7 Perspectives to Uncover the Truth Behind Self-Discharge
In the battery industry, “low voltage” is an issue that every engineer and salesperson has to face. Customers often complain: “These batteries dropped in voltage after sitting for a while — is there something wrong with your cells?”
In fact, in most cases, the root cause of low voltage is excessive self-discharge of the battery. So, what exactly is self-discharge? And what causes it? Today, the iRay Energy team will walk you through a systematic explanation.
1. What Exactly Is Battery Self-Discharge?
Simply put, self-discharge means that the battery’s voltage drops on its own when it is not being used. We usually measure the self-discharge rate in “mV/day” or using the “K value (mV/h).”
For example, if the voltage drops by 2 mV in one day, that is 2 mV/day. For a high-quality cell, the K value is typically within 0.08 mV/h.
Anything beyond this range can be considered excessive self-discharge, meaning that some “abnormal” internal consumption processes are occurring inside the battery.
2. Why Do Batteries Self-Discharge?
There are essentially only two core causes. In fact, all battery self-discharge issues can be attributed to these two categories:
Physical micro short circuits
Chemical reactions
These may sound technical, but once we break them down, they become quite easy to understand.
3. Physical Micro Short Circuits: Internal “Pathways” Form Inside the Cell
As the name suggests, a “physical micro short circuit” refers to tiny conductive paths that form inside the cell. Unlike an external short circuit, it won’t cause immediate ignition, but it continuously releases energy, causing the voltage to drop gradually. Common situations include:
a. Dust and burrs
This is the most common cause. When you disassemble a cell with high self-discharge, you will often see black spots on the separator.
If the black spots are in the middle area, they are usually caused by dust particles penetrating the separator.
If the spots are near the edges, they are most likely caused by electrode burrs piercing the separator.
These tiny defects are hard to see with the naked eye, but under a microscope, you can clearly observe local conductive marks.
b. Metal impurities in the cathode
Many people don’t realize that metal impurities in the cathode can also lead to physical short circuits. For example, in LFP materials, if iron impurities exceed the limit, they may migrate during charging and penetrate the separator. In such cases, similar black spots appear on the separator.
Therefore, controlling metal impurities in raw materials is the first barrier against self-discharge.
c. Metal impurities in the anode
Impurities in the anode are even more subtle. Some low-end artificial graphite or natural graphite contains trace metal particles. During cycling, these metal ions can dissolve and redeposit on the separator, causing micro short circuits.
This is why some domestic anode materials have good capacity performance but consistently show a high “low-voltage rate.”
d. Metal impurities from auxiliary materials
It’s not just the electrodes—tapes, aluminum-plastic film, and cover plates may all introduce metal contaminants. This issue is especially common in factories where cleanliness control is insufficient.
4. Chemical Reactions: The Cell’s “Internal Chemical Rebellion”
The second cause is chemical reactions occurring inside the cell. These reactions may take place slowly during storage. Although they are invisible and imperceptible, they continuously consume lithium and degrade the electrolyte, ultimately causing the voltage to drop. Common issues that lead to internal chemical side reactions include:
a. Moisture issues
Moisture is the “number one culprit” of self-discharge. Some may ask: “Wasn’t moisture already decomposed during formation?” Yes, but the problem is that its by-products continue to react!
During formation, moisture reacts with lithium salts to form by-products such as POF₃ and PF₅. These substances can regenerate moisture in subsequent cycles, effectively forming a catalytic reaction chain.
This explains why, even if the initial moisture is low, the cell voltage still drops over time. We have conducted experiments: letting the cell sit for 45 minutes after electrolyte injection before sealing significantly increased the self-discharge rate.
b. Unstable electrolyte solvents
Some high-conductivity solvents perform well but have poor oxidation resistance. During storage, they slowly decompose, producing by-products and consuming capacity.
In our R&D, we tested a solvent that significantly improved conductivity, but its self-discharge rate was three times faster than that of a normal system. This is a typical example of the “performance versus stability” contradiction.
c. Unstable SEI layer
The SEI layer acts as the anode’s “protective layer,” theoretically preventing continuous reactions. However, if the SEI is poorly formed, or if it is damaged or restructured during storage, ongoing side reactions can occur.
This is especially true in high-temperature warehouse environments, where the SEI layer is more likely to detach, triggering a series of issues including swelling and low voltage.
d. Poor sealing
Improper cell sealing can also cause self-discharge. For example:
Poor sealing around the tabs can lead to tab corrosion.
Insufficient edge sealing may allow the electrolyte to penetrate and corrode the aluminum foil.
Once the aluminum-plastic film is punctured, moisture enters, triggering a chain reaction.
In such cases, the symptoms of “low voltage + swelling” often appear simultaneously.
5. Why Are Dust and Burr Issues the Most Common?
In theory, everyone knows that dust must be controlled, but in actual production, it is very difficult to achieve.
On one hand, many high-performance formulations today use high surface area (high BET) materials and large amounts of conductive additives, which naturally shed particles more easily.
On the other hand, cleanliness management in workshops is often insufficient, especially during electrode transfer, slitting, and stacking processes, where dust can be easily introduced.
As a result, dust penetration remains the number one killer of self-discharge.
6. Isn’t Moisture Removed During Formation? Why Does Low Voltage Still Occur?
This is a common question among new engineers.
Indeed, moisture is electrolyzed during formation, but the by-products it generates (such as PF₅ and POF₃) do not disappear. These substances can react again during storage to produce trace amounts of water, forming a cyclical reaction.
In other words, these “chemical residues” act like catalysts inside the cell, continuously accelerating side reactions. This also indicates that controlling moisture is not only about the initial drying process—it is necessary to consider the atmosphere and timing throughout the entire sealing and electrolyte injection process.
7. How Do Metal Ions Cause Short Circuits?
The harmful effects of metal ions mainly occur during charging and discharging.
Take the cathode as an example: when it contains metal impurities, these impurities can be electrolyzed into ions during charging, migrate, and deposit onto the separator or anode, creating localized conductive paths.
The situation is similar for metal impurities in the anode, especially copper ions, which can dissolve and redeposit under abnormal conditions, causing micro short circuits.
Sometimes, we observe copper plating on the anode surface, which is direct evidence of metal ion deposition.
Conclusion
In summary, battery self-discharge is essentially the cell “leaking electricity” internally. Some of it occurs through physical pathways caused by dust and burrs, while other cases are due to chemical reactions involving moisture, electrolyte, the SEI layer, or poor sealing.
To truly address the issue, one must control the root causes—such as raw material purity, cleanliness management, and electrolyte injection processes—while also paying attention to storage conditions and sealing reliability. Every millivolt of voltage drop in a cell reflects the details of manufacturing processes and the rigor of management.
Sometimes, it’s not that the battery is faulty; it’s that we haven’t yet fully understood why it is “discharging on its own.”
