What Causes a 4S LiPo Battery to Swell or Fail?

Lithium Polymer (LiPo) batteries are the power source of choice for drones, RC cars, airplanes, and many high-performance electronics. Among them, the 4S LiPo battery—with four cells in series delivering 14.8V nominal voltage—strikes a balance between power and efficiency.

But despite their popularity, 4S LiPos often raise concerns. One of the most common issues users face is battery swelling or puffing, which signals internal damage and potential danger. A swollen LiPo isn’t just an inconvenience; it can lead to performance loss, fire risk, and even explosions if mishandled.

So, what causes a 4s LiPo battery to swell or fail? In this article, we’ll explore the root causes, warning signs, prevention methods, and safe disposal practices.

Understanding 4S LiPo Batteries

What is a 4S LiPo Battery?

A 4S LiPo battery contains four cells connected in series, giving a nominal voltage of 14.8V (16.8V when fully charged). This setup makes it powerful enough for racing drones, RC cars, FPV quadcopters, and robotics.

Why Are LiPo Batteries Sensitive?

Unlike NiMH or lead-acid batteries, LiPos have high energy density but low tolerance for mishandling. Their thin separators, flammable electrolyte, and delicate internal chemistry mean that even slight misuse can cause swelling or complete failure.

What Does Swelling in LiPo Batteries Mean?

When a LiPo battery swells, its casing becomes puffy due to gas buildup inside the cells. This occurs when chemical reactions inside the electrolyte break down and release gases like carbon dioxide.

Why swelling is dangerous:

• A swollen battery may rupture or catch fire if punctured.

• Performance drops significantly, with shorter run times and voltage sag.

• Continuing to use a swollen LiPo risks damaging your device and personal safety.

Common Causes of 4S LiPo Battery Swelling or Failure

Overcharging

• Charging a LiPo beyond 4.2V per cell pushes the chemistry past its limits.

• Faulty or cheap chargers without balance functions are often responsible.

• Overcharging causes electrolyte breakdown → gas generation → swelling.

Overheating

• Running batteries at high discharge rates without cooling.

• Leaving them in hot environments (cars, direct sunlight).

• Heat speeds up chemical degradation and increases swelling risk.

Over-discharging

• Draining below 3.0V per cell damages the anode and cathode.

• Cells lose capacity, and internal resistance rises.

• Repeated over-discharging leads to premature failure.

Charging/Discharging at the Wrong Rate

• C-rate defines how fast you can safely charge or discharge a LiPo.

• Charging too quickly causes heat buildup.

• Discharging too fast can cause voltage crashes and swelling.

Physical Damage

• Drops, punctures, or crashes can break internal separators.

• Short circuits inside the cell release heat and gases.

• Damage isn’t always visible but shows up as swelling later.

Poor Storage Practices

• Storing fully charged or fully drained for months stresses cells.

• Ideal storage voltage is 3.7–3.85V per cell.

• Hot storage conditions accelerate gas buildup.

Manufacturing Defects

• Low-quality brands may have impurities in materials.

• Weak casings or poor welds cause early failures.

• Even premium brands can fail, but risk is much higher with cheap knock-offs.

Warning Signs of a Failing 4S LiPo Battery

• Swelling or puffing of the pack.

• Noticeable heating during charge/discharge.

• Voltage imbalance between cells on a balance charger.

• Sudden voltage drops or reduced run times.

• Unusual odor or signs of electrolyte leakage.

How to Prevent Swelling and Failure

✅ Proper Charging Practices

• Always use a quality balance charger.

• Never exceed 4.2V per cell.

• Avoid leaving batteries unattended while charging.

✅ Safe Discharging

• Set low-voltage cutoffs in your RC equipment (around 3.5V per cell).

• Use voltage alarms for drones or RC cars.

✅ Temperature Management

• Keep charging/discharging within 0°C–60°C.

• Let batteries cool down before recharging.

• Use fans or airflow systems during high-drain usage.

✅ Correct Storage

• Store at 3.7–3.85V per cell.

• Place batteries in fireproof LiPo bags or metal containers.

• Keep them in cool, dry environments.

✅ Handling with Care

• Avoid crushing, bending, or puncturing packs.

• Rotate batteries so all packs get even use.

• Replace old or damaged packs promptly.

What to Do if Your 4S LiPo Battery Swells

• Stop using it immediately.

• Place it in a fireproof container.

• Discharge safely (resistor or bulb method; saltwater baths are outdated).

• Dispose at a certified recycling facility—never in household trash.

Myths vs. Facts About LiPo Swelling

• Myth: A slightly swollen battery is safe.

  • Fact: Any swelling indicates internal damage.

• Myth: Freezing batteries reverses swelling.

  • Fact: Freezing can worsen chemical breakdown.

• Myth: Only cheap batteries swell.

  • Fact: Even top-tier LiPos swell if misused.

Expert Tips to Extend 4S LiPo Lifespan

• Follow the 80% rule: never drain more than 80% capacity.

• Charge at 1C unless manufacturer allows higher.

• Label batteries with purchase dates and usage cycles.

• Invest in a LiPo-safe charging station.

• Inspect regularly and retire batteries showing wear.

Conclusion

Swelling in a 4S LiPo battery is never a minor issue—it’s a warning sign of chemical breakdown and potential fire risk. The main culprits are overcharging, overheating, over-discharging, improper storage, physical damage, and manufacturing flaws.

With proper handling—such as balanced charging, safe discharging, correct storage, and careful usage—you can extend your LiPo’s life, improve performance, and stay safe. Remember: once swelling occurs, the battery cannot be fixed and must be disposed of responsibly.

The bottom line: respect your LiPo batteries, and they’ll reward you with reliable, powerful performance.

FAQs

1. Can a swollen 4S LiPo battery still be used?

No. Using a swollen LiPo is dangerous and should be avoided.

2. How long does a 4S LiPo battery last?

With proper care, 150–300 cycles. Abuse shortens life drastically.

3. Why does my LiPo swell even with a balance charger?

Other factors like overheating, high discharge rates, or old age may cause swelling.

4. Can I store a slightly puffy LiPo at home?

It’s unsafe. Keep it in a fireproof bag until proper disposal.

5. Can LiPo swelling be reversed?

No. Swelling means permanent internal damage.

6. How should I dispose of a swollen LiPo?

Fully discharge, place in a fireproof container, and take to a recycling facility.