Understanding self-discharge of a Lithium-ion battery

5 min read

Goto Sunplus to know more.

What is self-discharge?

Battery self-discharge is caused by the internal reactions in a battery that reduce the energy stored without any connection with an external circuit. In other words, the battery loses the energy stored in it by itself due to its internal behaviour even when the connected application is not demanding any energy. Since the state-of-charge (SoC) is directly linked to the battery&#;s open-circuit voltage (OCV), self-discharge leads to a reduction of the SoC, which leads to the reduction of the OCV of the battery.

Self-discharge is undeniable, and it happens in every type of system (battery) that stores energy. However, the speed at which the self-discharge happens is of concern. This is one of the reasons why supercapacitors are not preferred in electric vehicle applications. Supercapacitors have a high self-discharge of up to 50% per month. Whereas Lithium-ion batteries have a self-discharge of up to 5% per month. But these values can change depending on the grade of cells.

What is the significance of self-discharge?

Grading

Self-discharge is an important parameter when the Lithium-ion cells undergo grading during cell manufacturing. However, many practitioners are unaware of the self-discharge parameter and only tend to check the capacity, OCV and IR values to understand the quality of the cell during the battery pack assembly process. Let us discuss the self-discharge characteristics of a popular type of cell used by many Indian battery pack assembly companies. For this exercise, let&#;s take the self-discharge grading parameters of an LFP cylindrical cell. The cell is to be charged to its nominal voltage of 3.2V and then kept at 45°C temperature for ten days. At this stage, the voltage drop to about 3.17V. The self-discharge test is performed hereafter by keeping the cell for 30 days in standard conditions. The following observations can be made based on their grades. A grade cell would see a voltage drop of less than 30mV. A minus grade cell would see a voltage drop between 30mV and 90mV. B grade cell would see a voltage drop of more than 90mV. The above values are for reference only and based on a technical exercise the author conducted with a particular cell manufacturer. These values are subject to change depending on the cell manufacturer and storage conditions for self-discharge. The above values will also vary for each type of cell chemistry.

Rating cell quality would mean the following:

A grade > A minus grade > B grade

The self-discharge parameter of the cell is often overlooked but is a critical factor when it comes to grading the cells. Many companies straightaway go for capacity grading and then for OCV and IR grading, but a little-known fact is that A and A-minus grades can have very similar values of capacity, OCV and IR and the distinguishing factor between them is self-discharge. This means that a company might think they are purchasing A grade cells, but in reality, they are receiving A-minus cells as self-discharge was not considered.

Balancing

A grade cell usage is essential for serious applications such as electric vehicles and long-duration energy storage systems. A grade cells have the least variation among themselves, and they can be used in battery packs with a high number of parallel connections. Such packs still manage not to have many balancing issues with ageing. This is why Tesla cars almost never face balancing issues.

A-minus grade cells can be used in slow charge and low power discharge scenarios such as smaller solar applications since slow charging gives more time for the BMS to balance the cells. They need time for balancing because the variation among A-minus cells is higher than A grade cells. It is advisable to use A-minus grade cells in a battery pack with a low number of parallel connections. If A-minus cells are used in battery packs with a high number of parallel connections, the battery pack tends to experience balancing issues after a few hundred cycles. B grade cells are not suitable to be used in battery packs with any number of parallel connections. This is because the variation among B grade cells can be very high and cause many balancing issues in a battery pack. The balancing issues in a battery pack lead to the inability of a battery pack to charge fully and discharge fully. This will mean that the vehicle range (in the case of an electric vehicle) or backup time (in the case of an energy storage system) will drastically be lower than the original value.

Choosing the right quality of cells along with a BMS with reliable components and the right parameters is the key to making a successful battery pack. Add thermal management to that, and you have a battery that can last a long time, avoiding on-field issues to a large extent.

About the Author

Rahul Bollini is a Lithium-ion cell and battery pack R&D expert with an industrial experience of over 7 years. He can be reached at +91- and .

This article was originally published in EVReporter July Magazine that can be accessed here.

Subscribe & Stay Informed

For more Low self-discharge rate low voltage lithium battery servicesinformation, please contact us. We will provide professional answers.

Subscribe today for free and stay on top of latest developments in EV domain.

Enter your address

Name

Name

Organisation

Organisation

Country

Country of Residence

Lithium Ion Self-Discharge Measurement System

What is a cell&#;s self-discharge? Self-discharge of an electrical cell is the loss of charge over time while not connected to any load. Some amount of self-discharge is a normal attribute resulting from chemical reactions taking place within the cell. Li-Ion cells typically lose about 0.5 to 1% of their charge per month.

The lithium-ion (Li-ion) cell market is experiencing explosive growth, and this growth creates stress on cell manufacturing operations, with pressures on process costs, inventories, and deliveries. It&#;s a challenge for Li-Ion cell manufacturers to quickly discern whether newly formed cells exhibit acceptable self-discharge behavior.

Lithium-Ion cell and battery performance testing is both a priority and a challenge for engineers in design or manufacturing. This is especially true for evaluating Li-ion cells for self-discharge. Lithium-Ion cells exhibiting high levels of self-discharge have higher likelihood of failure and must be sorted out and the cause identified. Unfortunately, this has traditionally been a long and tedious process to perform. Traditionally, self-discharge isn&#;t a complicated measurement &#; it&#;s relatively straightforward to measure how the open circuit voltage (OCV) of cells changes over time. The issue is how long it takes for that OCV to change enough to reliably tell whether the self-discharge of your cells is within acceptable limits.

Breakthrough Li-Ion Self Discharge Measurement Solutions

Now you can directly measure self-discharge current in 1&#;2 hours instead of monitoring cell open circuit voltage over weeks or months and quickly measure and analyze self-discharge current during cell design and evaluation.

Keysight&#;s new solutions of Li-Ion self discharge measurement solutions provide a revolutionary reduction in the time to measure and characterize self-discharge performance of Lithium-Ion cells. These Lithium-ion self discharge measurement solutions determine a cell&#;s self-discharge by directly measuring its self-discharge current.

Are you interested in learning more about low voltage lithuim battery China? Contact us today to secure an expert consultation!

• Directly measure self-discharge current in as little as 1&#;2 hours instead of monitoring cell open circuit voltage over days or weeks.
• Rapidly discern good vs. bad cell self-discharge performance in manufacturing in minutes. Dramatically decrease work-in-process inventory.
• Quickly measure and analyze self-discharge current during cell design and evaluation. Reduce design cycle time and get to market faster.