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Is The 32650 Lithium Battery Safe?
- Aug 20, 2018 -

Introduction to 32650 lithium battery

Lithium battery 26650, currently used to replace traditional nickel-chromium, nickel-hydrogen batteries, used in mining lamps, flashlights, power tools, toys, instrumentation, ups back power, communications equipment, medical equipment and military lights.

The definition of the model is: 26650 type, which refers to the battery with a diameter of 26mm and a length of 65mm. Generally used to name lithium batteries, including lithium primary batteries and lithium ion batteries. A common lithium battery made of nickel-cobalt-manganese cathode material and lithium iron phosphate material - INR26650-3.6V-4500mAh, IFR26650-3.2V-3200mAh.

Advantages of the 26650 lithium battery

26650 lithium battery, currently used to replace the traditional nickel-chromium, nickel-hydrogen battery, used in mining lamps, flashlights, power tools, toys, instrumentation, ups back power, communications equipment, medical equipment and military lights. Its advantages, compared with nickel-chromium and nickel-hydrogen batteries, are mainly reflected in the following aspects:

1. High energy density and low self-discharge rate

The capacity of the 26650 lithium battery is 1.5-2 times that of the same quality nickel-metal hydride battery. At the same time, the internal resistance of the domestic 26650 lithium battery is less than 60mΩ, which greatly reduces the self-consumption of the battery, and can extend the battery while prolonging the use time. The service life.

2, charge and discharge performance is stable

The 26650 lithium battery has no memory effect, does not decompose when exposed to heat, has high safety performance and long life cycle.

3, high voltage

The voltage of 26650 lithium battery is generally above 3.6 and 3.7V, which is much higher than nickel-chromium and nickel-hydrogen batteries.

4, can be used in series or in parallel to form a 18650 lithium battery pack

5, environmental protection and pollution

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Principle and process of charging and discharging 26650 lithium battery

The reason why the 26650 lithium battery can be charged and discharged is that it moves with the active lithium ions on its positive electrode. That is, when the battery is charged, active lithium ions are generated on the positive electrode of the lithium battery, moved to the negative electrode, and embedded in the layered structure of the negative electrode. The material system of the negative electrode is graphite, which is a layered carbon. It has many micropores. When lithium ions move to the negative electrode, they are embedded in the micropores. The more lithium ions are embedded in the micropores, the higher the charging capacity.

By the same token, when the battery is discharged, the lithium ions embedded in the carbon layer of the negative electrode are released and moved to the positive electrode, and the more lithium ions return to the positive electrode, the higher the discharge capacity. The size of the 26650 lithium battery we usually refer to is the discharge capacity.

Safety Analysis of Cylindrical 32650 Lithium Battery

A power lithium ion battery generally refers to a lithium ion battery that can power a device, an instrument, a vehicle, and the like through a large current discharge. Power lithium-ion batteries have been widely used due to their high specific energy, high current charge and discharge, and long cycle life. Power lithium-ion batteries are classified into three types: ternary, lithium cobaltate, lithium manganate, lithium iron phosphate, etc. according to different positive electrode materials; they are classified into prismatic batteries (prismaTIc) and cylindrical batteries (cylindrical) according to the shape. In order to improve the cruising range, the power of the power lithium-ion battery through the series-parallel combination is generally large, and the capacity ranges from a few ampere hours to several hundred ampere-hours, and the voltage ranges from a few volts to several hundred volts. As the carrying energy increases, the potential hazard of the battery increases. Therefore, how to improve the safety of power batteries has become an important prerequisite for the continuous development of electric vehicles. In the development of power lithium batteries, there have been two development directions. One direction is a large single cell, which is combined in small parallels; one direction is a small single cell, which is combined in a large number of parallels. South Korea LG, the domestic BYD representative of the company is a generous route; the United States A123, the domestic representative of Wattma is taking a small cylindrical route. These two routes are currently inconclusive, and different power battery manufacturers choose different process routes according to their own understanding. However, in terms of the indicator of safety, the difference between the results of the two routes is very large. This paper compares and analyzes the structure and performance of power batteries, especially safety, to illustrate the safety advantages of small cylindrical batteries in electric vehicles.

Comparative analysis of battery structure and performance

Cylindrical batteries and square batteries are currently the two mainstream directions in the industry. The basic structure of a cylindrical battery is shown in Figure 1. The positive and negative electrodes are separated by a separator, and a winding core is formed by winding. Usually, the positive and negative pole pieces are welded with positive and negative poles and are respectively led out through the both sides. The tabs are soldered to the positive and negative housings. The electrolyte is filled in the housing. Figure 2 shows a square battery structure. The structure of the square battery is divided into a laminated structure and a wound structure. The laminated chip type lithium ion battery is composed of n positive electrode sheets and n+1 negative electrode sheets to form a battery cell, and the positive and negative electrodes are separated by a diaphragm, and are reserved on one side of the positive and negative electrodes, respectively. The positive and negative ear regions are symmetrically extended from the sides of the core when the core cells are stacked. The winding structure of the prismatic battery is similar to that of the cylindrical battery, with the difference that the core is a flat shape rather than a cylindrical type. Due to the difference in shape of the cylindrical battery and the square battery, the structure is largely different. In general, cylindrical batteries cannot be made too large due to the core current density and heat dissipation. The square battery can increase the capacity by increasing the length and width on the premise that the thickness is appropriate. Its monomer capacity can generally exceed 10 times that of cylindrical batteries. Table 1 shows the performance comparison of cylindrical and square batteries. It can be seen that the two batteries have their own characteristics. The cylindrical battery structure is simple in design, the interface between the positive and negative electrodes is tight, the production line is mature, the cost is low, the heat dissipation in groups is good, and the safety performance is excellent. The disadvantage is that the internal resistance is relatively high and the group requirements are high. The advantage of the square battery is that the combination of large single capacity is simple. The disadvantage is that the production process is complicated, and the consistency of the large-capacity battery cells is difficult to control. In addition, the square shell is prone to stress concentration, the shell is easily broken, and the electrolyte splash causes safety hazards.

From the perspective of the global application market, large-capacity square batteries and small-capacity cylindrical batteries have applications in the power field. At present, the benchmark enterprise of lithium iron phosphate battery industry, the main product of American A123 is the cylindrical battery of 18650/26650/32113, the monomer capacity is 2.5Ah-5Ah.

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Security comparison analysis

1. Security comparison in extreme cases

The safety of power batteries under extreme conditions such as serious accidents in vehicles is the most concerned issue because it is directly related to the safety of life and property. The cylindrical battery has a small capacity and meets the requirements of the capacity and voltage of the power battery pack by serial combination. Take the current 32650 battery as an example, the battery capacity is only 5Ah. The capacity of the generous battery cells generally exceeds several tens of ampere, and some reach more than 100 Ah. In the extremely dangerous situation of battery collision, extrusion, etc., the cylindrical small battery releases much less energy than the square battery. At present, the electrolyte of the Waterma 5Ah battery is only 20 grams, while the large-type battery, such as 50Ah monomer, has an electrolyte volume of more than 200 grams. The amount of the electrolyte of the prismatic single cell is 10 times or more that of the small cylindrical battery. Once a single cell leaks in an accident, the degree of combustion caused by electrolyte leakage will be more than 10 times that of a small cylindrical battery. In this respect, small cylindrical batteries are much safer than large batteries. When a small cylindrical battery is destroyed, its burning power is much smaller than that of a large square battery. By separating and protecting a single battery, there is a problem with one battery and it does not affect other batteries. The safety of the battery is greatly improved by dispersing the energy.

Cylindrical and square batteries perform quite differently in terms of impact. The cylindrical battery has better deformation resistance than the square battery, and the force is uniform in all directions. The deformation retention capability is the best among all the current core technology. With the self-developed safety combination cap, the safety is extremely high. Great improvement. Even in the high-speed collision and extrusion process, the cylindrical battery has a certain deformation, but it will not burn. For the square battery, the larger side is easy to deform. During the high-speed collision and extrusion process, the cell casing can not guarantee the internal structure of the cell well, which easily leads to the misalignment of the internal positive and negative plates. The impact of the square cell cannot respond quickly.

In addition, for large square cells, because of their large side area, the probability of impacting other objects is much higher. Therefore, in a safety accident, the possibility of a single cell being damaged and causing a short circuit is higher than that. Small cylindrical batteries are much larger. For a small cylindrical battery combination, once the battery box is subjected to a violent impact, the first place where the small cylindrical battery is disconnected may be the riveting point of each individual battery cell, and since the cell volume is small, the larger possibility is to be smashed. The battery pack has failed. This is of great significance for improving the safety of the power battery pack. Therefore, a battery pack using a small-capacity cylindrical battery pack can provide a longer escape time in the event of an accident in a vehicle. According to Watmar's test, the battery was incinerated in a fire, and the electrolyte sprayed out for a period of 10 minutes.

2. Comparison of heat dissipation

In terms of heat dissipation of the unit, due to the different shapes of the cylindrical battery and the square battery, the heat dissipation performance is quite different. Taking a 50Ah square battery as an example, the surface area capacity ratio is about 1x10-3m2/Ah; and the 32650-5Ah cylindrical battery has a surface area capacity ratio of about 1.6x10-3m2/Ah; in comparison, it is 60% larger. Cylindrical small batteries have a natural advantage in terms of heat dissipation under the same external conditions.

When the cylindrical batteries are combined, there is a longitudinal gap between the batteries, which provides a natural heat dissipation path for the heat dissipation of the battery. Theoretically, the heat-dissipation cross-sectional area is at least 15.9% (closely aligned) and 21.9% (cubic arrangement). Figure 3 is a physical diagram of the combined structure of a Wattmar cylindrical battery. It can be seen that the combination has a good heat dissipation channel. The natural heat dissipation channel of the cylindrical battery combination ensures the heat dissipation effect of the battery and improves the safety of the battery.

3. Comparison of security mechanisms

Both battery configurations have an explosion-proof safety valve. The safety valve of the square battery is generally located on the end side, and the valve area is larger than the safety valve of the cylindrical battery. However, if the capacity of the battery, that is, the valve area per unit capacity, is taken into consideration, the square battery is much smaller than the cylindrical battery. Once the battery fails, especially in extreme impact situations, the square battery safety valve is less effective than a small cylindrical battery. The cylindrical battery combination cap has both an explosion-proof safety valve and a current cut-off device CID (CurrenTInterruptDevice), as shown in FIG. This is rarely used on square batteries. When an external short circuit occurs or when the internal pressure of the battery reaches a safety warning value of 1.2 MPa, the CID device is started first, the positive and negative poles are disconnected, the current is automatically cut off, and the internal circuit of the battery is disconnected; when the internal pressure of the battery is 1.8 MPa, the safety warning is issued. When the value is reached, the relief valve safety valve will open and the gas will be discharged to avoid the risk of explosion. At present, the safety combined cap technology is mature, and the safety of the cylindrical battery core is well guaranteed.

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4. Group consistency comparison

It is well known that the consistency of a single battery has a great influence on various aspects such as the life and safety of the battery pack. The complexity of the production process of the generous battery determines the consistency of the current single cell. The consistency of the battery after grouping directly affects safety. Because of the small capacity, the battery with high internal resistance is more exposed to the risk of overcharge and discharge. The production process of cylindrical batteries is mature and the battery consistency is high. After the battery is assembled, the probability of a low-capacity battery is low. Even with low-capacity batteries, the effects of inconsistency are largely eliminated by self-equalization due to multiple parallel connections.

5. Cylindrical battery safety test

In order to verify the safety of the cylindrical battery, the cylindrical unit battery and the battery pack were tested for safety. Fig. 5 is a photograph of a needle-punched experiment of a cylindrical battery pack. When the steel nail penetrates the battery, the electrolyte in the battery leaks, the surface temperature of the battery rises sharply, the voltage drops slowly, a small amount of white smoke is emitted from the vaporization of the electrolyte, and the insulating plastic sleeve for wrapping the battery is melted at a high temperature for about 10 minutes. After the phenomenon disappeared, the final short-circuit battery voltage dropped to zero, and the process maximum temperature rose to 141 °C. The battery does not explode and does not burn. Fully compliant with UL2580 (and SAEJ2464) standards. Figure 6 is a photograph of the cylindrical battery pack before and after the impact test. After the battery pack is hit by heavy objects, the battery is obviously damaged, but the battery has no fire, no leakage, no smoke or explosion, and the battery voltage is basically unchanged. Meets the UL1642 standard. Table 2 shows the safety test items for the battery pack. From the measured results, the cylindrical battery showed good safety performance. Figure 7 shows a more detailed photo of the incineration test process. From the measured results, the battery surface temperature of the battery rupture disk is about 240 ° C, the battery releases gas, the electrolyte is partially burned, and there is no explosion. The battery casing structure was not damaged after the flame was extinguished. According to the requirements of UL1642, "all and part of the cells should not penetrate the wire mesh during the experiment", and the safety results are shown to meet the UL standard.

At present, small cylindrical batteries have been widely used in electric vehicles. In order to verify the safety of batteries in automobile accidents, the vehicle has been tested for collision. The collision criteria are based on C-NCAP. The vehicle was subjected to a frontal collision with a speed of 50 km/h and a 100% overlap with a rigid fixed obstacle. The frontal offset collision with a speed of 56 km/h for a deformable obstacle with a 40% overlap rate, and a deformable movement obstacle with a speed of 50 km/h and a side collision of the vehicle. Figure 8 is a photograph of the entire vehicle after collision. After the collision, the vehicle was taken out for testing, and the battery pack was basically intact, no smoke, no burning. The vehicle crash test verified the safety advantages of small cylindrical batteries used in electric vehicles.

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The battery was filled at 280V (78 strings) at 3A (0.6C) at room temperature, and after standing for 30 minutes, it was freely dropped from a height of 1 m with a weight of 10 kg and hit the battery. (a) before testing; (b) after testing.

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Conclusion

The 32650-5Ah cylindrical lithium iron phosphate battery has high safety characteristics in power applications such as electric vehicles. This is due to factors such as small capacity of the single cell, reasonable structural design, and safe combination structure. The lithium iron phosphate power battery is applied to the power field in the form of series-parallel combination and is the future development direction.


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