BU-802b: What does Elevated Self-discharge Do?

Self-discharge is an inherent characteristic present in all batteries. It is important to note that self-discharge is not a defect stemming from manufacturing; however, poor manufacturing techniques and incorrect handling practices can exacerbate the issue. This phenomenon is permanent and cannot be corrected. Figure 1 below depicts the effects of self-discharge, which can manifest as fluid leaks.

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Figure 1: Impacts of Elevated Self-Discharge [1]
The self-discharge rate tends to increase with the battery's age, usage cycles, and exposure to high temperatures. If a battery shows a self-discharge rate of 30 percent within a 24-hour period, it should be discarded.

The rate of electrical self-discharge varies based on the battery type and its chemical composition. Primary batteries like lithium-metal and alkaline types have the best energy retention and can be stored for several years. In the realm of rechargeable batteries, lead acid holds a relatively low self-discharge rate, averaging a loss of approximately 5 percent monthly. However, as these batteries age and are utilized, they can accumulate sludge in the sediment trap, resulting in a soft short when this semi-conductive material reaches the plates (See BU-804a: Corrosion, Shedding, and Internal Short).

Energy loss from self-discharge is asymptotic; it peaks immediately after the battery has been charged and then gradually decreases. Nickel-based batteries can lose between 10 to 15 percent of their capacity in the first 24 hours following a charge, with an additional loss of 10 to 15 percent monthly. Figure 2 illustrates the typical storage loss for a nickel-based battery.

Figure 2: Self-Discharge Over Time [1]
The highest rates of self-discharge occur right after charging, diminishing over time. This graph depicts the self-discharge levels of a nickel-based battery, while lead and lithium-based systems exhibit lower rates of self-discharge.

Nickel-Metal Hydride (NiMH) and Nickel-Cadmium (NiCd) batteries are known for their high self-discharge rates. When left on the shelf for several weeks, they require recharging prior to use. High-performance versions of NiCd also demonstrate increased self-discharge compared to standard models. Furthermore, the self-discharge tends to increase as these batteries age, with crystalline formation (termed "memory effect") contributing to the issue. Performing regular complete discharge cycles can help manage this memory effect (See BU-807: How to Restore Nickel-Based Batteries).

Li-ion batteries experience about a 5 percent self-discharge in the first 24 hours after charging, followed by a monthly loss of 1 to 2 percent. Additionally, there’s a 3 percent monthly loss attributed to the protection circuit. A defective separator may lead to elevated self-discharge, which could create a current path, producing heat and potentially leading to thermal breakdown under extreme circumstances. In terms of self-discharge dynamics, lead acid batteries behave similarly to Li-ion batteries. Table 3 summarizes the anticipated self-discharge across 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: Self-Discharge Rate Over Time
Primary batteries exhibit considerably lower self-discharge rates compared to secondary (rechargeable) batteries.

The self-discharge rates for all battery chemistries tend to escalate at higher temperatures, often doubling with every 10°C (18°F) increment. Noteworthy energy losses can occur if a battery is left inside a hot vehicle. Increased cycle counts and aging further amplify self-discharge across all battery systems. Nickel-metal-hydride batteries can endure about 300-400 cycles, while standard nickel-cadmium batteries last over 1,000 cycles before self-discharge significantly impacts functionality. With older nickel-based batteries, self-discharge can become so high that the pack depletes from leakage rather than typical usage (See BU-208: Cycling Performance Demonstrating Capacity, Internal Resistance, and Self-Discharge).

Typically, the self-discharge rate for Li-ion batteries remains relatively stable during their operational lifetime; however, maintaining a full state-of-charge and high temperatures can increase self-discharge rates. These factors also impact overall lifespan. Importantly, a fully charged Li-ion battery is more susceptible to failure compared to one that is partially charged. Table 4 outlines the monthly self-discharge rates for Li-ion batteries at various temperatures and states of charge. The increased self-discharge observed at full charge and elevated temperatures can be surprising (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: Monthly Self-Discharge for Li-ion at Varying Temperatures and States of Charge
Self-discharge rises with increasing temperature and higher state-of-charge.

It is crucial not to discharge Lithium-ion batteries below 2.50V/cell. When this threshold is reached, the protection circuit disables, and most chargers will be unable to recharge the battery. A 'boost' program that applies a gentle charge current can often revive the protection circuit, restoring the battery to full capacity (See BU-803a: How to Awaken Sleeping Li-ion).

There are significant reasons behind Li-ion batteries entering a sleep state when discharged below 2.50V/cell. If allowed to remain in a low-voltage state for over a week, copper dendrites can develop, resulting in elevated self-discharge and potential safety hazards.

In manufacturing processes, self-discharge mechanisms must be carefully examined. Variations can arise due to corrosion or impurities in electrodes, leading to self-discharge fluctuations both within production batches and between individual cells. A reputable manufacturer will measure the self-discharge rates of each cell and discard those that do not meet accepted tolerances.

Regular cycling of charge and discharge may cause unwanted lithium metal deposits on the anode (negative electrode) of Li-ion batteries, culminating in capacity depletion and an increased risk of internal short circuits. Elevated self-discharge levels frequently precede the onset of internal short circuits, a critical area that requires further exploration to understand the specific self-discharge thresholds that may induce thermal runaway. These unwanted lithium deposits also escalate internal resistance, subsequently lowering load capability.

Figure 5 compares new Li-ion cells to those subjected to forced deep discharges, as well as those that were fully discharged, shorted for 14 days, and then recharged. Notably, the cell exposed to deep discharges below 2.50V/cell demonstrates higher self-discharge than a typical new cell, while the highest levels of self-discharge are observed in cells held at zero volts.

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Figure 5: Comparing Self-Discharge of New Versus Stressed Li-ion Cells [2]
Cells that face deep discharges and are kept at 0V have elevated self-discharge levels compared to new cells.

Figure 6 illustrates the self-discharge behavior of a lead-acid battery across different ambient temperatures. At a comfortable room temperature of 20°C (68°F), the self-discharge rate is about 3% monthly, allowing for up to 12 months of storage without a recharge. However, at an elevated temperature of 30°C (86°F), the self-discharge rises, necessitating a recharge after 6 months. If a battery’s state of charge dips below 60% for an extended period, sulfation may occur (See also BU-702: How to Store Batteries).

Figure 6: Self-Discharge of Lead Acid Based on Temperature [3]
Lead acid batteries should always maintain a charge higher than 60% of SoC and require more frequent charging in warm conditions.

References

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

Lithium-ion batteries are widespread rechargeable batteries used in electronics such as smartphones, laptops, and electric vehicles. Like any rechargeable battery, a Lithium-ion battery can lose charge over time when inactive; this phenomenon is referred to as self-discharge.

Self-discharge is triggered by a phenomenon known as parasitic load, originating from chemical reactions within the battery. Even when inactive, small currents flow within the battery, causing this self-discharge. The self-discharge rate is influenced by factors like temperature, state of charge, and the battery’s age.

In general, lithium-ion batteries exhibit lower self-discharge rates in comparison to other rechargeable battery types. However, self-discharge can still occur if the battery is not stored appropriately or is exposed to high temperatures over time.

To reduce self-discharge, it’s advisable to store Lithium-ion batteries at room temperature, ideally between 20°C and 25°C. Additionally, maintaining a partial state of charge, generally between 40% and 60%, helps alleviate stress on the battery and decelerate the self-discharge process.

In conclusion, self-discharge is a natural consequence for Lithium-ion batteries prompted by parasitic load. Various factors can influence the self-discharge rate, but adhering to proper storage conditions—such as maintaining a partial state of charge at room temperature—can significantly minimize self-discharge.

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