Li-ion Battery 101 continues with our fourth blog, "What’s the Right Li-ion Cell for my Device?" In this blog we will elaborate on the Li-ion cell types and why an application may require one cell type over another.
In our previous blog “Li-ion Cell Types,” we reviewed the most common type of battery cells - button or coin, prismatic, polymer or pouch, and cylindrical. But when designing a battery-powered device, how do you know which battery cell to use?
When selecting a battery cell, it is important to follow best practices and consider design implementation as well as the total system environment. The cells are a part of the battery pack, which is a part of the device, which is a part of the total system. All of these parts must work together. Before a battery cell is selected, we must understand the needs of the device, the architecture of the total system, and the environment in which all will operate, including the chargers, power supplies, accessories, and the end-users’ profile.
Some Questions to Ask When Choosing the Right Cell:
- Is the battery removable from the device? If so, how often is it removed?
- Is the battery removed by an end-user or a qualified technician?
- What is the form factor of the device the battery will be placed in?
- What is the device’s function?
- Are there weight and size requirements of the battery pack?
- Are there charge, storage, and temperature requirements of the battery?
- Are there specific run time and cycle life requirements of the battery?
- What are the power and performance needs of the battery?
- What is the life expectancy of the device?
- What is the availability of the cell and who will be the cell supplier?
Answering the above questions during the cell selection process will help determine the best cell option and can also aid in cell supplier selection. For the highest safety and reliability standards, choose tier 1 suppliers who follow the best manufacturing and quality processes, with fully automated lines and world class quality control. In other cases, it may make sense to choose a lower tier supplier, because top tier suppliers support markets differently. Stay involved in the process and ensure the cell supplier is audited regularly. Additionally, choose cells that have been properly agency tested and certified by UL (Underwriter Laboratories) and IEC (International Electrotechnical Commission).
When considering the expected market life of your device (i.e. 5, 10, 15 years) it is essential to know the cell's long-term availability needs. Typically, the cells available on manufacturer road maps are good for three years, but they may need to be changed out sooner. It is best to choose popular cells as they tend to have a longer supply life. You may need to plan for extra inventory capacity for your selected cell in case an End-of-Life (EOL) notification is issued. To alleviate this risk, we advise our customers to consider qualifying more than one cell for their device. You also should consider the device's market/application as it could impact the cell availability. Some cell suppliers have certain rules/restrictions for use in different markets. For example, some cell suppliers require that the charge voltage be reduced in medical applications. When designing a medical device, this would be another factor that should be considered during the cell selection process.
The Right Cell for an Application
There are tons of applications that run on batteries, and a cell type that might work for one application may not work for another. For example, with most cell phones and digital notepads “thin is in” and these applications need to keep a very thin profile. Due to the internal device structure, the designer needs to source thin profile battery cells, like a polymer pouch under 5mm. Other devices may require thin polymer, coin, or prismatic cells to stay under 10mm. Consider wearable medical or consumer devices that cannot stick out too far from the skin or body, such as glucose monitors, hearing aids, wireless earbuds, or smartwatches.
When a device does not require a thin profile design, cylindrical cells are the best choice because of their robustness, power, and energy. Cylindrical cells, like the 18650, are commonly found in battery packs that power medical instruments, Battery Backup Units (BBU), handheld military devices, e-bikes, power tools, electric vehicles, and more. If the battery pack thickness is over 20mm, a good practice to follow is to select a cylindrical cell as your first choice with a second option being a standard 103450 prismatic cell.
As you can see, there are several factors to consider when choosing the best cell for your device. At Inventus Power, we help our customer determine the best solution for their application based on our data experience and cell supplier relationships.
Since we launched Li-ion Battery 101 we’ve talked about what a Li-ion battery is, the different kinds of battery chemistries, and cell types. These topics primarily focused on the cell itself, which are the basic building blocks for Li-ion Batteries. In our next blog, we will learn how cells and other components come together in “The Construction of the Li-ion Battery Pack.”
