Home » Blog » The Science Behind LiPo Battery Swelling (Puffing)

How LiPo Batteries Work: Chemistry, Structure, and Functions

In the world of portable electronics, few things are as alarming as discovering that your sleek device has become distorted, the screen is lifting, or the battery compartment feels unusually tight. This phenomenon, colloquially known as “spicy pillows” or “puffing,” is a common failure mode of Lithium Polymer (LiPo) batteries. While it can look comical, it represents a serious chemical breakdown occurring inside the power source.

At Hanery, we believe that safety is the ultimate luxury. As a seasoned Chinese manufacturer specializing in the production of high-quality polymer lithium batteries, 18650 packs, and Lithium Iron Phosphate (LiFePO4) solutions, we see battery swelling not just as a defect, but as a scientific signal. It is the battery’s way of telling you that its internal chemistry has been compromised.

For Original Equipment Manufacturers (OEMs), engineers, and everyday consumers, understanding why this happens is the first step toward prevention. This comprehensive guide will dissect the science of gas generation, the dangers of swollen cells, and the rigorous standards Hanery employs to ensure our custom energy solutions stand the test of time.

Why Gases Accumulate Inside Cells: The Chemistry of the "Puff"

To understand swelling, we must first look inside the pouch. Unlike the rigid steel casing of an 18650 cylinder, a LiPo battery is encased in a flexible aluminum-polymer laminate. This allows for lightweight, custom shapes, but it also means the casing will expand if pressure builds up inside.

The root cause of this pressure is Electrolyte Decomposition.

A LiPo battery functions by moving lithium ions between the cathode (positive) and anode (negative) through a liquid or gel electrolyte. This electrolyte consists of organic solvents (like ethylene carbonate or dimethyl carbonate) and lithium salts. In a healthy battery, this process is seamless. However, when the battery is stressed, the electrolyte becomes unstable and breaks down.

The Chemical Reaction

When the electrolyte decomposes, it undergoes a reduction reaction that releases gases. The primary gases generated during this breakdown include:

Carbon Monoxide (CO)
Carbon Dioxide (CO2)
Hydrogen (H2)
Methane (CH4)
Ethylene (C2H4)

Because the battery is a sealed system (vacuum-sealed during manufacturing at Hanery), these gases have nowhere to go. They accumulate between the layers of the anode and cathode. As the volume of gas increases, it pushes against the flexible pouch walls, causing the battery to inflate like a balloon.

This gas generation is actually a safety mechanism of sorts. In a rigid steel battery, this pressure might lead to an explosion if the safety vent fails. In a LiPo, the pouch expands to accommodate the gas, giving the user a visual warning before a catastrophic failure occurs.

Signs of Early Swelling: Catching it Before the "Pop"

Swelling does not happen overnight. It is usually a gradual process. Detecting it early can save your device from permanent damage and prevent potential fire hazards.

Visual and Tactile Indicators

Screen Lift: In smartphones and tablets, the battery often sits directly behind the display. As it expands, it pushes the screen upward. If you notice your screen bulging in the center or separating from the frame, check the battery immediately.
The Spin Test: For removable batteries, place the battery on a flat table and try to spin it. A healthy battery is flat and will not spin easily. A swollen battery has a curved surface and will spin like a top.
Tight Fitment: If a battery that used to slide easily into a drone or camera compartment suddenly feels snug or requires force to remove, it has begun to swell.
Chassis Deformation: In laptops or plastic enclosures, the trackpad may stop clicking, or the keyboard may bulge upward.

The Olfactory Warning

A compromised battery often emits a distinct smell. If the pouch seal has been breached even microscopically, you may smell a sweet, chemical odor. This is the scent of the organic solvents in the electrolyte vaporizing. It is often described as smelling like nail polish remover or fermenting fruit. If you smell this, the battery is leaking and is an immediate safety risk.

Overcharging Factors: Pushing the Voltage Limit

One of the primary drivers of electrolyte decomposition is overcharging. Lithium chemistry is extremely precise. A standard LiPo cell has a nominal voltage of 3.7V and a maximum charge voltage of 4.20V.

The 4.20V Threshold

When a battery is charged, lithium ions move from the cathode to the anode. Once the battery reaches 4.20V, the anode is effectively “full” of lithium ions. If you continue to force current into the cell (overcharging), two destructive things happen:

Cathode Breakdown: The cathode structure begins to collapse because too many lithium ions have been removed. This structural collapse releases oxygen.
Electrolyte Oxidation: The excess voltage causes the electrolyte to react with the cathode, releasing massive amounts of CO2 gas.

This combination—oxygen and flammable gases—is a recipe for thermal runaway. Modern Battery Management Systems (BMS), which Hanery integrates into our packs, are designed to cut off charging precisely at 4.20V (or 4.35V for High-Voltage LiHv cells). However, using a cheap charger or a damaged BMS can bypass this safety, leading to rapid swelling.

Discharge and Temperature Effects

While overcharging is dangerous, how you use the battery (discharge) is equally critical.

The C-Rate Stress

Every battery has a C-rating, which dictates the maximum safe discharge current. If a battery is rated for 20 Amps but the device pulls 30 Amps, the internal temperature of the cell spikes. This phenomenon is known as Joule Heating.

Heat (Q) = Current (I²) x Resistance (R)

Excessive heat catalyzes the chemical breakdown of the electrolyte. Even if the battery is not overcharged, running it too hot (over 60°C / 140°F) will cause the liquid electrolyte to vaporize into gas, leading to puffing.

The "Deep Discharge" Danger

On the opposite end of the spectrum is over-discharge. Draining a LiPo battery below 3.0V per cell causes the electrolyte to decompose roughly. At very low voltages, the copper current collector on the anode can dissolve into the electrolyte. When the battery is recharged, this copper precipitates out as sharp dendrites (microscopic needles), which can cause internal shorts and gas generation.

Data Insight:

Operating Temp > 60°C: Increases swelling risk by 400%.
Discharging < 2.5V: Causes permanent chemical damage in 100% of cases.

Physical Damage Causes

Not all swelling is chemical in origin; some is mechanical. LiPo batteries are soft and vulnerable to impact.

Punctures and Dents

If a battery is dropped or pierced, the internal separator (a thin plastic sheet keeping the anode and cathode apart) can be damaged. Even a microscopic breach allows electrons to flow where they shouldn’t, creating a “micro-short.”

This localized short circuit generates intense heat in a pin-point area. This heat boils the surrounding electrolyte, creating a gas pocket. This is why a battery involved in a drone crash or a dropped phone may start to swell days after the incident.

The Compromised Seal

The aluminum pouch is vacuum-sealed. If the tabs are bent excessively or the seal edge is damaged during installation, air and moisture can enter the cell. Lithium reacts violently with moisture in the air to form Lithium Hydroxide and Hydrogen gas.

2Li + 2H₂O → 2LiOH + H₂ (Gas)

This hydrogen gas inflates the pouch rapidly.

How to Evaluate Risk: Keep or Toss?

One of the most common questions our support team at Hanery receives is, “My battery is slightly puffy; can I still use it?”

The strict engineering answer is: No. Once a battery has swollen, its internal chemistry is compromised, and its structural integrity is gone. The pressure inside increases the risk of the housing rupturing during the next charge cycle.

However, we understand real-world scenarios. Here is a risk matrix for evaluation:

Severity	Visual Sign	Risk Level	Action
Stage 1 (Minor)	Very slight squishiness; difficult to see but can be felt.	Low/Moderate	Stop use. Monitor. Do not fast charge. Plan for replacement immediately.
Stage 2 (Moderate)	Visible bulging (1-2mm). Battery spins on a table.	High	STOP USE IMMEDIATELY. Place in a fireproof container. Do not charge.
Stage 3 (Critical)	Battery looks like a balloon; casing is tight/hard. Seams may be splitting.	Extreme	DANGER. Do not touch with bare hands if leaking. Move to safe, outdoor location if possible.

Table 1: Risk Assessment Matrix for Swollen LiPo Batteries.

Safe Disposal Recommendations

A swollen battery is a hazardous waste item. It cannot be thrown in the regular trash, as the compactor truck could crush it, causing a lithium fire.

The Professional Route (Recommended)

The safest method is to take the battery to a certified e-waste recycler or a hazardous materials drop-off center. Many electronics retailers offer battery recycling bins. Tape the terminals (positive and negative) with electrical tape to preventing shorting during transport.

The "Salt Water Bath" (For Hobbyists)

Note: This method is debated and should be done with extreme caution and protective gear.

Some hobbyists use a saltwater bath to discharge a battery fully before disposal. The salt water acts as a conductive load, slowly draining the battery to 0 volts over several days.

Fill a plastic bucket with water and mix in salt (roughly 1/2 cup per gallon).
Submerge the battery fully.
Leave for 1-2 weeks.
Hanery Disclaimer: This creates a corrosive sludge and hydrogen gas. It must be done outdoors in a well-ventilated area. We generally recommend professional recycling over this method for safety.

Prevention Methods: Best Practices

Prevention is better than cure. To extend the life of your Hanery battery and prevent swelling, follow these golden rules:

Storage Voltage: Never store a LiPo fully charged (4.2V) or fully empty. The ideal storage voltage is 3.80V to 3.85V per cell. At this voltage, the chemistry is most stable.
Avoid Heat: Store batteries in a cool, dry place (Room temperature, approx 20-25°C). Never leave them in a hot car.
Quality Charging: Use a high-quality balance charger. “Balance charging” ensures every cell in a pack is at the exact same voltage, preventing one cell from overcharging while others catch up.
Don’t Leave Unattended: Never charge a battery while you are sleeping or away from the house.
Respect C-Ratings: Do not use a low-output battery for a high-performance device.

Impact on Device Performance

A swollen battery is not just a safety risk; it is a performance bottleneck.

Reduced Capacity: The gas occupies space that should be taken up by the electrode stack. The electrolyte decomposition means there are fewer ions available to store energy. A swollen battery might only hold 50% of its original charge.
High Internal Impedance: The gas creates a barrier between the anode and cathode layers. This increases internal resistance. When the device asks for power, the voltage will sag instantly, causing the device to shut down even if the battery reads “80% charged.”
Structural Damage: The pressure can crack circuit boards, break waterproof seals, and crush internal components of the device it powers.

Manufacturer Testing Standards: The Hanery Difference

At Hanery, we prevent swelling before the battery even leaves the factory. Our OEM/ODM production lines adhere to strict quality inspection certifications (ISO 9001).

The "Aging" Process

Every single battery manufactured by Hanery undergoes an Aging Test. After the battery is assembled and filled with electrolyte, we charge it and let it rest in a temperature-controlled room for several days (typically 7-14 days).

Purpose: This allows the SEI (Solid Electrolyte Interphase) layer to form fully.
Screening: If a battery has a manufacturing defect or internal short, it will swell or lose voltage during this aging period. We discard these defective cells immediately, ensuring that only stable, healthy batteries are shipped to our clients.

High-Temp Storage Test

We randomly sample batches and subject them to high temperatures (e.g., 60°C or 85°C) for extended periods to verify the stability of the electrolyte formula. If a batch swells under test conditions, the entire lot is rejected and re-analyzed.

Frequently Asked Questions

Can a swollen battery be fixed or “deflated”?

No. Never attempt to puncture the pouch to release the gas. The gas is toxic and flammable. Puncturing the pouch will likely pierce the electrodes, causing an immediate fire. A swollen battery is permanently damaged.

Is it safe to fly with a swollen battery?

No. Airlines strictly prohibit damaged or defective lithium batteries. The lower air pressure in the cabin (even though pressurized) and the vibrations of flight can exacerbate the swelling and lead to a fire mid-air.

Why did my battery swell even though I didn’t use it?

If a battery was stored fully charged (100%) for months, it will degrade and swell. Alternatively, if it was stored empty and self-discharged below critical voltage, the electrolyte broke down. Always store at 3.8V.

How long does it take for a battery to swell?

It depends on the cause. A severe overcharge can cause ballooning in minutes. Poor storage practices usually result in gradual swelling over months.

Are 18650 batteries immune to swelling?

18650 cells have a rigid steel can, so they do not “puff” visibly like pouch cells. However, pressure still builds up. If it gets too high, the safety valve (CID) will trip, permanently disabling the battery, or it will vent gas/liquid. They are mechanically safer but chemically similar.

Does fast charging cause swelling?

Frequent fast charging generates excess heat. If the battery gets too hot (>45°C) during charging repeatedly, it accelerates electrolyte decay and can lead to premature swelling.

Is the gas inside toxic?

Yes. While it is mostly CO2 and CO, it can also contain hydrofluoric acid (HF) vapor if moisture is present, which is highly toxic and corrosive to lungs and skin.

Can I put a swollen battery in the freezer to shrink it?

No. This is a myth. Freezing might temporarily reduce the volume of the gas (PV=nRT), but it does not fix the chemistry. Furthermore, moisture condensation from the freezer can cause short circuits.

What should I do if a battery starts smoking?

Do not inhale the smoke. If safe, move the device to a non-flammable surface (concrete, outside). Do not throw water on a large lithium fire; use a Class D fire extinguisher or cover it with sand/dirt to suffocate it.

Why does Hanery use pouch cells if they swell?

Pouch cells offer the highest energy density and form factor flexibility (thin, light, custom shapes). The swelling risk is managed through high-quality manufacturing and proper Battery Management Systems (BMS). When treated right, they are safe and superior for portable electronics.

Summary & Key Takeaways

The phenomenon of LiPo battery swelling is a reminder of the potent chemical energy stored within our devices. It is caused by the generation of gases—primarily CO, CO2, and Hydrogen—resulting from the decomposition of the electrolyte. This breakdown is triggered by heat, overcharging, deep discharging, or physical trauma.

Key Takeaways:

Respect the Limits: Keep voltage between 3.0V and 4.2V.
Watch the Heat: Heat is the enemy of battery longevity.
Inspect Regularly: Look for screen lift or tight fitment.
Dispose Safely: Never puncture; recycle at certified centers.
Trust Quality: Manufacturing standards matter.

Partner with Hanery for Safer Energy Solutions

At Hanery, we mitigate these risks through rigorous R&D, advanced electrolyte formulations, and 100% aging tests. Whether you are an OEM looking for a custom battery design that prioritizes safety, or a business seeking reliable 18650 packs, our team is ready to serve.

Don’t compromise on safety. Choose a manufacturer that understands the science behind the cell.

Ready to start your project? Reach out for a consultation on custom battery packs, safety certifications, and bulk orders. Let us power your innovation safely.

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