BU-802b: What does Elevated Self-discharge Do?
BU-802b: What does Elevated Self-discharge Do?
All batteries are affected by self-discharge. Self-discharge is not a manufacturing defect but a battery characteristic; although poor fabrication practices and improper handling can increase the problem. Self-discharge is permanent and cannot be reversed. Figure 1 illustrates self-discharge in the form of leaking fluid.
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The amount of electrical self-discharge varies with battery type and chemistry. Primary cells such as lithium-metal and alkaline retain the stored energy best, and can be kept in storage for several years. Among rechargeable batteries, lead acid has one of the lowest self-discharge rates and loses only about 5 percent per month. With usage and age, however, the flooded lead acid builds up sludge in the sediment trap, which causes a soft short when this semi-conductive substance reaches the plates(See BU-804a: Corrosion, shedding and Internal Short)
The energy loss is asymptotical, meaning that the self-discharge is highest right after charge and then tapers off. Nickel-based batteries lose 10&#;15 percent of their capacity in the first 24 hours after charge, then 10&#;15 percent per month. Figure 2 shows the typical loss of a nickel-based battery while in storage.
NiMH and NiCd belong to rechargeable batteries that have the highest self-discharge; they need recharging before use when placed on a shelf for a few weeks. High-performance NiCd has a higher self-discharge than the standard versions. Furthermore, the self-discharge increases with use and age, of which crystalline formation (memory) is a contributing factor. Regular full discharge cycles keeps memory under control(See BU-807: How to restore Nickel-based Batteries)
Li-ion self-discharges about 5 percent in the first 24 hours and then loses 1&#;2 percent per month; the protection circuit adds another 3 percent per month. A faulty separator can lead to elevated self-discharge that could develop into a current path, generating heat and, in an extreme case, initiate a thermal breakdown. In terms of self-discharge, lead acid is similar to Li-ion. Table 3 summarizes the expected self-discharge of different battery systems.
The self-discharge of all battery chemistries increases at higher temperature, and the rate typically doubles with every 10°C (18°F). A noticeable energy loss occurs if a battery is left in a hot vehicle. High cycle count and aging also increase self-discharge of all systems. Nickel-metal-hydride is good for 300&#;400 cycles, whereas the standard nickel-cadmium lasts for over 1,000 cycles before elevated self-discharge starts interfering with performance. The self-discharge on an older nickel-based battery can get so high that the pack goes flat from leakage rather than normal use(See BU-208: Cycling Performance demonstrating the relationship of capacity, internal resistance and self-discharge)
Under normal circumstances the self-discharge of Li-ion is reasonably steady throughout its service life; however, full state-of-charge and elevated temperature cause an increase. These same factors also affect longevity. Furthermore, a fully charged Li-ion is more prone to failure than one that is partially charged. Table 4 shows the self-discharge per month of Li-ion at various temperatures and state-of-charge. The high self-discharge at full state-of-charge and high temperatures comes as a surprise(See BU-808: How to Prolong Lithium-based Batteries)
Lithium-ion should not be discharged below 2.50V/cell. The protection circuit turns off and most chargers will not charge the battery in that state. A &#;boost&#; program applying a gentle charge current to wake up the protection circuit often restores the battery to full capacity(See BU-803a: How to Awaken Sleeping Li-ion)
There are reasons why Li-ion is put to sleep when discharging below 2.50V/cell. Copper dendrites grow if the cell is allowed to dwell in a low-voltage state for longer than a week. This results in elevated self-discharge, which could compromise safety.
Self-discharge mechanisms must also be observed in manufacturing. They vary from corrosion to impurities in the electrodes that reflect in self-discharge variations not only from batch to batch but also form cell to cell. A quality manufacturer checks the self-discharge of each cell and rejects those that fall outside tolerances.
Regular charge and discharge causes an unwanted deposit of lithium metal on the anode (negative electrode) of Li-ion, resulting in capacity loss through a depletion of the lithium inventory and the possibility of creating an internal short circuit. An internal short is often preceded with elevated self-discharge, a field that needs further research to learn what levels of self-discharge would pose a hazard that can lead to a thermal runaway. Unwanted lithium deposition also increases the internal resistance that reduces loading capability.
Figure 5 compares the self-discharge of a new Li-ion cell with a cell that underwent forced deep discharges and one that was fully discharged, shorted for 14 days and then recharged. The cell that was exposed to deep discharges beyond 2.50V/cell shows a slightly higher self-discharge than a new cell. The largest self-discharge is visible with the cell that was stored at zero volts.
Figure 6 illustrates the self-discharge of a lead acid battery at different ambient temperatures At a room temperature of 20°C (68°F), the self-discharge is roughly 3% per month and the battery can theoretically be stored of 12 months without recharge. With a warm temperature of 30°C (86°F), the self-discharge increases and a recharge will be needed after 6 months. Letting the battery drop below 60 percent SoC for some time causes sulfation(See also BU-702: How to Store Batteries)
Reference
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.
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
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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
- 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.
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