Self-discharge refers to the gradual loss of charge in a battery when it is not in use. While self-discharge cannot be completely eliminated, it must be effectively managed. A high self-discharge rate can limit a battery's potential applications. The causes of self-discharge can vary based on the battery's chemistry and construction.
For more information, please visit Sunplus.
This overview compares the self-discharge rates of different primary and secondary battery chemistries, addresses challenges in measuring self-discharge, and explores chemistry-specific factors affecting self-discharge. Additionally, it discusses the achievement of ultra-low self-discharge rates in some primary lithium batteries and recent research on how tape used in Li-ion battery packs significantly impacts self-discharge rates.
Impact on Battery Shelf Life
Self-discharge can significantly reduce the shelf life of batteries. Factors such as ambient temperature, battery state of charge, construction, charging current, and others influence the self-discharge rate. Generally, primary batteries exhibit lower self-discharge rates compared to rechargeable chemistries. However, specially designed rechargeable nickel metal hydride (NiMH) batteries can achieve self-discharge rates as low as 0.25% per month.
Measuring Self-Discharge
No singular method exists for measuring self-discharge. The method chosen depends on battery chemistry and the desired accuracy of the measurement. One common approach is to measure the open circuit voltage (Voc) to determine a battery's state of charge and self-discharge over time. However, this can pose challenges.
For instance, the discharge voltage curves of popular rechargeable lithium chemistries like Li-manganese, Li-phosphate, and Li nickel manganese cobalt are relatively flat. These batteries need to be around 80% discharged before a significant voltage drop occurs. Using Voc to measure the state of charge in such cases is not practical, as it only indicates a full charge or approximately 80% discharge states. Fortunately, these chemistries generally exhibit lower self-discharge rates compared to other rechargeable options. For most Li rechargeable chemistries, coulomb counting during charging and discharging is typically used to determine state of charge.
Measuring Voc can be a simple way to gauge the state of charge and self-discharge in lead-acid batteries. However, this approach is complicated by the fact that lead-acid Voc varies with temperature. Furthermore, the use of additives, such as calcium in lead-acid battery plates, can increase voltage readings by up to 8%. In addition, a brief discharge can cause voltage to drop to a more realistic level, as increased surface charge elevates Voc immediately after charging. Absorbent glass mat (AGM) lead-acid batteries, for example, show higher Voc than their flooded counterparts.
It is crucial to note that using a Voc measurement relies on the assumption that the circuit is open and the battery voltage is 'floating' without an attached load. In contemporary systems, this assumption often does not hold true. Parasitic loads for various housekeeping functions (like digital clocks in cars) are often present, meaning the battery is rarely completely disconnected.
Understanding Various Battery Types
Lead Acid Batteries
The self-discharge rate in lead-acid batteries varies based on type and ambient temperature. AGM and gel-type lead-acid batteries generally see a self-discharge rate of around 4% per month, while less expensive flooded batteries can experience rates of up to 8% per month. Ambient temperature is a significant factor affecting self-discharge in these batteries, particularly in applications like industrial uninterruptible power supplies (UPS) and automobiles where batteries may endure high temperatures.
Nickel Metal Hydride (NiMH) Batteries
NiMH batteries have internal designs optimized to offer varying capacities and self-discharge rates. High-capacity NiMH batteries utilize thinner insulators that allow for more active material, resulting in over 25% improved capacity compared to standard designs. However, the trade-off is a shorter recharge life and higher self-discharge.
A standard NiMH battery retains 70% of its charge after 10 years of storage and can undergo over 2,000 charge cycles. In contrast, high-capacity NiMH batteries may lose up to 15% of their charge annually and typically have a recharge life of 500 cycles. There's also a lightweight NiMH variant that is about 30% less weight than standard designs. Although it has about 30% lower capacity, it can be recharged up to 3,000 times and features a self-discharge rate between that of the standard and high-capacity options.
Lithium-Ion (Li-Ion) Batteries
The breakdown of organic electrolytes is a common self-discharge source for Li-ion batteries. Factors such as high temperatures and humidity exacerbate electrolyte degradation, leading to increased self-discharge rates. Excessive heat can also damage the solid electrolyte interface (SEI), contributing to higher rates of electrolyte breakdown and lithium loss. Moreover, moisture in the battery can create an electrolytic imbalance that further raises self-discharge rates.
Self-discharge in Li-ion batteries can be managed but not fully eliminated. Storing batteries in cool, dry conditions helps mitigate electrolyte breakdown, with optimal storage temperatures typically ranging from 10 to 25 °C. Adopting proper charge management protocols to prevent overcharging can reduce microcrack formation in the separator.
Lithium Thionyl Chloride Batteries
Primary lithium chemistries such as lithium thionyl chloride (LiSOCl2) offer notably low self-discharge rates, potentially resulting in multi-decade battery lifespans. These batteries are available in spiral-wound and bobbin configurations, with spiral-wound cells supporting high discharge rates while bobbin cells boast higher capacities and self-discharge rates of under 1% annually, allowing for battery lives of up to 40 years.
The passivation that occurs within LiSOCl2 batteries, where a thin lithium chloride (LiCl) film forms on the lithium anode, is a key factor in their low self-discharge rates. This passivation layer provides a barrier that inhibits self-discharge-related chemical reactions. However, it may also induce high initial resistance and cause a temporary drop in cell voltage at the onset of discharging. As the discharge progresses, the passivation layer dissipates, resulting in increased current availability.
Tape Impact on Li-Ion Self-Discharge
Recent research indicates that the tape used in Li-ion battery assembly can contribute significantly to self-discharge. The polyethylene terephthalate (PET) tape, which holds electrodes together, undergoes chemical decomposition that produces a compound leading to self-discharge.
This conclusion was drawn when cells exposed to various temperatures were disassembled. Cells housed at temperatures ranging from 25 to 70 °C exhibited visible changes, with temperatures above 25 °C yielding increasing discolors, from slight discoloration at 40 °C to a dark red hue at 70 °C.
At high temperatures, PET tape decomposes and generates a redox shuttle molecule that transfers between electrodes, contributing to self-discharge. This molecule remains active even during storage, continually depleting the battery's charge. Replacing PET tape with alternative materials may reduce self-discharge rates in Li-ion batteries.
Conclusion
Self-discharge is an inherent property of all batteries, yet the rate varies significantly across different chemistries. Additionally, the quality of materials and construction details play critical roles in influencing self-discharge rates. Recent findings highlight that the tape used in Li-ion batteries can significantly affect self-discharge levels.
For further inquiries regarding Low self-discharge rate low voltage lithium battery wholesaler, please contact us. We will provide professional answers.
Comments
0