1 What is BMS active balancing
BMS active balancing is different from passive balancing. Passive balancing generally discharges the energy of high-voltage batteries through resistors to maintain an equal state with the power of low-voltage batteries. This method has disadvantages such as low energy utilization efficiency, heat dissipation, low balancing current, and slow effectiveness. Active balancing, on the other hand, is the process of transferring energy from high-energy batteries to low-energy batteries, much like cutting off the strengths and weaknesses of a wooden board.
At present, there are various active balancing solutions, except for the Fit capacitor solution which has not become mainstream due to its low number of applicable strings and limitations in transfer, there are also transformer solutions and battery specific DCDC conversion chips designed by semiconductor manufacturers that have been introduced to the market. The benefits of active balancing are obvious, with high efficiency, energy transfer, and only transformer coil losses, accounting for a small proportion; The balanced current can be designed to be large, reaching several amperes or even 10A level, and the balancing effect is fast.
2 The role of BMS active balancing in energy storage systems
(1) Enhance the overall performance of the battery pack
1. Relieve the power imbalance caused by inconsistent individual cells in the battery pack, and improve the overall capacity and power performance of the battery pack.
For example, transformers are widely used in active balancing to balance the energy distribution within the battery pack through bottom and top balancing strategies. There are also balancing methods based on energy storage components such as capacitors and inductors, or based on DC-DC converters. These non energy consuming balancing methods mainly transfer energy between individual cells or between individual cells and the entire battery pack through capacitors, inductors, or DC-DC converters. Compared with energy consuming balancing structures, they are more complex, but have higher energy utilization efficiency, flexible energy transfer, and can effectively improve the overall performance of the battery pack.
2. By detecting the status of each individual cell in the battery pack, use balancing methods to maintain the voltage or state of charge between individual cells within a certain range.
The BMS active balancing system evaluates the working status of the battery by continuously monitoring key parameters such as voltage, current, and temperature of each battery cell. When a difference in voltage or state of charge between individual batteries is detected, the balancing method is activated. For example, using a bidirectional forward DC-DC converter as the balancing main circuit, when energy is transferred from the high voltage side to the low voltage side, the four switching transistors work according to specific driving signal conduction timing to achieve bidirectional energy transfer from the low voltage side U1 of the unit to the high voltage side U2 and from the high voltage side U2 to the low voltage side U1 of the unit, thereby maintaining the voltage or state of charge between the unit batteries within a certain range.
(2) Extend the lifespan of the battery pack
1. Inhibit the occurrence of consistency between battery cells and reduce the impact of battery dispersion on the lifespan of the battery pack.
2. It can increase the available capacity of the battery system and significantly improve the cycle life.
BMS active balancing technology effectively eliminates the problem of capacity imbalance between battery cells through energy transfer, improving the performance of the entire battery pack. Adopting the independently developed bidirectional DC-DC active balancing chip by Kelie, compared with traditional balancing chips, the innovative embedded advanced intelligent algorithm quickly and effectively compensates for the differences generated by the battery pack through energy transfer, ensuring battery consistency, extending the service life and mean time between failures of the battery pack, and effectively improving the economic benefits of the entire product life cycle. Long term cyclic testing data shows that this active balancing technology can increase the available capacity of the battery system by more than 10%, improve the cycle life by more than 20%, and the more series connected, the more significant the improvement effect.
3 The working principle of BMS active balancing
(1) Composition of equilibrium system
The BMS active balancing system mainly consists of a series battery module, a 12V battery pack, a switch array, a balancing main circuit, a voltage acquisition circuit, and a microcontroller control circuit.
1. Switch array:
The switch array consists of battery cell gating switches and battery polarity gating switches, which can achieve gating of the cells that need to be balanced. For example, for a 7-cell series connected battery pack, there are specific switch combinations to select different batteries. Taking the selection of battery 1 and battery 2 as an example, when battery 1 is selected, switches K1, K2, KP3, and KP4 are turned on, and other switches are turned off, forming a specific charging and discharging circuit; When selecting battery 2, switches K2, K3, KP1, and KP2 are turned on, and other switches are turned off, forming a corresponding charging and discharging circuit. Odd cell gating can refer to the gating switch combination of battery 1, and even cell gating can refer to the gating switch combination of battery 2.
2. Balanced main circuit:
Adopting a bidirectional forward DC-DC converter to achieve bidirectional energy transfer. This topology mainly includes a transformer T, two sampling resistors R1 and R2, two filtering capacitors C1 and C2, a clamping capacitor C3, a filtering inductor L, and four switching transistors Q1 to Q4.
(2) Working mode
1. Energy is transferred from the low pressure side of a single unit to the high pressure side.
2. Energy is transferred from the high voltage side to the low voltage side of a single unit, which is divided into four stages. The transfer and release of energy are achieved through the conduction and disconnection of the switch tube:
Stage 1: From time t1 to t2, the switching tubes Q2 and Q3 are turned on. At this time, the input current I1 flows into the same terminal of the transformer's high voltage side winding, and the output current I2 flows out of the same terminal of the transformer's low voltage side winding. High voltage side U2 simultaneously transfers energy to low voltage side U1 and inductor L.
Stage 2: From time t2 to t3, switches Q2 and Q3 are turned off, I2 is continued by the body diodes of switches Q1 and Q2, IT2 gradually decreases, IQ1 gradually increases, and the energy stored in inductor L and the residual magnetic energy of the low-voltage winding are released to the low-voltage side.
Stage 3: From time t3 to t4, switch Q1 is turned on, I2 is continued by switch Q1, and the energy stored in inductor L is released to the low-voltage side U1.
Stage 4: From time t4 to t5, switch Q1 is turned off, I2 is continued by the body diode of switch Q1, and the energy stored in inductor L continues to be released to the low-voltage side U1. Among them, stage two and stage four are both dead zone stages, in order to prevent Q1 from short circuiting the low-voltage winding when Q2 and Q3 are conducting. The switch Q4 is connected in series with the clamping capacitor C3, and parallel connected at both ends of the switch Q3 for active clamping and transformer magnetic reset.
4 The current application status of BMS active balancing in energy storage systems
(1) Market participating enterprises
The installed capacity of BMS produced by vehicle manufacturers accounts for about 21.3% of the total, the installed capacity of BMS produced by power lithium battery factories accounts for about 45.4%, and professional BMS manufacturers account for about 33.3% of the share. Although vehicle manufacturers and battery manufacturers still hold important positions, with the trend of technological advancement and specialized division of labor, professional BMS manufacturers are rising strongly and have become dominant in the commercial vehicle field, and are expected to make a big impact in the energy storage field.
(2) Existing issues
1. China's energy storage BMS started relatively late, with incomplete standards and no unified control strategy. Although there are framework standards in place, each company has different requirements for high-voltage boxes and energy storage wiring harnesses, which leads to high installation and commissioning costs, multiple faults, and difficult operation and maintenance of energy storage systems. The relevant departments of the country are formulating industry related standards, which are expected to further regulate the BMS industry, ensure the safety and service life of batteries, and reduce the cost of energy storage systems through standardization and scaling.
2. The reliability of active balancing technology still needs to be further improved, and the cost needs to be further reduced. At present, the structure of active equilibrium is complex and the cost is much higher than that of passive equilibrium. For example, the common method of active balancing using transformers for DC-DC charging and discharging is complex in structure, and the design and control of switch matrices and drivers are difficult, which also limits the full integration of active balancing function into dedicated ICs. Moreover, complex structures inevitably lead to complex circuits, and the increase in cost and failure rate is inevitable, which also limits the promotion of active balancing BMS.
3. The energy storage BMS algorithm is just starting, and there is still room for improvement in estimating progress, algorithm convergence, and robustness. Especially the battery warning algorithm is very important in energy storage systems, but it is still almost a blank in the industry in China. Overall, the BMS industry in the energy storage field has a low overall level, with a variety of BMS production enterprises and uneven product quality. Some enterprises have insufficient understanding of energy storage systems. This leads to BMS always ranking high in the component failure ranking of the entire energy storage system.
5 The development trend of BMS active balancing in energy storage systems
(1
With the continuous improvement of performance requirements for energy storage systems, the advantages of active balancing technology are becoming increasingly prominent. It can effectively improve the consistency of the battery pack, thereby enhancing the overall performance of the energy storage system. In practical applications, active balancing technology can transfer energy from high-energy batteries to low-energy batteries, achieving energy balance in the battery pack, just like cutting off the strengths and weaknesses of wooden boards. This technology not only has high efficiency and low losses, but also has a large balanced current and quick results. Therefore, active balancing technology will be widely applied in energy storage battery management systems and become a future development trend.
(2) Localization of key components
(3) Improved product integration