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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Figure 1: Effects of high self-discharge [1]
Self-discharge increases with age, cycling and elevated temperature. Discard a battery if the self-discharge reaches 30 percent in 24 hours.

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.

Figure 2: Self-discharge as a function of time [1]
The self-discharge is highest right after charge and tapers off. The graph shows self-discharge of a nickel-based battery. Lead- and lithium-based systems have a lower self-discharge.

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.

Battery System Estimated Self-Discharge Primary lithium-metal 10% in 5 years Alkaline 2&#;3% per year (7-10 years shelf life) Lead-acid 10&#;15% in 24h, then 10-15% per month Nickel-based Li-ion, NiCd, NiMH Lithium-ion 5% in 24h, then 1&#;2% per month (plus 3% for safety circuit) Table 3: Percentage of self-discharge in years and months
Primary batteries have considerably less self-discharge than secondary (rechargeable) batteries.

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)

Type 0°C (32°F) 25°C (77°F) 60°C (140°F) Full Charge 6% 20% 35% 40&#;60% Charge 2% 4% 15% Table 4: Self-discharge per month of Li-ion at various temperatures and state-of-charge
Self-discharge increases with rising temperature and higher SoC.

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.

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Figure 5: Self-discharge of new and stressed Li-ion cells [2]
Cells that had been stressed with deep discharges and kept at 0V show a higher self-discharge than a new cell.

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)

Figure 6: Self-discharge of lead acid as a function of temperature [3]
Lead acid should never drop below 60% SoC. Charge more often when warm.

Reference

[1] Courtesy of Cadex
[2] Source: TU München
[3] Source: Power-Sonic

Lithium-ion batteries are rechargeable batteries that are commonly used in electronic devices such as smartphones, laptops, and electric vehicles. Like any other rechargeable battery, a Lithium-ion battery can lose charge over time when it is not in use. This is known as self-discharge.

Self-discharge occurs because of a process known as parasitic load, which is caused by the chemical reactions that occur within the battery. Even when a Lithium-ion battery is not being used, small currents are still flowing within the battery that can cause self-discharge. The rate of self-discharge varies depending on a number of factors, such as the temperature, the state of charge, and the age of the battery.

Generally, Lithium-ion batteries have a low self-discharge rate compared to other types of rechargeable batteries. However, self-discharge can still occur over time, particularly if the battery is not stored properly or is exposed to high temperatures.

To minimize self-discharge, it is recommended to store Lithium-ion batteries at room temperature, ideally between 20°C and 25°C. It is also recommended to store the battery at a partial state of charge, typically between 40% and 60%. This helps minimize the battery's stress and slow down the self-discharge process.

In summary, self-discharge is a natural process that occurs in Lithium-ion batteries due to parasitic load. The self-discharge rate can vary depending on various factors, and storing the battery at a partial state of charge and at room temperature can help minimize self-discharge.

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