Lithium ion batteries

Bold Valuable Technologies offers custom battery design and manufacturing services, including lithium ion batteries. We have developed several packs for motorsport applications for years and offer this experience to our customers seeking a reliable and safe operation of their battery powered products. Lithium ion batteries are used extensively across many sectors for their energy density and durability.

Battery fundamentals

Before digging into lithium ion battery we need to know some basic battery fundamentals. The smallest unit of a battery is called a cell. A cell is composed of an anode (-), a cathode (+) and is able to receive, store and provide electric energy. Cells are arranged in series or paralel to form what is known as battery.

Cells have different characteristic voltages depending of its chemistry. This voltage is the nominal voltage and the cell can produce higher or lower voltage depending of the state of charge. Most lithium ion batteries have its nominal voltage above 3,0 V.

One of the most important fucntions of a battery is to store energy and charge. Charge capacity is the charge a cell is able to store. Its units are Ah (or in IS, Coulomb – 1Ah = 3600 C). Similarly to the voltage characteristics, different chemistries have different capacities. The energy a cell is able to store is know as energy capacity, or nominal energy capacity. Its units are Wh (Watt * hour) or J (Joules).

The discharge or charge rates of the cell are know as C-rate. This is the theoretical charge that the cell can supply for 1h. For example, a 3Ah battery is able to supply 3A for 1 hour. This would be called 1C discharge rate. At 2C, it will supply 6A for 30 min. The relationship is not completelly linear due to phisical properties of the cell but this will not be covered in this article.

A lithium ion battery with cells of 3,6V and 2Ah, has roughly 7,2 Wh energy storage capacity. The power at which the battery can be discharged is a relationship of its internal resistance and is difficult to quantify.

If our lithium ion battery has 10 lithium ion cells of 2Ah and 3,6 V connected in series, the battery will have a charge capacity of 2Ah and an energy storage capacity of 72 Wh, and a voltage of 36V. If the same cells are connected in paralel, the voltage of the battery will be 3,6V, a charge capacity of 20Ah and an energy storage capacity of 72 Wh.

The energy density is the energy capacity of the battery per unit mass or volume. This gives an indication for comparison of how small a battery can be packaged to achieve the same energy capacity.

Introduction to lithium ion cells

Huge advances have been found in the battery chemistry in the last decade. This has allowed a number of new applications to benefit from the use of batteries, including the electric vehicle industry. Lithium ion cells are the star amongst the chemistries that have been developed the most recently.

Characteristics of lithium ion cells

These are some characteristics of lithium ion cells:

• Lithium ion cells operate at higher voltages (3,7V nominal voltage) than other common chemistries (e.g. 1,2V for NiMH). This makes them suitable for more applications since less cells are required to achieve a given voltage.
• Although other cell chemistries do not require a battery management system, lithium ion cells need it to operate within safety conditions. In practical terms, a BMS is a piece of electronic circuitry that measures voltage and other parameters of all or a number of cells in the battery. An algorithm in the electronic device determines whether the voltage of a particular cell needs to be adjusted which is actioned by the same device. The fact that lithium ion cells need a BMS, is a small drawback compared to other chemistries, However, for high performance or large capacity applicaions, all chemistries need a BMS, hence this is no longer a negative.
• Lithium ion cells have lower self-discharge rate than other chemistries. Even after months of built date, Li-ion cells will retain most of its capacity. Compared to other chemistries such as NiMH or NiCd where the self discharge rate is in the region of 1%-5% this is a big advantadge.

Disadvantadges of lithium ion cells

These are the disadvantadges compared to other types of chemistries:

• Lithium ion cells are in general more expensive than NiCd and NiMH. This is due to economies of scale, and thus it should become less expensive as Li-Ion cells are produced in larger quantities.
• As stated before, BMS are needed to safetly operate a lithium ion battery. The cost of this circuitry might not scale down at the same rate as cells when those are produced in larger quantities. Thus it might keep the overall price of the battery system higher than other chemistires. The BMS is needed due to the fact that lithium ion cells are very sensitive to over charge or over discharge, which in both cases would lead to the battery being damaged permanently or self-combustion in some lilthium ion chemistries.
• There are standard cell sizes (AA, C, D) for other chemistries that do not apply to lithium ion cells. Instead, common li-ion cell types are “18650” or “26650” that define its form factor. For instance, a 18650 means the cell is 18 mm in diameter and 65 mm in length.

Lithium ion cell chemistries used in lithium ion batteries

Lithium ion batteries have a number of main chemistries used in the industry at the moment. Each chemistry offers different characteristics that make it more suitable to each application. Cell phones, laptops, power tools all use this chemistry all use this type of chemistry. The main reason is they have the highest energy to weight ratio.

They have relatively long cycle life, from 300 cycles to more than 1000 cycles depending on operating conditions.

Lithium Ion cells are in the middle when it comes to cost. They are available in many different form factors and thus can be easily suited for many applications. One of the most common formats is 18650. The nominal voltage is 3,7V and voltage range is 2,5V to 4,2V.

The main types of Lithium Ion (Li-Ion) cells chemistries are: Lithium Manganese Oxide (Li-Manganse), Lithium Cobalt Oxide (Li-cobalt), Lithium Polymer (Li-Po), Lithium Nickel Manganese Cobalt Oxide (NMC), Lithium Nickel Cobalt Aluminium Oxide (NCA or NCR) and Lithium Iron Phospate (Li-Phosphate).

Lithium Manganese Oxide

Lithium Manganese Oxide was one of the first commercially available chemistries. This type of chemistry can manage high power discharge rates for short periods of time but it is also stable thermally. This is due to its low internal resistance.

The drawback of this chemistry is the relatively short life cycle times compared to other lithium ion types. The applications for this chemistry include electric vehicles, power tools and medical instruments.

The chemistry can be tweeked to obtain longer life span, higher capacity or high power rates.

Lithium Cobalt Oxide (Li-cobalt)

It is one of the oldest froms of commercially available lithium ion cells. It generally exhibits high capacity at low cost. However, it has lower current rating and only moderate life. Since it has a low thermal runaway temperature, it could be less safe than oth
er forms of cell.

Lithium Cobalt cells are also the basis for what is know as lithium Polymer cells used in applications with super-high discharge rate. These ones use a modified chemistry that provides this improved characteristics for applications such as drones.

Lithium Nickel Manganese Cobalt Oxide (NMC)

A relatively new type of chemistry that performs well in most categories, showing good life cycle, capacity and safety. The balance between nickel, manganese and cobalt determines the exacty performance in each category. Although other chemistries will surpase the performance in one of these chategories, NMC perform well in most of them simultaneously.

This makes NMC batteries suitable for many applications offering good safety and performance characteristics.

Lithium Nickel Cobalt Aluminium Oxide (NCA or NCR)

It is similar to NMC with the difference of having aluminium in the cathode instead of Manganese. This makes NCA cells to have the highest capacity of all lithium ion cells. Hence, these cells are an excellent candidate for electric vehicle applications to achieve large capacities. It also offers small volume and weight for its capacity, being one of the highest energy dense chemistries.

Lithium Iron Phosphate

Lithium Iron Phosphate are heavier and have lower energy density than most li-ion cells. They are also more expensive than other types, around 20% when compared with Li-ion cells of the same capacity. Their discharge rate depends vastly on the supplier, however premium makers offer high power levels which are used by OEM in automotive.

The advantadges of Lithium Iron Phosphate cells are that they are one of the most safe chemistries to operate. Fires are extremely rare due to the low oxidation reactivity of this electrolyte. Furthermore, they have large cycle life and low self discharge rate.

Lithium Iron phosphate cells have a nominal voltage of 3,2V and a range of 2,5V to 3,65V.

Summary of lithium ion cells

As a quick summary, the table below shows the main characteristics of the chemistries described before.

Many other chemistries are available and Bold Valuable Technology can offer you specialist advice to select the best cells for your application.

Bold can offer cell block design including:

• Cell chemistry and supplier selection
• Cell incoming inspection
• Cell testing and characterization
• Cell block dimensioning and simulation (thermal and electromechanical)
• Cell block mechanical design for operational requirements and UN38.3
• Cell block safety detail design and working procedure manual creation
• Cell block cooling system design

Cell construction formats in a lithium ion battery

The basic manufacturing method for all lithium ion chemistries is similar. They have a cathode material as positive active material, anode material as negative active material, a membrane separating positive and negative material layers, and an electrolyte. The way those elements are packaged gives different construction formats. The main three are pouch cells, prismatic cells and cylindrical cells.

Pouch cells

Pouch cells are the simplest form of construction. The name comes from the way the active material is stored in a pouch made out of thin aluminium foil and covered by a layer of insulating material. The active materials inside are stacked in layers which determine the capacity and thermal performance. To form a battery, the pouches are stacked and clamped externally to work under pressure. This prevents the cell from expanding during operation.

Due to the thin construction they are the best volumetric efficient cells and can be the lightest depending on how the structure is completed around the stack of cells. The tabs can be both on the same side or on opposite sides.

Prismatic cells

Prismatic cells are similar to pouch cells in shape. The main difference is that they have a rigid container around them. This means they are more durable than pouch cells and do not require an external clamping structure to apply pressure to the cells. The terminals usually have a boss with a threaded hole which allows a cable to be easily connected to it. These modules can be used directly for electric vehicles or house storage batteries.

Cylindrical cells

Cylindrical cells are the most common type of cells used by many consumer products. Their equivalent with lithium ion cells are for example 18650, 26650, 38120 and others depending on them manufacturer. This construction has the active material rolled inside a metallic cylindrical can that is applying pressure and provides protection. The terminals are either side of the cylinder and are usually welded to reduce electrical resistance and improve reliability.

Battery pack design for lithium ion batteries

In lithium ion batteries, cells are arranged in series or parallel to form a cell block. This unit is where the electrochemical energy is stored. The cell block is also formed by the BMS to monitor the health of the system.

In order for the energy to be used in a vehicular application, there is the need for an inverter which converts electricity in both directions from a DC signal to AC signal. This device also receives energy from the harvesting system, usually an electric motor, back into the battery to charge it. This allows a vehicle to convert the kinetic energy into electric energy under breaking and in turn charge the battery.

Bold Valuable Technology has experience in inverter design. Besides the electronic design of the unit to meet the operational and performance requirements, we can bring the experience in designing cooling systems for correct heat management. This includes liquid coolers with pumps and heat exchangers. We can also provide CFD analysis for optimization of the cooling system design and pump dimensioning.

Structural analysis, especially modal analysis, helps in avoiding failures of the electronic elements and bus bars. Bold has worked closely in resolving potential issues detected in Finite Element Analysis and has implemented solutions which where then validated by physical tests.

The last important aspect of the pack design around the inverter is high voltage protection. In combination with our composites knowledge, we can provide a lightweight and flexible solution to achieve the structural requirements with low weight.

Com
posite materials offer the possibility of combining non conductive materials within the structure to make it completely insulated in case of the enclosure coming into contact with the high voltage devices.

Bold offers complete battery pack design including:

• Cooling design using CFD for water cooled or air cooled system optimization
• Structural analysis of battery pack enlcosures
• Battery pack custom AVM design and supply
• High voltage custom connector design or commercial connector selection
• Battert pack venting system design
• Port design and auxiliary equipment selection for Inert gas purging
• Transport container selection, adaptation and commissioning
• Cooling system bleeding equipment design and commissioning
• Battery pack test matrix definition and commissioning of testing

Lithium ion battery pack assembly

Bold can set up lithium ion battery assembly lines and design the equipment necessary for production of battery packs, from prototypes to high volume. We can design and produce the jigs for the different stages of manufacturing, including insertion of the cells into the cradle or cooler, welding, testing, etc. We always put safety first, especially when it comes to high voltage and lithium ion cell handling.
Also, we can provide services for dielectric strength test, continuity testing, resistance measurements, pressure testing, vibration testing, amongst others.

Bold Valuable can arrange for the transportation legislation UN38.3 testing to be carried out. We have designed and commissioned a large amount of packs that have undertaken this tests satisfactorily. This includes impact testing, vibration testing and climatic testing amongst others and is compulsory for batteries that have to be air freighted at some point in its life.

If you have a project regarding lithium ion batteries, do not hesitate to contact us.