Understanding the Self-charge and discharge mechanism ...

Lithium-ion batteries are rechargeable batteries that use lithium ions to store energy. They are known for having a low self-discharge rate compared to other rechargeable batteries, typically losing only about 5% of their monthly charge. This means that they can be left unused for long periods of time without losing their charge. The movement of lithium ions between the anode and cathode of the battery mainly controls the charge and discharge of lithium-ion batteries. During charging, lithium ions move from the anode to the cathode, storing energy. During discharge, the lithium ions move from the cathode to the anode, releasing energy. This cycle can be repeated multiple times for the same battery. The rate at which lithium-ion batteries charge and discharge depend on several factors, including the type of electrolyte used, the size and composition of the electrodes, and the battery's temperature. The rate of charge and discharge is also affected by the design of the battery, such as how the electrodes are arranged. Overall, the charge and discharge of lithium-ion batteries is a complex process that can be affected by many different factors. However, lithium-ion batteries are still a very popular choice for many applications due to their high energy density and low self-discharge rate.

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Self-charge, and discharge mechanism of a lithium-ion battery (Image by: Dreamstime.com)

Importance of self-discharge

The self-discharge of lithium-ion batteries is an important factor in ensuring the long-term performance of the battery. Self-discharge occurs when the battery is not in use and is a natural process that occurs with all battery types. A lithium-ion battery typically self-discharges at a rate of about 5% per month, depending on the type and temperature of the battery. This self-discharge rate can be reduced by maintaining the battery storage voltage above the minimum voltage and storing the battery at lower temperatures. By managing self-discharge, the battery can maintain its charge capacity over time and provide a longer lifespan.

Self-discharge mechanism

Self-discharge is a phenomenon in which the stored electrical energy of a battery is gradually lost over time even when the battery is not being used. This phenomenon occurs due to a variety of factors, including chemical reactions, leakage, and temperature. In order to reduce the rate of self-discharge, various methods such as using lower temperatures and special coatings are used.

Effect of self-discharge of a storage battery

The self-discharge of a storage battery is the loss of charge that occurs over time, even when the battery is not in use. This can affect the battery&#;s performance and shorten its lifespan. Self-discharge can be caused by internal chemical reactions, environmental factors, and other factors. It can reduce the battery&#;s capacity and performance and can also lead to early battery failure. Self-discharge can be minimized by proper storage and maintenance, but the effects of self-discharge cannot be completely eliminated.

The distinction between chemical and physical self-discharge

Chemical self-discharge occurs when an electrochemical reaction reduces the voltage of a battery over time. This happens when the chemicals in the battery react with the electrolyte, producing other elements. Physical self-discharge occurs when the terminal voltage of a battery drops as a result of heat, vibration, or other mechanical action. This process can be accelerated by exposure to high temperatures, which causes the ionic charge carriers to move faster, leading to a quicker voltage loss.

Self-discharge test

Self-discharge tests are performed to measure the rate at which a battery discharges itself over time. This is important in order to determine the battery's capacity, reliability, and performance. The test involves disconnecting the battery from an electrical circuit and measuring the voltage level over a certain period of time. The voltage level should decrease steadily as the battery discharges. If the voltage level does not decrease as expected, then the battery may be faulty or have a shorter lifespan than expected.

Types of self-discharge tests

A self-discharge test of a battery is a type of test used to measure the rate at which a battery loses its charge over time when it is not connected to a load or other devices. This test is typically done by measuring the voltage of the battery over a given period of time and can be used to assess the health of a battery. It can also be used to determine the battery's capacity and expected life span. Below mentioned are the type of self-discharge tests:             

1. Continuous self-discharge test

2. Intermittent self-discharge test

3. Short-term self-discharge test

4. Long-term self-discharge test

5. Vibration self-discharge test

6. Submersion self-discharge test

7. High-temperature self-discharge test

8. Low-temperature self-discharge test

Influencing factors and control points of self-discharge

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•  The internal resistance of the battery: The internal resistance of the battery affects the self-discharge rate since it determines the amount of current that can be drawn from the battery. High internal resistance results in a lower self-discharge rate.

Control point: Select a battery with lower internal resistance.

•    Temperature: Temperature affects the electrochemical reaction inside the battery, which in turn affects the self-discharge rate. Higher temperature results in a higher self-discharge rate.

Control point: Keeping the temperature at an optimum level for the battery.

•   State of charge: Self-discharge rate increases as the state of charge of the battery decreases.

Control point: Maintaining the state of charge of the battery at an optimum level.

•   Age: As the battery ages, its self-discharge rate increases.

Control point: Regular maintenance and replacing the battery after its life cycle.

Battery Discharge Rate:

Rate refers to the current value required for the battery to discharge its rated capacity within a specified time. Numerically, it is equal to the multiple of the rated capacity of the battery, usually denoted by the letter C. Generally, 0.2-2C represents capacity lithium batteries, abbreviated as LCR, while 2C and above represent power lithium batteries, abbreviated as LNR/LMR.

High-rate batteries typically refer to lithium batteries, which mainly rely on the movement of lithium ions between the positive and negative electrodes to operate. During the charging and discharging process, Li+ is inserted and extracted between the two electrodes: during charging, Li+ is extracted from the positive electrode and embedded in the negative electrode via the electrolyte, and the negative electrode is in a lithium-rich state; during discharging, the process is reversed. Batteries with lithium-containing materials as electrodes are representative of modern high-performance batteries.

Lithium batteries can be divided into high-rate batteries and lithium-ion batteries. Currently, lithium-ion batteries are used in mobile phones and laptops, which are commonly referred to as high-rate batteries by people. True high-rate batteries are rarely used in daily electronic products due to their high risk.

Lithium-ion batteries have high energy density and a uniform high output voltage. They have low self-discharge, with good batteries having less than 2% self-discharge per month (recoverable). They do not have memory effects. They have a wide operating temperature range of -20°C to 60°C. They have excellent cyclic performance, can be quickly charged and discharged, have a charging efficiency of up to 100%, and have a high output power. They have a long service life. They do not contain toxic or harmful substances and are known as green batteries.

Battery Self-Discharge Rate:

Self-discharge rate, also known as charge retention capacity, refers to the ability of a battery to retain its stored charge under certain conditions while in an open circuit state. It is mainly influenced by factors such as battery manufacturing processes, materials, and storage conditions. It is an important parameter for measuring battery performance.

Since it is impossible for the raw materials used in battery production to be 100% pure, impurities are inevitably present, resulting in self-discharge phenomena.

The drop in voltage of lithium power lithium batteries is measured by how many mV it decreases per day, denoted as mV/day. Qualified lithium power lithium batteries should not have a voltage drop of more than 2mV per day.

It is represented by the value K, which indicates how much the voltage drops within a unit time for lithium power lithium batteries, denoted as mV/h. For qualified lithium power lithium batteries, the K value is generally within 0.08mV/h. The K value for lithium power lithium batteries is calculated as follows: K = (V1 - V2) / ΔT, where V1 is the voltage of the lithium power lithium battery one hour before, and V2 is the voltage one hour later.

It is expressed as the self-discharge rate, which represents the percentage decrease in the capacity of lithium power lithium batteries within a specified time: Y% = (C1 - C2) / C1 × 100, where Y% is the self-discharge rate, C1 is the capacity of the lithium power lithium battery before storage, C2 is the capacity after storage, and T is the storage time, usually expressed in days, weeks, months, or years.

Due to factors such as electrolyte compatibility, graphite negative electrode characteristics, and inconsistent assembly, lithium power lithium batteries often experience voltage drop during use or storage. A significant portion of the voltage drop is caused by the self-discharge of the lithium power lithium battery cells themselves.

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