Why Do Lithium Battery Packs Swell and How to Prevent It?

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What Exactly Happens When a Lithium Battery Pack Swells?

When people notice their lithium battery pack getting thicker or the case starting to bulge, they usually panic. Swelling, also called bloating or puffing, means gas is building up inside the cells. In a normal lithium battery pack, the pouch or metal can is sealed tightly. Once gas forms and has nowhere to go, pressure pushes the layers apart and the whole pack expands. In severe cases, the pack can double in thickness and even split open.

Most Common Causes of Swelling in Lithium Battery Packs

Swelling rarely happens for just one reason. Usually several factors work together. The good news is that almost all cases come from a handful of well-known triggers that are easy to avoid once you know them.

Overcharging – The #1 Enemy

Overcharging is by far the most frequent cause. When voltage goes above 4.2 V–4.25 V per cell (depending on chemistry), the electrolyte starts to break down. This process, called electrolyte decomposition, creates gases like CO₂, CO, and hydrogen. Even a small overcharge of 0.1 V repeated many times will slowly fill the cell with gas. Cheap chargers without proper cut-off or damaged BMS boards are the usual culprits.

Over-Discharging and Deep Discharge Damage

Draining a lithium battery pack below its safe voltage (usually 2.5–3.0 V per cell) dissolves the copper current collector. When you later charge it again, copper ions can form dendrites that pierce the separator and cause micro-short circuits. These tiny shorts generate heat and gas every cycle until the pack visibly swells.

Extreme Temperatures Speed Up Gas Build-Up

High temperature accelerates almost every unwanted chemical reaction inside the cell. Above 45 °C during charge or discharge, SEI layer growth and electrolyte oxidation become much faster, producing gas quickly. Charging below 0 °C is equally bad – it leads to lithium plating on the anode, which also generates gas when the plated metal reacts later.

Physical Damage and Internal Short Circuits

Dropping an e-bike battery, over-tightening screws on a power tool pack, or vibration in industrial equipment can crush or puncture the jelly-roll inside. Even a tiny deformation can press the cathode and anode together through the separator. The resulting internal short creates intense local heat and rapid gas formation.

Normal Ageing and Cycle Life Limits

Every lithium battery pack has a limited number of cycles before the chemistry starts to degrade. After 500–1000 full cycles (depending on depth of discharge and temperature), the SEI layer thickens and electrolyte slowly breaks down. Gas generation is part of natural ageing. A well-treated pack may stay flat for years; a heavily used one will start to puff even if never abused.

Poor-Quality Cells or Bad Assembly

Low-grade cells often contain impurities or have uneven coating. These defects act as catalysts for side reactions from day one. Poor welding, contaminated electrolyte, or missing safety vents in cheap packs make swelling almost inevitable within the first year.

Early Warning Signs You Should Never Ignore

Catch it early and you still have time to act. Look for:
• The case feels warm after normal use
• Slight bulge on one side (compare with a new pack)
• Reduced runtime even though the voltage reads normal
• Unusual hissing sound or smell
Any of these means the lithium battery pack is already producing gas.

What to Do If Your Lithium Battery Pack Is Already Swelling

Stop using it immediately. Move the pack to a fireproof area away from flammable materials. Do not puncture, squeeze, or try to charge it again – that can lead to venting with flames. Discharge it slowly to 0 % if possible (using a resistor or proper discharger), then take it to a certified recycling facility. Never throw a swollen lithium battery pack in the trash.

Practical Ways to Prevent Lithium Battery Pack Swelling

Most swelling can be prevented with simple habits:
• Always use the original charger or a certified replacement
• Never leave the pack on the charger for days
• Store at 30–50 % charge in a cool place (15–25 °C) if not used for weeks
• Avoid full discharge; recharge when you still have 20 % left
• Keep away from direct sun and car dashboards in summer
• Check the pack visually every few months

Quick FAQ about Swelling Lithium Battery Packs

Q: Is a slightly swollen lithium battery pack still safe to use?
A: No. Even mild swelling means internal damage has started. Continued use can lead to sudden failure or fire.

Q: Can I push the gas out and keep using it?
A: Never. Puncturing releases flammable electrolyte vapor and almost always starts a fire.

Q: Will swelling stop if I just store it?
A: No. The chemical reactions continue slowly even at rest.

Q: How long does it take for a lithium battery pack to swell after abuse?
A: From a few days (severe overcharge) to several months (mild overcharge + heat).

Understanding why lithium battery packs swell helps you treat them right and avoid scary surprises. Treat them with basic care and they will serve reliably for years.