Electric Vehicles Battery Cells: All you need to know
There have been a lot of technological advancements happening in the area of Energy storage for Electric vehicles. These energy systems are consisting of three major components a Battery, a Battery Case, and a Battery Management System. Most of the EVs currently running on the roads uses lithium-ion and lithium-polymer battery. These batteries contain thousands of Li-Ion Cells which are working together. And these cells come in different formats and shapes. Let’s understand these different battery cells in a little more detail through this post
What is a cell?
A battery cell is a basic unit that converts electrochemical energy into electric energy. A cell is typically an enclosure that is available in different shapes and sizes. Further, a cell has a single cathode and anode separated by some electrolyte. Such a combination of components produces electric energy in form of voltage and current through a chemical process. A combination of cells packaged together in series and parallel can produce different capacities. The battery cell used in any electric vehicle has a nominal voltage of 3.1 to 4.2V depending on the chemical composition. A battery pack of an EV can contain hundreds of these cells arranged in different configurations. e.g. a 3S2P battery configuration means 3 cells connected in series and two of such 3 cell packs are connected in parallel producing roughly 12Volts
Difference Between A Battery and A Cell
| Cell | Battery |
|---|---|
| A cell is a single unit made up of different chemical configurations | A combination of more than one cell in series or parallel makes a battery pack |
| A cell is a very basic unit and smaller in size | A Battery is bigger and bulkier |
| Typical cell voltage lies somewhere between 4.2V to 3.7V | Batteries come in different capacities depending on the application |
| Provides energy for a shorter duration | Suitable for longer duration |
| Used in small electric tools | Used mostly in rechargeable electrical appliances |
| Cheaper | Costly |
What is an Ideal Energy Storage device
- High Capacity
- High Power
- Small Size
- Lightweight
- Cost efficient
- Easy to Manufacture
It comes down to cost and performance in the end for practical usage.
Types of Cells
There are three basic types of battery cells used in electric vehicles: cylindrical cells, prismatic cells, and pouch cells. The Coin Cells are mostly used for research purposes.
Cylindrical Cells
They are inexpensive to manufacture given the level of maturity it has attained over the period of time. The cylindrical structure is very sound and provides good mechanical resistance. They offer limited power because of this structure and hence required more quantity. They are also smaller in size to provide better thermal dissipation which ultimately extends the battery life. On average EVs have around 5000 to 9000 Cylindrical cells.
Cell Formats
- 18650: 18mm in diameter and 65mm in length and weighs approximately 47 grams. At a nominal voltage of 3.7volts, each cell can be charged as high as 4.2 volts and discharged as low as 2.5 volts, with each cell storing up to 3500 mAh.
- 21700: 21mm in diameter and 70mm in length and weighs approximately 68 grams. At a nominal voltage of 3.7volts, each cell can be charged as high as 4.2 volts and discharged as low as 2.5 volts, with each cell storing up to 4800 mAh. Because of the larger size, there is a longer path for the charge to travel resulting in more resistance and ultimately more heat dissipation.
- 4680: Also known as ‘tabless’ cell. Shorter electrical path hence low resistance and hence less heat generation. 46mm in diameter and 80mm in length. Each cell is rated at around 9000 mAh. 4680 is expected to provide five times more energy storage with 6 times more power
Prismatic Cells
This type of cell is enclosed in a rigid rectangular shape. Utilizing prismatic design helps battery pack manufacturers efficiently package the cells and optimize the overall space requirement. This way, manufacturers can easily reduce the overall weight as well as the cost associated with the battery component on an EV. However, due to reduced mechanical stability, the prismatic cells need thicker walls resulting in a small capacity drop. The prismatic cells compensate for higher manufacturing costs by providing flexibility in design. These cells are also less efficient when it comes to thermal management and have a lower life cycle. Prismatic cells are ideal for LFP chemistry. There is no standard size format for these cells and their size varies according to the requirements and application.
Pouch Cells
These cells are even more flexible in terms of packaging. Rather than using a mechanical container, the pouch cell has a layer of aluminum plastic film on the outer side. This way compared to other cells, the weight is lighter and these types of batteries are available in different shapes and sizes.
The major advantage of pouch cells is their higher density and performance. However, they are more complex to produce and require higher technical requirements. For the same capacity battery, an EV would need a lesser number of pouch cells than cylindrical cells.
Upcoming Technology
BYD Blade cell
BYD’s Blade battery is based on LFP chemistry and has a prismatic structure, however it is thinner and longer than prismatic Li-ion cells. This new design offers structural advantage and better support for cell to pack configuration. The cell has a bigger surface area and it allows better heat dissipation. This new cells are very safe and can withstand more external pressure than the traditional batteries. Blade battery has passed ‘Nail penetration test’, one of the most stringent tests for a battery.
Tesla Tabless 4680
These new structure is similar to cylindrical cells but with enhanced thermal optimization. The basic principle here is to achieve the conventional tab functionality through a conductive portion than runs through the electrode. With this new design, the distance an electron should travel is the height rather than length. The new design means simpler manufcaturing, fewer parts and better thermal benefits.
OEMs are also exploring Fuel Cells which needs hydrogen for generating electricity. Find more here: Fuel Cell Vehicle
