Why do lithium battery packs bulge?There are three main reasons for lithium battery bulging:①Manufacturer production process problemsThe battery coating is uneven, and dust particles are mixed into the electrolyte. These may cause the lithium battery pack to bulge when the user uses it.②User daily use habitsIf the user uses lithium battery products improperly, such as overcharging and overdischarging, or continuous use in extremely harsh environments, it may also cause lithium batteries to bulge. Among them, the reasons for bulging caused by overcharging and overdischarging are as follows:Bulging caused by overcharging: Overcharging will cause all the lithium atoms in the positive electrode material to run into the negative electrode material, causing the originally full grid of the positive electrode to deform and collapse, which is also a major reason for the decrease in lithium battery power. In this process, the number of lithium ions in the negative electrode increases, and excessive accumulation causes lithium atoms to grow crystals, causing the lithium battery to bulge.Bulging caused by overdischarge: During the first charge and discharge process of liquid lithium-ion batteries, the electrode material and the electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material. The formed passivation layer can effectively prevent the passage of electrolyte molecules, but Li+ can be freely embedded and removed through the passivation layer, which has the characteristics of a solid electrolyte. Therefore, this passivation film is called "solid electrolyte interface", or SEI for short. The SEI film protects the negative electrode material, making the material structure less likely to collapse, and can increase the cycle life of the electrode material. The SEI film is not static, and there will be a little change during the charge and discharge process, mainly because some organic matter will undergo reversible changes. After the battery is over-discharged, the SEI film is reversibly destroyed, and the SEI that protects the negative electrode material is destroyed, causing the negative electrode material to collapse, thus forming a bulging phenomenon.These two factors will cause a violent reaction similar to a short circuit to occur inside the battery during use, generating a large amount of heat, which will cause the electrolyte to decompose and gasify, and the battery will bulge.③ Long-term non-use and improper storageThe battery has not been used for a long time and has not been well preserved. When it is exposed to air for a long time and is not used, and the battery is fully charged. Since air is conductive to a certain extent, if the battery is left for too long, it is equivalent to the positive and negative poles of the battery being in direct contact, causing a chronic short circuit. Once a short circuit occurs, heat will be generated, and some electrolytes will decompose or even vaporize, leading to bulging.The capacity of bulging lithium batteries is seriously damaged and there are safety hazards. It is recommended not to use them.Waste lithium batteries should be processed through professional recycling channels to prevent environmental pollution.

1. The impact of low battery temperature on battery discharge capacityCapacity is one of the most important parameters of lithium batteries, and its size changes with temperature. The two curves in the figure below are temperature capacity curves obtained by discharging the battery at 0.1C and 0.3C at different temperatures.Obviously, as the temperature rises, the capacity gradually increases. The capacity at -20℃ is only about 60% of the capacity at 15℃. In addition to capacity, the open circuit voltage of the battery will also decrease with increasing temperature. We all know that the energy contained in the battery is the product of capacity and terminal voltage. When both multiples decrease, the energy in the battery must be the superposition of the two decreasing effects.When the battery temperature is low, the activity of the positive electrode material decreases, which reduces the number of lithium ions that can move and bring discharge current, which is the fundamental reason for the decrease in capacity.2. The influence of low battery temperature on battery internal resistanceThe relationship between lithium battery temperature and resistance is shown in the figure below. Different curves represent different charging levels of the battery itself. In any charging situation, the internal resistance of the battery will increase significantly as the temperature decreases. The lower the charge, the greater the internal resistance, and this trend remains unchanged with temperature changes.When the battery temperature is low, the diffusion and mobility of charged ions in the positive and negative electrode materials become worse, and it is difficult to pass through the passivation film between the electrode and the electrolyte. The transfer speed in the electrolyte is also reduced, and a large amount of heat is generated during the transfer process.After the lithium ions reach the negative electrode, the diffusion inside the negative electrode material also becomes unsmooth. Throughout the process, the movement of charged ions becomes very difficult. From the outside, it means that the internal resistance of the battery cell has increased.3. The impact of low battery temperature on battery charging and discharging efficiencyThe following curve is the curve of charging efficiency changing with temperature. We can observe that the charging efficiency at -20℃ is only 65% of that at 15℃.Low battery temperature brings about changes in the above-mentioned electrochemical properties, and the internal resistance increases significantly. During the discharge process, a large amount of electrical energy is consumed on the internal resistance and generates heat.Lithium battery low temperature preheating technologyFaced with the restrictions on the use of lithium batteries when the battery temperature is low, technicians have found a countermeasure for charging and preheating. Although it is a stopgap measure, it has a significant effect on improving the discharge capacity and long-term life of lithium batteries.Before charging or using lithium batteries in an environment with low battery temperature, the battery must be preheated. The way the battery management system (BMS) heats the battery can be roughly divided into two categories: external heating and internal heating.Compared to external heating methods, internal heating avoids long-path heat conduction and the formation of local hot spots close to the heating device. As a result, internal heating can heat the battery more evenly, resulting in better heating with higher efficiency and is easier to implement.

Effect of charge and discharge rate on battery performance1. Problems with rapid charge and discharge① Battery heatingRapid charge and discharge will generate a large current inside the battery, causing the battery to heat up. Excessive temperature may affect the performance and life of the battery.② Increased internal stressLarge current charge and discharge will generate greater stress inside the battery, which may cause deformation or even damage to the battery structure.③ Poor contact between active material and current collectorRapid charge and discharge may cause poor contact between active material and current collector, reduce the conductivity of the battery, and affect the charge and discharge efficiency of the battery.2. Limitation of high-rate dischargeWhen discharging at a high rate, the energy output of the battery will be limited. This may not meet the needs of certain high-performance applications, such as acceleration and climbing of electric vehicles. Effect of charge and discharge rate on battery capacityThe effect of charge and discharge rate on battery capacity is mainly reflected in the following aspects:1. When charging quickly, the chemical reaction inside the battery may not proceed evenly. This will cause the active material in some areas to over-react, while other areas will not react fully. If this continues for a long time, the battery capacity will gradually decrease.2. Rapid discharge will also have an adverse effect on battery capacity. High-rate discharge will accelerate the chemical reaction inside the battery, generating more heat and pressure. This may cause damage to the internal structure of the battery, deactivate the active substances, and thus reduce the capacity of the battery. For example, in some high-power application scenarios, such as power tools, frequent rapid discharge will cause the battery capacity to decay rapidly.3. Unreasonable charge and discharge rates will also affect the cycle life of the battery. Frequent rapid charge and discharge will cause the chemical structure inside the battery to change continuously, accelerate the aging of the battery, thereby reducing the number of times the battery can be charged and discharged, and further reducing the battery capacity.

Why do we need to balance?Due to the differences in battery use process and materials, as well as the differences in temperature, humidity and other environments during the actual use of the battery, there are differences in the SOC of the single cells in the battery pack. The difference in SOC is intuitively reflected in the different voltages of the batteries.Assuming that the SOC of a battery in the battery pack is higher than that of other cells, this battery will be fully charged first during the charging process, causing the charging of other cells to stop before reaching the rated capacity; similarly, assuming that the SOC of a battery is lower than that of other cells, it will reach the discharge cut-off voltage first during the discharge process, causing other cells to have residual capacity that cannot be released;Therefore, we can draw a conclusion that batteries are different.1. Definition and significance of battery balancingDefinition: Battery balancing refers to the use of specific technologies and methods to make each battery cell in the battery pack reach a relatively consistent state in voltage, capacity and state, thereby improving the performance and life of the entire battery pack.Significance: Improve battery pack performance: Through balancing, the performance degradation of the entire battery pack caused by the degradation of individual battery performance can be avoided.Extend battery life: Balancing can reduce the voltage and capacity differences between battery cells, reduce the internal resistance of the battery, and thus extend the battery life.Improve safety: Balancing can prevent overcharging or over-discharging of battery cells and reduce the risk of safety hazards such as thermal runaway.2. Battery balancing methodThe following mainly introduces the BMS balancing function. Through the BMS balancing function, the inconsistency between each battery cell can be reduced and the available capacity of the battery pack can be increased. At present, the main balancing methods used are passive balancing (energy dissipation balancing) and active balancing (non-energy dissipation balancing, energy transfer balancing).There are two main methods for battery balancing: active balancing and passive balancing.Active balancing: Active balancing is a technology that achieves voltage balancing between battery cells by energy transfer. It achieves more accurate balancing by transferring energy from a single cell with a higher capacity to a single cell with a lower capacity. This transfer can be achieved through technologies such as capacitors and transformers. During the charging process, if a single cell reaches the upper limit of the operating voltage first, the BMS will identify the single cell with a lower capacity and transfer energy from the high-voltage battery to the low-voltage battery through the balancing circuit.Advantages: high energy utilization, fast balancing speed, and can improve the overall performance of the battery pack.Disadvantages: complex control algorithm and high production cost.Passive balancing: Principle: By consuming energy, the excess energy in high-voltage or high-capacity battery cells is dissipated in the form of heat energy, thereby reducing its voltage and capacity and achieving balance between battery cells.Passive balancing (energy dissipation balancing) is achieved by shunting the parallel resistance of the single battery. The energy of the battery with a higher state of charge in the battery pack is consumed through the parallel resistance to achieve balance with other batteries in the group.Typical passive balancing is implemented as follows: the voltage of each single battery is measured at the high or low end of the SOC. When the voltage of some single batteries exceeds the average voltage of the battery pack, the estimated balancing time is calculated based on the voltage difference or the single SOC difference, and then the parallel resistance of these high-energy batteries is turned on, so that part of their energy is consumed on the parallel resistance, and finally the balance of the entire battery group is achieved.Advantages: simple implementation and low cost.Disadvantages: large energy loss, slow balancing speed, and heat may be generated to cause the battery pack temperature to rise.3. Battery balancing processThe battery balancing process usually includes the following steps:Detection: Detect the voltage, current, temperature and other parameters of each battery cell in the battery pack through the BMS.Judgment: Determine whether there are differences between battery cells and the degree of difference based on the detection results.Perform balancing: Select the appropriate balancing method based on the judgment results and perform the balancing operation. For active balancing, it may be necessary to accurately calculate the amount of energy transfer through the control algorithm; for passive balancing, it may be necessary to control the on and off time of the switch to consume excess energy.Monitoring: Continuously monitor the parameter changes of battery cells during the balancing process to ensure the effectiveness and safety of the balancing operation.End balancing: When the difference between battery cells reaches the set threshold, the balancing operation is ended.4. Precautions for battery balancingSelect the appropriate balancing method: Select the appropriate balancing method according to the actual situation and performance requirements of the battery pack.Control the balancing speed and degree: Avoid damage to the battery cells or performance degradation due to excessive balancing speed or excessive degree.Monitor battery parameters: Continuously monitor the changes in parameters such as voltage, current, and temperature of battery cells during the balancing process to ensure the safety and effectiveness of the balancing operation.Preventing heat buildup: For passive balancing methods, measures need to be taken to prevent heat buildup that could cause the battery pack temperature to rise

BOSA was on North America’s Battery Show Oct.7-11th, 2024. It’s one of the BOSA’S highlights in 2024. The Battery Show is North America’s largest advanced battery event,  it brings together engineers, business leaders, top-industry companies, and innovative thinkers to discover ground-breaking products and create powerful solutions for the future. More than 19,000 attendees are expected to take advantage of four full days of educational sessions, networking opportunities and, of course, explore the latest market innovations from over 1,150 exhibitors across one of the world’s largest battery technology trade shows.As one of China’s leading manufacturers of lithium ion batteries, BOSA has a wide range of lithium battery products and is committed to providing customers with the most cost-effective and sustainable energy. Safety, integrity, responsibility and reliability are the keywords for BOSA. In this Battery Show, BOSA showed standard battery modules, battery packs and battery systems. Our products are widely used in electric marine, energy storage system, ev, light tower, heavy machine and so on. We are delighted to meet all our new and old customers on site and discuss our products together, looking forward to our next meeting!

The operating principle of lithium-ion batteries is based on the embedding and de-embedding process of lithium ions between positive and negative electrode materials. In the charging state, lithium ions are de-embedded from the positive electrode material and migrate to the negative electrode through the electrolyte solution, where the negative electrode material is in a lithium ion-rich state. Conversely, during the discharge process, lithium ions are de-embedded from the negative electrode material and migrate back to the positive electrode. The capacity of the battery is mainly determined by the number of reversibly embedded and de-embedded lithium ions, and the performance parameters of the battery will change under different charging and discharging conditions or during the cycle. So, where does the lithium go?Lithium ions inside the battery are mainly stored in the following places:(1) Active lithium that moves and deintercalates between the positive and negative electrodes;(2) Unusable lithium that is embedded in the negative electrode layer and cannot be removed;(3) SEI film formed on the surface of negative electrode particles;(4) Lithium precipitated on the surface of negative electrode coating;(5) Lithium solvated in the electrolyte;(6) Lithium fluoride formed by decomposition of electrolyte;(7) CEI film formed on the surface of positive electrode particles;(8) Unusable lithium that cannot be removed from the inside of positive electrode materials.Figure 1 Schematic diagram of lithium distribution inside a lithium-ion batteryDuring the cycle of lithium-ion batteries, or under different charge and discharge rates and temperature conditions, the distribution of lithium inside the battery changes. Researchers at the University of Münster in Germany studied this.During the study, a T-shaped battery structure was first constructed. After the battery has been cycled or aged, it is disassembled and the following steps are performed:Figure 2 T-shaped battery structure diagram.The positive and negative electrodes are first cleaned with DMC, and then dissolved in water through microwave reaction. After the cleaning solution and dissolving solution are diluted with distilled water, the lithium content is determined using ICP technology..The diaphragm is soaked in hydrochloric acid, and the soaking solution is diluted and then tested for ICP lithium content..The nylon membrane used in battery assembly is washed and diluted with distilled water, and then tested for ICP lithium content..The other main components of the battery are also tested for ICP lithium content after being washed with distilled water.In addition, as a control, the researchers tested the lithium content of the battery that was cycled once at a 0.1C rate, assuming that a solid electrolyte interface (SEI) and a positive electrode electrolyte interface (CEI) film were formed on the electrode surface. The original negative electrode sheet does not contain lithium, so the lithium content of the negative electrode sheet of the control battery is regarded as the lithium content that forms the SEI film. The lithium content of the positive electrode sheet of the control battery minus the lithium content of the original electrode sheet is regarded as the additional lithium content that forms the CEI film. The accuracy of all test results is ±7%.