A swollen LiPo is one of the most common field complaints in consumer electronics programs. The pouch enclosure that makes lithium-polymer cells thin and shapeable also makes them visible gas-pressure indicators. Understanding why a cell swells — and which type of swelling matters — stops engineering teams from either ignoring a real safety risk or panicking about a normal manufacturing artefact.
The basic mechanism: gas inside a sealed pouch
A lithium-polymer cell is a stack of electrode layers sealed inside an aluminium-composite laminate pouch. The electrolyte fills the spaces between layers. When side reactions occur inside the cell, they produce gas — typically CO₂, CO, methane, or ethylene, depending on the cathode chemistry and the specific reaction. Because the pouch is sealed, that gas has nowhere to go except to inflate the pouch itself.
The key question is what triggered the gas. Three sources dominate in practice:
Source 1: Formation residual gas (normal, benign)
During the first charge after electrolyte fill — the formation cycle — the electrolyte reacts with fresh electrode surfaces to form the solid electrolyte interphase (SEI) layer. This produces a small amount of gas as a byproduct. Most manufacturers handle this by puncturing and resealing the pouch after formation, or by designing a "degassing" step into the production process. If this step is incomplete or skipped (a cost-cutting shortcut in some commodity cells), the finished cell arrives with a small amount of trapped formation gas.
Formation residual gas causes very mild swelling — typically less than 0.3 mm thickness increase on a 4 mm cell. It does not grow with use and is generally harmless. The tell: a cell that arrives from the factory with a very slight dome that does not change over the first 50 cycles.
Source 2: Electrolyte decomposition from overcharge or over-temperature
When a cell is charged above its rated voltage — even briefly — the electrolyte begins to oxidise at the cathode. The decomposition products include CO₂ and other gases, and the reaction is not self-limiting: once started, continued overcharge accelerates decomposition. Similarly, extended exposure above 60 °C (for standard LiPo) accelerates the same electrolyte breakdown independently of voltage.
This type of swelling is progressive: the cell gets thicker with each charge cycle. It is the most common cause of failure in consumer electronics products that charge at high rates in hot enclosures. The BMS protection parameters responsible are the charge voltage ceiling (which must never exceed the cell's rated max voltage, typically 4.20 V or 4.35 V for HV-LCO) and the temperature cutoff during charging.
Source 3: Calendar ageing and SEI growth
Even a cell stored at room temperature slowly builds up additional SEI material over time. The byproducts of this slow reaction include trace gas. In a correctly manufactured and operated cell, calendar-ageing swelling over 2–3 years is typically less than 0.5 mm. In a cell stored at elevated temperature (40–60 °C, common in vehicles or outdoor devices in summer), calendar ageing accelerates significantly and the associated swelling can become mechanically problematic within 18 months.
How to assess whether swelling is dangerous
Not all swelling requires cell replacement. The risk framework we use with product teams:
| Swelling category | Thickness increase | Risk level | Action |
|---|---|---|---|
| Formation residual (new cell) | < 0.3 mm | Negligible | No action needed |
| Normal calendar ageing (> 1 year, ambient storage) | 0.3–0.8 mm | Low | Monitor; note in EOL planning |
| Progressive electrolyte decomposition | > 1 mm, growing per cycle | Moderate | Investigate root cause; apply charge voltage / temperature fix |
| Rapid venting (visible dome or audible hiss) | Large (> 3 mm or deformed) | High | Remove from device immediately; do not charge |
| Vented cell with electrolyte smell | Any | Critical | Isolate; follow MSDS disposal procedure; do not charge |
Design changes that prevent swelling
Mechanical relief space. Every enclosure around a LiPo cell should budget at least 1–1.5 mm of expansion room on the widest face of the cell. Cells pressed flat against a rigid housing with zero clearance can buckle in their electrical connections or delaminate internal tabs when they swell.
Charge voltage margin. Specifying the charge cutoff at 4.18 V instead of 4.20 V on a standard LiPo (and 4.33 V instead of 4.35 V on an HV-LCO variant) reduces electrolyte oxidation rate by approximately 40% with a capacity penalty of only 2–3%. For products that prioritise longevity over peak capacity, this is one of the most effective single changes available.
Temperature charging inhibit. Disable charging above 45 °C (rather than the cell's rated 60 °C) in the BMS. Most of the overcharge-related electrolyte decomposition at high voltage occurs much faster above 50 °C. The combined overvoltage + over-temperature scenario is where thermal runaway risk begins.
Ventilation path design. For devices that produce significant heat (induction chargers, high-discharge drone ESCs), a ventilation path that moves air past the cell face reduces the enclosure temperature enough to meaningfully extend calendar life and reduce swelling rate.
What to do in the field
If a customer reports a visibly swollen device, the safe procedure is: power off, do not charge, remove the battery if the device allows it, and follow the cell's MSDS disposal guidance. Swollen cells should never be placed in household recycling bins — they require lithium battery disposal at a certified point. For RMA analysis, ship the returned cell with the swelling dimension recorded and the charge history from the device if available. The charge history often reveals whether overcharge or over-temperature was the root cause.