Materials and standards for lithium ion battery pack

At present, there is no standard for lithium ion battery pack size due to different product specifications and different application scenarios of different manufacturers. Battery pack is usually made of stamped stainless steel or aluminum, cast aluminum, fiber glass, polymers or some composites.

This article will introduce in detail the various lithium ion battery pack methods and some commonly used standards of battery pack . After reading this article, you will have a new understanding of this aspect of lithium-ion battery.

Metal packaging

Battery pack uses a variety of materials. Plastic, stainless steel, aluminum, fiberglass and composite materials used in the battery module interior and shell.In most cases, the energy storage system is this combination.

Stainless steel case and aluminum case

Stainless steel is relatively inexpensive and provides high mechanical strength, but stainless steel shells are usually heavy and require welders or accessories to increase strength. These additions add time to the process and cost more money.

Currently,  lithium ion battery company is trying to develop stainless steel shells with high mechanical strength and light weight in order to compete with lighter aluminum. Nanosteel Company is one of them, offering advanced stainless steel that has a similar weight to aluminum but greater mechanical strength.

Aluminum shells are made by stamping or casting. Aluminum has a light weight but requires a relatively large thickness to satisfy mechanical properties, especially for aluminum shells prepared by stamping. Aluminum shells prepared by high-pressure Die Casting (HPDC) have optimal mechanical strength, porosity and weight, but require expensive tools.

Sand casting is relatively cheap, but some of the finished products produced are of poor quality, requiring additional process steps to perfect.

Gypsum casting is a cheap process and the surface flatness of products is comparable to that of high-pressure casting. However, gypsum casting products are poorly controlled in terms of porosity and often have defects. Gypsum casting takes a relatively short time to produce, so it is frequently used in rapid prototyping.

High-pressure Die Casting (HPDC) can be combined with other characteristics during the casting process, such as installation of specific air channels, combined with material support structures, etc. These additional properties make aluminum shells useful in many lithium-ion battery pack and system.

Coating

If the battery system is based on a variety of metal materials, consider whether to coat the surface. There are many reasons for coating and types of coating. One of the main reasons for coating is to prevent short circuit in contact with the ground or with electronic devices and cells;

In other words, the coating acts as a barrier to the battery pack, and this function is suitable for a variety of materials. The most commonly used coating technology is directly affixed to the surface with a sticky film, which has only one side of the sticky film and can be directly adhered to the metal surface to meet certain isolation effects.

The second coating method is liquid and powder coating. Both coatings provide environmental protection rather than isolation.

Evaluate the mechanical strength of the metal

For metal enclosures, the mechanical strength of the metal must be assessed during design of battery pack. When casting a metal shell, it is often necessary to strengthen the shell structure by means of nut reinforcement, welding, or coating. The degree of strengthening can be determined by finite element analysis and impact and vibration simulation.

Watch out for incompatible metals

Caution must be exercised with the use of incompatible metals, which can cause unexpected chemical reactions (such as galvanic corrosion). This is important for the long-term operation of electrochemical systems, because the system carries a partial charge during metal corrosion, which can impair the long-term stability of the battery pack.

Compound encapsulation

Plastic and polymer packaging

Some small battery pack also use plastic packaging. If the structure of the battery is not required, it is likely that the battery pack will be externally packaged with a polymer film. For example, some hybrid and start/stop vehicle batteries only need to be sealed to protect the lithium-ion battery cell, and the mechanical requirements for the casing are low.

Larger batteries not only use more of the polymer, but may also use a compound that complements the metal matrix. For example, The Chevrolet Volt of General Motors uses a vinyl ester resin (a type of fiberglass) containing nano-clay and 40% glass fiber as the packaging shell. In other words, the Chevrolet Volt uses vinyl ester resin to package the battery back.

The Volt use flexible lithium-ion battery cells and polymers derived from BASF nylon and 1503-2F NAT, containing 33% glass fiber and hydrolytic stabilizers, as the end and core partitions.

Polymers are also commonly used in interconnected circuit boards. Interconnected circuit boards often incorporate cell assembly points, temperature and voltage control wire harness, electronic monitoring loops, cell mechanical support, and cell vent management.

Due to its versatility, the supermodel pressing plastic plate will have some nickel or copper clad bus bars. The general electric Volt circuit board uses polyamide as the connector for the base and housing, and 35% fiberglass polyamide as the control circuit board.

Flame retardancy of polymer

The flame retardancy of polymers in battery systems also needs to be considered. According to UL-94 standard, flame retardancy includes vertical direction and horizontal direction. In the vertical direction, the flame retardancy rate range is VO-V2, and there are also some horizontal flame retardancy rate standards.

Firstly, ensure that the polymer used is not easy to burn, so that the spread of thermal runaway can be more easily controlled when the thermal runaway of lithium ion battery core is out of control. If the polymer flame retardant rate is Vo, the initial effect of battery thermal runaway on the polymer is small, so it is easier to control the cell failure.

Standard of battery pack

Battery pack must consider the battery application conditions and installation position. In practical applications, all battery systems need to meet the protection standards specified by IEC IP. For power system batteries, liquid and dust isolation (IP6K9) needs to be met. The IP rating, which is subject to the IEC standard, is 1EC60529.

The requirements of battery pack must first be able to prevent dust and physical intrusion, in other words, do not allow dust to enter the battery interior and prevent objects (such as fingers) from entering the battery interior.

Secondly, the battery needs to have a good ability to isolate liquid, such as the battery can not only tolerate liquid drops on the battery, but also can be fully immersed in the liquid or undergo water spraying without failure. For electric vehicle battery systems, sealing is required to prevent liquids and dust from entering the battery pack.

IEC IP standard for battery pack

The package grade must meet NEMA standards. NEMA sets a series of standards for indoor and outdoor electrical equipment that are often used in stationary energy storage systems and grid systems. Many lithium-ion battery manufacturers can produce standardized NEMA packaging materials.

For some small energy storage systems, materials of battery pack that meet NEMA standards are readily available. NEMA and IEC IP standards are similar, but NEMA grade requirements and protection scope is broader, including mechanical damage, explosion risk, and damp, corrosive gases, fungi, birds and animals and other special conditions protection; IECIP protects only dust and liquids.

There are many manufacturers that produce enclosures that meet NEMA standards. For smaller energy storage devices, casings are already available on the market.

Battery pack standard of Electrical Manufacturers Association of America