Why-do-batteries-require-mandatory-internal-short-circuit-testing

Why do batteries require mandatory internal short circuit testing?

This article will explain the types of lithium battery internal short circuit, how they differ from battery external short circuits, why it is mandatory to test for battery internal short circuits, and ways to detect battery internal short circuit.
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4 response types of internal short circuit

Internal short circuit. the topic may sound a bit specialized and complicated, but in reality, it is divided into four main and simple types of reactions. Next, we will study them one by one. 

  • Intenal short circuit between copper and aluminum foils

First, let’s look at a short circuit between copper and aluminum foil. In this case, electrons will flow from the battery copper foil to the aluminum foil because aluminum has better conductivity than copper.

This type of short circuit can cause a drop in battery performance and may even cause safety issues. To prevent this from happening, battery manufacturers use special insulating materials at the connections of the battery to minimize this possibility.

  • Intenal short circuit between cathode and anode

Next is a internal short circuit between the cathode and anode. In this case, electrons will flow directly from one end of the battery to the other, causing a rapid increase in current. This short circuit can also cause li ion battery safety issues, so battery manufacturers also use special protective designs inside the battery to prevent this from happening.

  • Intenal short circuit between cathode and aluminum foil

The third case is an internal short circuit between the cathode and the aluminum foil. In this case, electrons flow from the cathode to the aluminum foil. This short-circuit may result in reduced battery performance because the flow of electrons affects the charging and discharging efficiency of the battery.

To prevent this from happening, battery manufacturers use special protective materials at the anode to minimize the flow of electrons.

4-response-types-of-internal-short-circuit

  • Intenal short circuit between anode and copper foil

The last type is an internal short circuit between the cathode and the copper foil, in which electrons flow from the cathode to the copper foil. This type of short circuit may also cause a decrease in battery performance, but for a different reason than a short circuit between the anode and the aluminum foil.

This is because a chemical reaction occurs at the cathode rather than a flow of electrons. To prevent this from happening, battery manufacturers use special protective materials at the cathode and optimize the design of the battery to reduce the likelihood of a chemical reaction.

What is the difference between internal short circuit vs external short circuit?

Battery short circuit is a more common battery failure, there are external short circuit and internal short circuit. There is a certain difference between the two:

Internal short circuit that lithium power battery internal cathode and anode direct contact, of course, the degree of contact is different, triggered by the subsequent reaction also varies greatly.

The main factors that cause internal short circuit of lithium power battery are: conductive dust on the surface of the seperator, misalignment of cathode and anode, burrs on the pole piece and uneven distribution of electrolyte and other process factors; metal impurities in the material;

Low-temperature charging, high-current charging, anode performance degradation is too fast resulting in lithium precipitation on the surface of the anode, vibration or collision, etc.; caused by mechanical and thermal abuse of large-scale internal short circuit.

What-is-the-difference-between-internal-short-circuit-vs-external-short-circuit

If the internal short circuit is a defect in the manufacturing process, this internal short circuit occurs slowly and takes days or even months to develop into a spontaneous internal short circuit.

The mechanisms involved in the long gestation process are quite complex, very long, and there is no telling when it will experience thermal runaway. However, it is relatively mild, generates very little heat, and does not immediately trigger a TR (Thermal Runaway Temperature TR).

Internal short circuits that originate from mechanical and thermal abuse will trigger a TR directly, with a sharp release of energy that varies with the degree of seperator rupture and the length of time from the internal short circuit to the TR.

External short circuit may originate from deformation caused by automobile collision, immersion in water, conductor contamination or electric shock during maintenance. The important link in the middle from external short circuit to thermal runaway of lithium power battery is high temperature.

When the heat generated by the external short circuit can not be well dissipated, the temperature of lithium power battery will rise, and high temperature triggers thermal runaway. Therefore, cutting off the short-circuit current or dissipating the excess heat are ways to inhibit the external short circuit from producing further harm.

In actual vehicle operation, the probability of external short-circuit is very low, because the whole vehicle system is equipped with fuses and lithium battery management system BMS, lithium power battery can withstand short-time high current impact.

Internal-short-circuit-vs-External-short-circuit

In extreme cases, the short-circuit point crosses the fuse of the whole vehicle, and at the same time, the BMS fails, and a longer external short-circuit generally leads to the burning of the weak point of the connection in the circuit, and seldom leads to the thermal runaway event of the lithium power battery.

When a short circuit occurs in a battery, it will cause the battery to continue to exothermic, and the temperature is very high enough to melt the general metal, which may cause a fire or even an explosion, posing a serious threat to property and life safety. Therefore, more and more businessmen pay more attention to ensure the safety of batteries and their products in order to avoid personal injury and brand reputation loss caused by battery safety accidents.

So product testing and inspection is a good means of prevention, only to continue to improve, sound equipment product testing standards, and demand, in order to maximize the safety of product quality, to avoid the occurrence of accidents.

Why is internal short circuit testing mandatory?

An important way to measure the inherent safety of the electric core is to force its internal short circuit test whether it catches fire or explodes as well as the pressure drop and temperature rise of the electric core. Power battery large-scale manufacturing start, the safety issue has become hanging over the industry’s head of the sword of Damocles, urgently need to be resolved.

Battery safety can generally be categorized into the following three levels: the inherent safety of the cell, the safety of different levels of protection, and the (relatively) acceptable safety of regulations/standards. At present, the inherent safety of the cell is still the priority issue for battery safety.

An important way to measure the inherent safety of a cell is to force its internal short circuit, testing it for fire or explosion as well as the voltage drop and temperature rise of the cell.

Why-is-internal-short-circuit-testing-mandatory

The significance of mandatory internal short circuit testing

There are many factors that can cause an internal short circuit of a battery cell, including seperator characteristics, development and design, battery production process, packaging and transportation, assembly and application, use, maintenance and reuse, etc. All of these scenarios may cause an indirect or direct internal short circuit of the battery cell.

The mandatory internal short circuit test of the battery cell is to verify whether the battery cell has an internal short circuit and whether it will catch fire or explode when an internal short circuit occurs by simulating the extreme situation, so as to identify the intrinsic safety characteristics of the battery cell as well as the safety characteristics of the battery system.

Application of forced internal short circuit testing

Most battery standards regulate the mandatory internal short circuit test for battery cells, and many brands of batteries and battery applications have adopted this test program as their corporate specification.

The mandatory internal short circuit test for battery cells is widely used in the evaluation of battery materials, development, design, manufacturing process, and even the compliance of battery modules and battery systems.

Standards for mandatory internal short circuit testing

IEC 62133-2:2017/AMD1:2021 (Portable batteries)
Appendix 9 (edition 1) (Portable batteries)
JIS C 62133-2:2020 (Portable batteries)
IEC 62660-3:2016 (Power batteries)
IEC 62619:2017 (Industrial, energy storage batteries)

Standards-for-mandatory-internal-short-circuit-testing

Methods of forcing internal short circuit test on battery cell

  • Pre-treatment of the battery cells
  1. The battery cells need to be charged at 20 °C ± 5 °C according to the manufacturer’s recommendations and then discharged at 0,2 It A to the discharge cut-off voltage defined by the manufacturer;
  2. The batteries are left for 1-4 hours at the upper and lower charging temperatures defined by the manufacturer;
  3. Charge the battery cell at the maximum charging current and the upper limit charging voltage until the current is reduced to 0,05 It A.
  • Dismantling of the battery cells

The disassembly of the cell is a critical step in the mandatory internal short circuit test of the cell, but it is also extremely challenging and controversial.

  • Extrusion of cores
  1. After the surface temperature of the core has stabilized within 2 degrees of the upper and lower charging limit temperature, it is ready to be squeezed;
  2. The press is pressed to the core at a speed of 0,1 mm/s, while the core voltage is monitored at a speed of more than 100 times per second;
  3. When a voltage drop caused by an internal short circuit is detected, the extrusion shall be stopped immediately and the pressure shall be released after holding the press tool in this position for 30 seconds. If the voltage drop compared to the initial voltage exceeds 50 mV, it is determined that an internal short circuit has occurred. before the voltage drop reaches 50 mV, if the pressure reaches 800 N (for cylindrical cells) or 400 N (for square cells), stop squeezing immediately.
  4. Determination of compliance: No fire during testing (companies may have more stringent allowable acceptance criteria).
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