EV battery safety assessment – future evolution (2022)

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EV battery safety is a major challenge in the development of electric vehicles with lithium-ion battery systems. Internal quick circuit, electric overload or mechanical harm can reason an out of control upward push in battery temperature electric vehicle battery safety standards.

If a crucial temperature is passed withinside the mobileular, self-accelerating exothermic chemical reactions produce big quantities of warmth in a completely quick time. As a result, this big warmness dissipation also can overheat the adjoining cells. In a series response, it could overheat the battery module and, withinside the worst case, the whole battery pack. This technique, additionally called thermal runway, can bring about extreme harm to the battery machine or the whole automobile from fires and explosionsEV battery safety .

EV battery safety :

EV battery safety

In this webinar, Juergen Schneider, Solution Manager for Virtual Battery Development, and Bernhard Brunnsteiner, Lead Engineer Battery Simulation, gift a completely unique simulation approach that permits to investigate this hassle in detail. Measures to prevent this chain response or at the least significantly sluggish down the thermal growth may be in comparison to its effectiveness. This technique considers all of the outcomes from the thermal get away of a unmarried mobileular via the venting technique and from the melting of the battery additives to the format of the protection vent. Different layout structures may be almost examined to save you or postpone thermal growth to satisfy the protection necessities of EV battery safety.

Li-ion Cell Design and Safety Performance :

EV battery safety

As described earlier, the fundamental building block of any battery is the cell. Cells package the cathode, anode, current collectors, electrolyte, and separator components into a practical functional subsystem that stores energy in electrochemical form EV battery safety .

Cells provide electrical poles to support electrical charge and discharge. Cells provide mechanical and environmental protection for the components. Cells can also incorporate safety features intended to prevent or mitigateserious failures. Cell electrochemistry defines the fundamental voltage, and design and construction determine the energy storage capacity EV battery safety.

As the fundamental building blocks, multiple cells can be assembled in modules to deliver the required voltage capacity required for an application.
And modules can be further assembled into battery packs and systems to supply the necessary energy. Alternatively, cells can be assembled directly into battery packs.This chapter describes the design and construction of Li-ion battery cells, focusing on theirelectrical, thermal, mechanical, and EV battery safety

Cell Design and Construction :

Li-ion cells in car batteries come in a variety of shapes and sizes. These cell types are known as cylindrical, prismatic, elliptical, sac and large format. All the cells relevant to this discussion contain liquid electrolytes.

Some battery pack manufacturers build on cylindrical cells because of their high reliability and versatility in the market. Other manufacturers favor the prismatic type because of its greater strength and potential for energy concentration and geometric compatibility with the overall pack design.

Elliptical cells are similar in construction to prismatic cells, and sac cells are usually a type of Li-ion polymer that can use a combination of dry and gel electrolytes. Other forms of such cells may be present or emerging in the market EV battery safety .

Cylindrical Cells :

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Cylindrical cells have a well-known cylindrical shape and have long been used as single cells. Electronic devices such as cameras and small series and other parallel configurations. Electronic devices such as laptops EV battery safety .

Most cylindrical cells have the same basic design. Cylindrical cells come and go in different shapes Usually defined by their dimensions. For example, a model number 18650 specifies a diameter18mm and 65mm length.

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Electrical connections and ceilings require positive and negative poles, insulators and gaskets. Positive temperature coefficient devices, current barrier devices, and exhaust or vent discs are safety features that are designed in the cell package EV battery safety .

Prismatic Cells :

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Prismatic cells are built in a wound or flat plate configuration. The distinction lies in the production process. Wound prismatic cells are typically built with the aid of using wrapping the layers round a bobbin or mandrel, just like that of a cylindrical mobileular.

In a flat plate mobileular, discrete layers are stacked aspect with the aid of using aspect and pressed collectively or folded. The folded layout is just like that of a stacked mobileular besides the layers are connected in a single big sheet and are folded returned over one every other and pressed collectively to shape the mobileular EV battery safety.

The layers are contained in a inflexible case that gives terminal connections and typically has a vent plug. Similar to cylindrical cells, prismatic cells are typically described with the aid of using their measurement in which the primary digits confer with the thickness, the subsequent or 3 digits constitute the width, and the last digits constitute the duration ofthe mobileular EV battery safety.

The identical fundamental additives of cylindrical cells are used to assemble prismatic Li-ion cells; however, prismatic cells designed for automobile programs will have an awful lot large capacities than cylindrical cells.So called “big layout cells,”. If a mobileular is big (more than approximately 10 Ah) the conditions, inclusive of temperature and SOC, aren’t uniformwithin the mobileular throughout charging or discharging EV battery safety .

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This multiplied potential of those big layout cells calls for the protecting gadgets to be designed and scaled accordingly. Therefore, a few defensive gadgets that paintings properly for smaller cylindrical cells might not be realistic or powerful for big prismatic cells.

pouch Cells :

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Pouch cells are essentially prismatic cells with bendy polymer covered aluminum packaging as opposed to metallic cans. The idea originated with the Belco “polymer” mobileular wherein the electrodes have been layered collectively with the “polymer” separator EV battery safety.

Today endless cells are made with trendy dividers and are nevertheless taken into consideration as sac cells. The person layers are stacked or folded, packed below a vacuum and held collectively via way of means of a pouch. If overcharged, the sac mobileular releases fueloline and the sac may also expand, inflicting the layers to separate. Some producers declare that it is able to be used as a protecting function due to the fact keeping apart the layers closes the cell working.

When the cells are packed together in a battery, there may not be enough space in practice to balloon the cell. In automotive applications, the pouch component provides little physical protection, so the module or battery housing must provide an external structure to support, restrain, and protect the pouch cells.

The tabs or conductors of a sac cell are usually connected in an axial or radial fashion. Axial placement allows the tabs to widen further, which, for the thickness of the given material, reduces the total series resistance and increases the ability to conduct heat from the package.

Large Format Cells :

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Several small Li-ion cells have been integrated in series and in parallel grouping to create a battery pack with enough voltage, current and storage capacity to power a motor vehicle.

Technical authors typically identify cells larger than 10 Ah as large. A high-capacity pack consisting of one large cell or many smaller cells. The definition of large cells is associated with transport regulatory requirements that have been subject to change. Based on recent UN model regulation, a large format room has more than 20 hours of power (e.g., more than 5 Ah capacity with a nominal voltage of 3.7), while a larger format battery pack has more than 100 Wh (e.g., a battery pack so Twelve has more than 2.2 hours of cells) EV battery safety.

The internal structure of the cell In a large chamber the length of the pouch is long enough and the pouch is folded or wrapped around itself to fit prismatically. A common chemical in large cells is LiFePO4 (lithium iron phosphate or LFP). An important consideration in the design of large cells is that the internal conditions cannot be assumed to be identical due to the presence of a small cell. Especially during charging or discharge. The current distribution in a cell is shown as non-uniform when charging and varies with the progress of the charging cycle. Similarly as a large cell excretion, SOC can vary from one point of the cell to another.

Design Considerations :

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Exceeds the rate of heat dissipation by the thermal cell of a Li-ion cell. Rapid heating may be caused by internal or external short circuit, safety risk from overcharging Li-ion batteries.The design of a cell strongly influences its ability to control and remove heat, which in turn affects its safety performance EV battery safety.

Thermal Design Considerations and Thermal Management :

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Li-ion batteries depend on a variety of factors, including the formation of heat dissipation in cells. Materials, and location and type of heat source as well as heat sinks.
The material and thickness of the outer packaging of the cell significantly affects the ability to dissipate heat. Failure to lose heat at a rate higher than the rate of heat production in the cell can lead to heat escape. Cylindrical cells rely heavily on a right direction (height to diameter) ratio for adequate heat dissipation EV battery safety .

When excessive charge or discharge rate or excessive heat is generated in the cell from an external short circuit, the porosity of the divider is lost, which stops the flow of Li-ions and stops further current-induced heating.

This security feature, when enabled, permanently disables the cell. If the heat is from SEI breakdown, high external temperature, or other factors which do not involve current flow (ionic transport), preventing ionic transport cannot prevent exothermic reactions.

Mechanical Design Considerations :

Each cell has a distinct mechanical design and production process. Mechanical design affects the thermal properties of a cell, how cells are assembled into modules and packs, to prevent mechanical damage and to withstand internal stress without rupture. Cylindrical cells have a concentric round stand of layers that are usually packaged in aluminum or steel cans, but other packaging types can be used EV battery safety .

The cylindrical design produces a symmetry that applies an even distribution of pressure across the cell layers; However, maintaining a uniform compressive strength in prismatic cells is even more challenging. Prismatic cells do not have a rigid case often packaged inside a case that provides structure and protection from shock, vibration and other mechanical disturbances EV battery safety .

Charge and Discharge Management :

Charging and discharging are the normal energy cycles applied to replenish or extract the electrical energy available in Li-ion cells, respectively. Proper management of electrical loads within the cells of a pack helps maintain the overall charge and discharge efficiency within an acceptable range of electrical balance and prevents the condition of excess discharge or excess charge EV battery safety .

Since temperature is a key indicator of the cell’s electrical performance (e.g., hot cells can discharge or charge faster than cold cells), heat management techniques are often integrated into battery system design to monitor charging and discharging events and mitigate potentially problematic situations.

Cell Safety and Failure Mitigation Measures :

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Li-ion chemistry has a precise usability range, combining different cathode, anode, and electrolyte components. When cells and batteries operate within their prescribed charge and discharge system and within the appropriate ambient temperature range, the risk of cell failure decreases. Conditions outside of normal design parameters, unplanned “catastrophic effects” or improper production strategies that cause cell failure EV battery safety .

A failure inside a Li-ion cell or battery often leads to more serious consequences than failure inside the battery with other chemicals. The catastrophic failure of Li-ion cells may be more severe than that of other rechargeable cells of similar size, because Li-ion cells have higher energy concentrations and release rates, and Li-ion cells contain flammable organic solvents as part of the electrolyte.

Li-ion batteries have fire safety concerns primarily due to the use of a combustible (organic) solvent component of the electrolyte, which can ignite when exposed to high temperatures or electrostatic sparks, and when charged to the lithium SEI layer and anode, which is in the air. Turn around. External short circuit, internal short circuit, cell overcharging, cell over-discharging or external exposure to high environment,

Self-resetting PTC devices work using materials whose resistance increases with temperature. PTC resistance is low at normal operating temperatures, but resistance increases rapidly in the presence of high temperatures. Thus, when an external electrical stimulus such as a short circuit causes the PTC material to heat up rapidly, the PTC reacts with an increase in resistance, resulting in a decrease in current flow. When the stimulus is removed, the PTC cools and self-resets.

PTCs usually trigger temperatures above 100 C and can be used several times before resetting stops. If PTC fails, it remains in its high-resistance state, preventing cell operation until PTC is replaced.

In such cases, there must be other methods of protecting the cell or relying on adequate heat transfer in the environment to prevent the cell from overheating. Such excessive heat can cause cell degradation rather than instantaneous thermal escape EV battery safety.

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