As is well known, the safety of battery cells and battery packs during use has a significant impact on the safety of users. This project mainly studies the impact of lithium-ion cells and battery packs on users after being punctured by foreign objects. Lithium ion battery is a high-capacity rechargeable battery developed in the 1990s. It can store more energy than nickel hydrogen battery. It has large specific energy, long cycle life, low self-discharge rate, and no memory effect. It can meet the needs of passenger vehicles with high requirements for volume, life, power, etc. It has become an ideal product for future pure electric vehicle applications.

Not only in the electric vehicle industry, single or several dozens of batteries are favored by consumers, and lithium-ion batteries have been widely used in many industries. However, regardless of the industry or user, there are high requirements for safety during use, especially for power battery systems that require the combination of multiple individual cells into a battery pack. If there is serious interference from external objects during use, the battery pack will have a high degree of safety hazards, causing harm to personal and property. Given this situation, it is necessary to study the degree of impact of external interference on the use of individual cells and battery packs, in order to improve the safety of battery pack usage.

1.Test

Test object: 30P battery pack composed of cylindrical lithium-ion cells with a nominal capacity of 2700mAh and cylindrical lithium-ion cells with a nominal capacity of 2700mAh.

Test equipment: copper cable, relay, constant voltage source, and DGBELL Nail Penetration Test Chamber(the needle is φ 5mm, with a penetration depth of at least 2/3 of the battery cell).

2.Single Cell Nail Penetration Test

Single cell 100% SOC, conducted nail penetration test in a non discharge state.

Whether it is horizontal nail penetration or vertical. The highest temperature of the cells after nail penetration is around 100 ℃. The battery cells do not explode, do not catch fire, and emit slight smoke. From this, it can be seen that in the state of no discharge (i.e. full energy), the single cell will not pose a threat to human life and property after being nail penetration test in different directions from the outside.

3 Battery Pack Nail Penetration Test

The circuit components are: (1) Battery pack: lithium-ion single cell 30P, 100% SOC, 8lAh, ACR=1.7mn; (2) Copper cable: 10mm, 18m in diameter, with a total resistance of 14m (in order to make the current in the circuit close to the current of the battery pack 1C); (3) Relay; (4) Constant voltage source: relay switch.

Simulate the test of the battery pack being subjected to external interference during use using the DGBELL Nail Penetration Test Chamber.

There are two types of experiments determined:

• (1) The circuit belongs to the path state, that is, the battery pack is punctured while discharging to observe the final state of the battery pack;
• (2) The circuit is in an open circuit state, that is, without discharging, needle the battery pack and observe the final state of the battery pack.

3.1 Nail Penetration Test in Circuit Path State

According to the above circuit diagram, combine various components to form a circuit, and set the switch in a safer place. Then place the battery pack on the needle bed of the acupuncture device and secure the protection. Collect the total voltage of the battery pack, the temperature at 4 points near the needle position and in the circuit, and the current in the circuit.

When ready, turn off the relay switch to create a path for the circuit. Press the needle device switch to insert the needle into the interior of the battery cell and hold for 10 seconds. Remove the needle and observe the battery pack. The punctured battery cell emits smoke. After 1 minute, the smoke decreases and the nail penetrates the interior of the battery cell again and holds for 10 seconds. Repeat this process 3 times.

During the acupuncture process, the battery pack only emitted a small amount of thick smoke without any other abnormalities; Until the end of the puncture, there was no abnormal phenomenon such as explosion or fire in the battery pack.

The above experiment shows that when the battery pack is subjected to external interference during use, as long as it maintains continuous discharge and releases internal energy, it can eliminate safety hazards and avoid the occurrence of danger.

3.2 Nail Penetration Test in Open Circuit State

The experiments are all the same as the needle punched battery pack in the 3.1 circuit path state, with the only difference being that the relay switch is always in the open state, and the circuit is not a path, that is, the battery pack is needle punched in a non discharge state.

The result of the acupuncture was that the highest temperature around the acupuncture cell reached 260 ℃, and five cells around it exploded and caught fire, causing the entire battery pack to burst. From the above tests, it can be found that puncturing the battery pack without releasing energy poses a serious threat to users.

4.Conclusion

The above experimental results indicate that even if a single cell is punctured by external objects without releasing energy, there will be no abnormal phenomena such as explosion or fire, and users can use it with confidence; The battery pack will not experience any abnormal phenomena such as explosion or fire when being punctured by external objects while releasing energy, and there are no safety hazards to the battery pack; However, if the battery pack is punctured by foreign objects without releasing energy, it will catch fire and explode, posing a certain threat to users.

Lithium ion batteries have been produced in large quantities both domestically and internationally, and their performance is constantly improving. The application field of battery packs is gradually expanding. But the safety of battery packs has always been a top international concern. At present, completely eliminating the safety hazards of lithium-ion battery packs is still a challenge, which requires users to pay attention to correct operation and use to avoid safety accidents.