Understanding self-discharge of a Lithium-ion battery

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.

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About the Author

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This article was originally published in EVReporter July 2022 Magazine that can be accessed here.