Advantages and Disadvantages of 18650 Lithium Batteries
Speaking of using 18650 cells as the power batteries for new energy electric vehicles, the first pioneer that comes to mind is Tesla. During the development of electric vehicle batteries, Tesla tested many types of batteries and ultimately focused on the 18650 battery. Wonder why they chose the 18650 battery? First, we need to know about the advantages and disadvantages of the 18650 batteries.
Here is a brief introduction to the 18650 cylindrical lithium-ion cell:
18650 refers to the battery’s physical dimensions. It is a standard battery model established by SONY to save costs. The number 18 represents a diameter of 18mm, 65 represents a length of 65mm, and 0 indicates a cylindrical shape.
Originally, 18650 batteries referred to both nickel-metal hydride (NiMH) and lithium-ion batteries. However, since NiMH batteries are now less commonly used, 18650 mostly refers to lithium-ion batteries. A single 18650 lithium-ion battery typically has a nominal voltage of 3.6V or 3.7V; the minimum discharge cutoff voltage is generally between 2.5V and 2.75V. The common capacity ranges from 1200mAh to 3300mAh.
Next, we will analyze the advantages and disadvantages of 18650 lithium batteries from the perspectives of consistency, heat dissipation capability, energy density, safety, and cost.
1. Consistency of 18650 Lithium Batteries
Among lithium-ion batteries, 18650 batteries are the most mature and stable, widely used in electronic products. Over the years, battery manufacturers have accumulated extensive experience in the production process of 18650 batteries, achieving very high levels of consistency and safety.
In contrast, stacked lithium-ion batteries are far less mature. Common types include prismatic and pouch batteries, some of which even lack uniformity in size, dimensions, and tab positions. The production processes of manufacturers of these types of batteries often do not meet the necessary standards, relying heavily on manual control, and the consistency of these batteries cannot match that of 18650 batteries. If battery consistency does not meet requirements, managing large battery packs formed by series and parallel connections of numerous cells cannot optimally leverage the performance of each cell. However, the 18650 cell can solve this problem.
In summary, 18650 batteries have a small individual capacity, requiring a large number of cells (the Model S has 7,104 cells), but they offer excellent consistency. Stacked batteries can have a larger capacity (20Ah~60Ah), reducing the number of cells needed, but their consistency is poor. Currently, it is challenging to invest significant human and material resources to improve the production processes of stacked batteries in collaboration with battery suppliers. Therefore, during the development of the Roadster and Model S, Tesla’s only option was to purchase batteries from the market and develop its own battery system. Developing a system to manage over 6,000 highly consistent cells is technically less challenging than developing a system to manage over 200 poorly consistent cells. Even though the number of individual cells increases, it is easier to manage if the performance of these cells is reliable.
2. Heat Dissipation Capability of 18650 Lithium Batteries
Stacked batteries are thin and of large surfaces, providing good uniform heat distribution and dissipation capabilities. Therefore, the Nissan LEAF boldly adopts a passive thermal management system (essentially no management), relying on natural air convection to dissipate heat.
From the individual cells to the battery modules composed of two parallel and two series-connected cells, and finally to the battery pack consisting of 48 series-connected modules, the LEAF does not have any fans, coolant pipes, or other thermal management systems.
Then, let’s turn to Tesla. The 18650 batteries are relatively small, and the temperature difference within a single cell during normal charging and discharging is not significant. However, maintaining the temperature difference of over 6,000 individual cells within a range of no more than 5℃ is very challenging. But Tesla achieved it. How did they do it? What advantages does Tesla’s Battery Management System (BMS) have compared to other electric vehicles?
As shown in the above image, these pipes are coolant channels, which are evenly distributed in the middle of the battery module, allowing each cell to have good contact with the pipes. This way, the heat dissipated during cooling is almost the same for each cell, effectively controlling the temperature difference within a very small range.
In summary, due to the use of small-capacity 18650 batteries, the complexity of Tesla’s thermal management system has greatly increased. This means that, if only considering heat dissipation capabilities, using small-capacity 18650 batteries is not the optimal choice.
3. Energy Density of 18650 Lithium Batteries
When it comes to energy density, we need to first distinguish between the energy density of individual cells and that of battery packs.
In terms of the energy density of individual cells, 18650 batteries are higher than stacked lithium-ion batteries. The energy density of the 33Ah lithium-ion battery used in the Nissan LEAF is 157Wh/kg, while that of the stacked batteries used in the GM Volt is approximately 150Wh/kg. The energy density of the 18650 batteries used in the Roadster is approximately 211Wh/kg.
However, the management system of 18650 batteries is more complex, and the additional weight resulting from this complexity will significantly reduce the energy density of the battery pack compared to that of individual cells. The battery pack of the Roadster weighs 450kg, with an energy density of 118Wh/kg, while the LEAF battery pack weighs 225kg, with an energy density of 107Wh/kg. In terms of battery packs, the energy density of the two is comparable.
The Importance of Battery Energy Density for Electric Vehicles
It’s like discussing how important a heartbeat is to the human body. The performance, range, and reliability of a purely electric vehicle, or more precisely, a battery-powered vehicle, depend on the number of batteries in its battery pack and the energy density of each cell. That is: Total energy = Number of batteries × Energy density of each cell.
The structural design of modern cars is quite rigid, leaving limited space for the battery pack, mostly under the passenger compartment floor. This design is also for safety and space considerations.
Therefore, the number of batteries is restricted to a certain range and cannot be significantly increased. To enhance the performance of the battery pack, the only way is to improve the energy density.
4. Safety of 18650 Lithium Batteries
Since stacked lithium-ion batteries generally use aluminum-plastic film (which is thin with poor mechanical strength) for packaging, in extreme situations such as car collisions, the aluminum-plastic film is prone to rupture, leading to safety accidents. This is also why Nissan adds an aluminum shell outside the battery module composed of four individual units.
18650 batteries typically have a steel shell, which provides better safety. As mentioned earlier, with the continuous improvement of the production technology of 18650 batteries, their safety is also constantly improving. Tesla has also invested a lot of effort in dealing with potential safety accidents involving 18650 batteries. If an individual battery experiences abnormal conditions such as high temperature, depending on the severity of the anomaly, the module containing this battery will disconnect the circuit to prevent the spread of accidents. Since the individual capacity is small, as long as the accident does not spread, the severity of the accident will be relatively low.
5. Cost of 18650 Lithium Batteries
The 18650 lithium-ion battery is characterized by its large capacity, long lifespan, and high safety performance. It is also favored by consumers because of its small size, light weight, and convenience. With the deepening research on 18650 battery technology, the consistency and stability of the battery have reached very high standards. As the earliest lithium-ion battery, the 18650 battery is also the most mature and stable battery in the world, and it still occupies a leading position in its application field.
Tesla’s adoption of the 18650 battery allows it to utilize the production lines of Japanese manufacturers such as Panasonic. In the increasingly competitive market for 18650 batteries used in consumer electronic products, manufacturers like Panasonic have upgraded their products in collaboration with Tesla, modifying existing production lines for the production of power batteries. Industrial production benefits from economies of scale, and when the scale of production reaches a certain level, costs will be significantly reduced. A new energy vehicle requires thousands of 18650 batteries, making the procurement cost of individual batteries manageable.
Conclusion
It can be said that using the 18650 battery as the power source for new energy electric vehicles is the optimal choice at the current stage. With the maturity of battery technology and processes, perhaps new types of batteries will emerge for use in new energy electric vehicles.