When designing battery packs, the choice between series and parallel can have a significant impact on battery performance, safety, and longevity. The design of the battery pack is complex and delicate, requiring in-depth understanding of some key technical issues. In this paper, the technical challenges in battery series and parallel design are discussed from ten aspects, such as battery consistency, voltage and current equalization, and thermal management system design.
1. The battery is inconsistent
Battery consistency refers to the similarity of each battery unit in terms of capacity, voltage, internal resistance, discharge curve, etc. Differences in battery performance can cause problems in series or parallel battery packs. For example, in a series configuration, the overall performance of the battery pack is affected by the worst cell; In a parallel configuration, performance differences can lead to uneven current distribution, which affects the life and efficiency of the entire battery set.
Problem: In the series configuration, if one battery is low, the entire battery pack will not be able to discharge completely, which will cause power waste and efficiency reduction; In parallel configuration, the current load of different batteries is different, which is easy to cause local overheating and performance degradation of batteries.
Solution:
Battery screening: In the production process, high-precision testing equipment should be used to strictly screen each battery unit to ensure a high degree of consistency of parameters such as capacity, voltage and internal resistance. Common screening methods include OCV test (open circuit voltage), internal resistance test and capacity test. These tests effectively weed out substandard batteries and avoid assembling inconsistent battery cells together.
Balancing circuit design: Integrate active or passive balancing circuits in the battery management system (BMS). The active equalization circuit can transfer electricity through switching inductance or capacitance to realize energy redistribution in the battery pack. Passive equalization adjusts the battery voltage by consuming excess energy. Active balancing is complex but efficient, suitable for large-capacity battery packs, and passive balancing is suitable for small and medium-sized battery packs.
2. Balance voltage and current
In series batteries, the problem of voltage equalization is the key to ensure uniform charging and discharging of each cell. A battery pack without a voltage equalization circuit will overcharge or undercharge part of the battery, which will affect the life of the entire battery pack. The parallel battery pack faces the problem of current balance, and the difference of internal resistance leads to uneven current distribution, which is easy to make some batteries bear greater current load.
Specific problems: Uneven voltage series battery packs can cause some batteries to overcharge and damage, or early depletion during discharge; The unbalanced current of the parallel battery pack will accelerate the aging of the battery unit and shorten the life of the battery.
Solution:
Active balancing circuit: Through the combination of inductors, capacitors and control chips, intelligent power transfer, to achieve efficient voltage balancing. This method can effectively reduce the internal consumption of the battery and improve the charge and discharge efficiency of the whole battery pack. Common balancing methods include flying capacitance balancing and switching inductance balancing. You need to select an appropriate solution based on the battery application scenario.
Passive equalization circuit: Through the resistance to consume the excess power of the high voltage battery. This method is simple, low cost, but low efficiency and heat generation, suitable for balanced treatment of small battery packs. During the design, attention should be paid to the power and heat dissipation performance of the resistor to prevent local overheating during the equalization process.
3. Thermal management system design
The battery will generate a lot of heat in the work, especially when the large current discharge is more obvious. If the heat is not effectively distributed, the battery temperature will gradually increase, resulting in reduced battery performance, shortened life, and even the risk of thermal runaway.
Specific problems: Uneven temperature will cause some battery units to overheat, resulting in increased internal resistance, electrolyte decomposition and other problems, and even lead to battery pack fire.
Solution:
Thermal conductivity materials and heat dissipation design: In the design of the battery pack, materials with high thermal conductivity can be added, such as aluminum alloy heat dissipation plates, thermal silicone pads, etc., which can quickly export the heat generated by the battery unit. Optimize the air flow channels inside the battery pack to improve heat dissipation by natural convection or fan forced convection. For high power applications, liquid cooling heat dissipation can be considered, and heat dissipation efficiency is higher by absorbing heat through circulating coolant.
Temperature monitoring and active cooling: A temperature sensor is integrated into the BMS to monitor the temperature of each cell in real time. If the temperature is too high, the system automatically reduces the load or adjusts the heat dissipation scheme. At the same time, the intelligent cooling system can automatically start the fan or liquid cooling system according to the temperature status of the battery to avoid battery failure caused by overheating.
4. Structural design of battery pack
The structural design of the battery pack should not only ensure the stability of the battery unit, but also optimize the ease of electrical connection, heat dissipation and maintenance. Unreasonable structural design will lead to poor battery contact, vibration damage, and even short circuit and other problems.
Specific problems: Unreasonable structure may cause the battery to loosen, wear, increase the internal contact resistance, resulting in local heating and performance degradation.
Solution:
Modular design: The modular battery pack design enables a single battery module to be independently packaged and easily connected, and the damaged module can be quickly replaced to improve the maintenance convenience of the battery pack. The seismic structure design is adopted between modules to reduce the influence of external force on the battery unit.
Protective materials and design enhancement: Use shock-absorbing foam, rubber pads and other materials to wrap the battery module to reduce external shock and shock. The housing materials should be fireproof, waterproof, corrosion-resistant high-strength materials such as aluminum alloy, stainless steel or engineering plastics, and ventilation holes and heat sinks should be designed to optimize thermal management.
5. Battery Management System (BMS)
The BMS (Battery Management System) is the control center of the battery pack, which is responsible for real-time monitoring and management of the battery's power, voltage, current and temperature parameters. The functions of BMS include battery balancing, fault diagnosis, charge and discharge control, etc. Its performance determines the overall safety and efficiency of the battery pack.
Specific problems: If the BMS is not properly designed, the abnormal state may not be detected in time, resulting in problems such as overcharging, overdischarging, or overheating of the battery pack.
Solution:
High-precision monitoring sensors: High-precision voltage, current, and temperature sensors are embedded in the BMS, which can accurately detect the status parameters of the battery and transmit them to the BMS control unit in real time. The control unit analyzes the battery's charging and discharging behavior and balance adjustment through built-in algorithms to ensure system safety.
Intelligent algorithms and data analysis: Advanced battery management algorithms, such as neural networks, machine learning and other technologies, are used to analyze the usage data of the battery to predict the health status and remaining life of the battery. Based on the algorithm analysis results, BMS actively optimizes the charging and discharging strategy to maximize the battery life.
6. Battery protection circuit design
The battery protection circuit is an important safety measure to prevent overvoltage, undervoltage, short circuit and overcurrent of the battery pack. If the battery pack does not have a suitable protection circuit, it is easy to cause danger in extreme cases, and it will cause fire or explosion in serious cases.
Specific problems: The missing or poorly designed protection circuit may cause the battery to be damaged during charging and discharging, or fail to cut off the current during a short circuit.
Solution:
Multiple protection mechanism: Design and integrate multiple protection circuits, including overvoltage protection, undervoltage protection, overcurrent protection, short circuit protection, etc. Each protection circuit is designed to work independently of the BMS, ensuring that the battery is protected in the event of a master failure. For example, overcurrent protection can be designed through the combination of MOS tube and fast fuse to achieve instantaneous power failure function.
Two-way protection and isolation design: The two-way protection circuit design can simultaneously monitor the charging and discharging process of the battery to prevent the battery from being affected by abnormalities at the charger and load end. The isolation circuit between battery packs is designed to avoid the current backflow between battery cells and improve the overall electrical safety.
7. Efficiency and energy loss
The energy loss in series parallel battery design mainly comes from the internal resistance, the resistance of the connector, and the power consumption of the BMS and the protection circuit. At high current operation, these losses will further increase, directly reducing the overall efficiency and endurance of the battery pack.
Specific problems: Low efficiency will lead to shorter battery life, affecting the actual use experience of the device, and long-term energy loss will also increase the heat of the battery pack.
Solution:
Optimized connection materials: Use low resistance, high conductivity connection materials such as copper foil, tinned copper strip or aluminum alloy strip. Optimize the welding process, through laser welding, ultrasonic welding and other advanced technologies to ensure the fastness and low resistance of the connection point, reduce the loss of electric energy at the connection point.
Improved BMS energy efficiency: BMS should be designed with energy efficiency optimization in mind to avoid unnecessary power consumption. Reduce the energy consumption of the BMS itself through the use of low-power chips and smart sleep modes. Energy recovery modules can also be added to the BMS to recover and reuse the redundant energy inside the battery pack to improve the overall energy efficiency.
8. Safety and reliability
Battery packs need to deal with a variety of harsh environments such as vibration, impact, and high temperature in actual use, and the safety and reliability design of batteries is crucial. The failure of a single battery can cause problems with the entire battery pack, leading to equipment failure and even endangering user safety.
Specific problems: The failure of any one unit in the battery pack can spread, resulting in the overall failure of the battery pack or a safety accident.
Solution:
Multi-layer protection and redundant design: When designing battery packs, multi-layer protection should be considered, such as adding safety components such as fire partitions and explosion-proof valves to the structural design. The redundant design allows the rest of the battery to work properly when some units fail, thereby improving overall safety.
Strict testing and certification: battery packs need to undergo strict environmental adaptability testing before production, including vibration test, drop test, thermal cycle test, etc. Through UL, CE, UN38.3 and other international certifications, to ensure that the battery pack meets international safety standards, enhance user confidence.
9. Dynamic response features
Battery packs need to respond quickly to load changes to ensure device stability. A battery pack with a slow dynamic response will cause unstable device operation and significant voltage fluctuations, affecting user experience.
Specific problems: Insufficient battery response can cause the device to speed up and slow down, or the phenomenon of voltage drop when the load changes dramatically.
Solution:
High-rate battery selection: Select high-rate battery cells with excellent dynamic response characteristics, which can quickly adjust the output when the load changes rapidly. Combined with the fast response algorithm of BMS, the output dynamic characteristics of the battery can be further optimized.
Inductance filter and capacitance compensation: The inductance filter and capacitance compensation network are designed at the output end of the battery pack to reduce voltage fluctuations and improve the dynamic response ability of the battery. This design mitigates voltage fluctuations and ensures stable operation of the equipment under high load variations.
10. Connection technology and material selection
The connection part of the battery pack needs excellent electrical and thermal performance. Poor connection will lead to increased resistance, contact heating and even melting, which may cause battery pack failure in serious cases.
Specific problems: Improper connection materials or poor process may increase the resistance of the battery pack, causing local overheating, resulting in energy loss and safety hazards.
Solution:
Select high quality connectors and welding technologies: Welding methods such as laser welding or ultrasonic welding are commonly used in battery pack connections. These methods ensure low resistance and high mechanical strength at the connection site and avoid poor contact due to vibration or temperature changes.
Use heat-resistant insulation materials: The connection parts in the battery pack should be wrapped with heat-resistant and wear-resistant insulation materials, such as heat-resistant tape and Teflon insulated tubes. These materials can effectively prevent short circuit or leakage at the connection and improve the safety and service life of the battery pack.
