temperature, the life of the lithium-ion battery will be reduced by half. Due to the large number of car battery systems connected in series, generally between 88 and 100 series, their capacity is generally between 20 and 60 kWh. The location of each string of batteries is different and there will be a temperature difference. Even in the same battery box, there will be a temperature difference due to the location and heating of the battery, and this temperature difference will have a major negative impact on the battery life, causing the battery to appear unbalanced, reducing the cruising range and shortening the cycle life. 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.
Picture 1: Battery system loss and equalization effect
As shown in picture 1, the initial capacity of the battery system is 100%. During use, the battery will gradually decay due to various reasons (mainly temperature), which is a characteristic of lithium batteries. This part of the attenuation cannot be recovered by equalization. The main reason for the decrease in system capacity is the system loss caused by the unbalanced battery capacity. System loss does not mean that all battery capacity is reduced, but that the battery system has capacity and cannot be used due to imbalance.
Under normal circumstances, when the battery capacity drops to 70% to 80%, the battery will be replaced to maintain the continuous range. The longer the battery capacity remains above 70%, the lower the cost of the electric vehicle. Without equalization and general passive equalization technology, the capacity of the battery system will drop below 70% in less than 3 years (full charge once a day). A better passive balance can barely maintain the battery capacity at 70%. In sharp contrast to this, a better active equalization can minimize system losses. Such active balancing can effectively reduce the system loss caused by the unbalanced capacity, thereby prolonging the service life of the battery system, delaying the replacement period of the battery system, and increasing the cruising range at the same time. Passive equilibrium and active equilibrium
The lithium battery protection board, which plays an important role in the battery system, has gradually gained attention as an effective means to extend the battery life. Among them, the lithium battery protection board equalization system that plays a key role has also attracted widespread attention. At present, there are two ways of balancing and multi-series battery systems in the market: traditional passive balancing and active balancing.
1. Passive balance
Passive balancing generally discharges the battery with a higher voltage through resistance discharge, and releases the power in the form of heat, so as to obtain more charging time for other batteries. In this way, the power of the entire system is limited by the battery with the least capacity. During the charging process, lithium batteries generally have a charging upper limit protection voltage value. When a certain string of batteries reaches this voltage value, the lithium battery protection board will cut off the charging circuit and stop charging. If the voltage during charging exceeds this value, which is commonly known as "overcharge", the lithium battery may burn or explode. Therefore, lithium battery protection boards generally have an overcharge protection function to prevent overcharge of the battery.
Picture 2: Reasons for system loss caused by battery protection
Picture 3: Working principle during passive equalization charging
As shown in Figure 2, during the charging process, the No. 2 battery is first charged to the protection voltage value, which triggers the protection mechanism of the lithium battery protection board to stop the charging of the battery system, which directly causes the No. 1 and No. 3 batteries to be fully charged. The full charge capacity of the entire system is limited by the No. 2 battery, which is the system loss. In order to increase the power of the battery system, the lithium battery protection board will balance the battery during charging. As shown in Figure 3, after the equalization starts, the lithium battery protection board will discharge the No. 2 battery, delaying the time for it to reach the protection voltage value, so that the charging time of the No. 1 and No. 3 batteries will be extended accordingly, thereby improving the entire battery system Power. However, 100% of the discharged power of the No. 2 battery is converted into heat release, resulting in a lot of waste (the heat dissipation of the No. 2 battery is a loss of the system and a waste of power).
Picture 4: Unable to balance when passively balanced discharge
As shown in picture 4, in addition to overcharging will have a serious impact on the battery, overdischarging will also cause serious damage to the battery. Similarly, the lithium battery protection board has an over-discharge protection function. When discharging, when the voltage of the No. 2 battery reaches the discharge protection value, the protection mechanism of the lithium battery protection board is triggered to stop the system discharge, which directly causes the battery remaining capacity of the No. 1 and No. 3 batteries to be not fully used, and the system will be improved after the equalization starts. Over-discharge.
The advantages of passive balancing are low cost and simple circuit design. The disadvantage is that balancing is based on the lowest battery residual capacity, which cannot increase the capacity of batteries with low residual capacity, and 100% of the balanced power is wasted in the form of heat.
2. Active balance
Active balancing is based on the way of power transfer, which has high efficiency and low loss. Different manufacturers have different methods, and the equalizing current also ranges from 1 to 10 A. At present, many active equalization technologies appearing on the market are immature, leading to battery over-discharge and accelerating battery degradation. Most of the active equilibrium in the market adopts the principle of voltage transformation, relying on the expensive chips of chip manufacturers. In addition to the equalization chip, this method also requires expensive transformers and other peripheral components, which is larger in size and higher in cost.
Picture 5: Principle of active equalization in variable voltage mode
As shown in picture 5, every 6 series of batteries is a group, and the total power of the 6 series of batteries is transferred to a battery with a small capacity. Inductive active balancing is based on physical conversion, integrating power switches and micro-inductors, and adopts a two-way balancing method to balance the battery through charge transfer between adjacent or adjacent batteries, and it can be used regardless of whether the battery is discharged, charged or left standstill. Perform equalization, and the equalization efficiency is as high as 92%.
Picture 6: Working principle of inductive active equalization charging
Picture 7: Working principle of inductive active equalization discharge
The working principle of discharging and charging, as shown in Figure 6 and Figure 7, the No. 2 battery transfers the power to the No. 1 and No. 3 batteries. Efficient charge transfer keeps the voltage of the three batteries in an equilibrium state during charging, so that all batteries can be fully charged. The lithium battery protection board can also balance the battery when discharging. The No. 1 and No. 3 batteries transfer the power to the No. 2 battery, and the voltages of the 3 batteries are always discharged in a balanced state, so that all the batteries can be used up.