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.

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

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