Why LiPo packs swell, how to assess the damage, when the pack must be retired, and how to dispose of it safely.
A puffed LiPo has gas build-up from internal electrolyte decomposition — a sign of irreversible damage. Stop using it. Any visible swelling means the pack is degraded; moderate or hard swelling means retire it immediately. Never charge a puffed pack. Discharge fully to zero, submerge in salt water for 24–48 hours, then take to a battery recycling drop-off. There is no repair.
LiPo (Lithium Polymer) cells use a semi-solid polymer electrolyte that reacts to heat, overcharge, and over-discharge by decomposing and releasing gas — primarily carbon dioxide and other volatile compounds. Because the cell is sealed, this gas has nowhere to go and inflates the soft outer casing.
The process is cumulative. Each cycle that pushes the cell outside its operating limits — voltage too high, current too high, temperature too high — causes incremental decomposition. The gas builds slowly until visible swelling appears. By the time swelling is obvious, significant capacity loss and internal resistance increase have already occurred.
⚠️ Do not puncture a puffed LiPo to "release the gas." The gas inside is flammable, and puncturing introduces oxygen to a reactive electrolyte — this is how LiPo fires start. Handle puffed packs carefully and avoid compression or impact.
| Grade | What You See | Action |
|---|---|---|
| Grade 1 — Barely visible | Cells slightly rounded; hard to tell without comparing to a new pack | Monitor closely; check cell balance after every flight; expect retirement soon |
| Grade 2 — Visible | Cells clearly inflated; pack no longer flat; fits poorly in battery tray | Retire. Stop flying this pack. |
| Grade 3 — Hard / firm | Pack rigid, drum-tight; significant deformation visible | Retire immediately. Do not charge. Handle with care. |
| Grade 4 — Extreme | Outer wrap splitting, seams separating, visible leakage | Treat as active hazard. Store outdoors in fireproof container. Dispose same day if possible. |
Grade 1 packs that still have balanced cells and acceptable capacity can fly briefly while you source a replacement — but the pack is on borrowed time. Grades 2 and above are retired, no exceptions.
Inspect every LiPo for swelling and voltage balance after any hard landing or crash — even if the pack looks fine externally. Internal damage from impact can trigger gradual decomposition that shows up as puffing over the next several charging cycles.
Use a LiPo discharger set to discharge mode, or connect a 12V automotive light bulb across the main leads and wait until no reading is detectable with a multimeter. Do not use your balance charger's discharge function on a visibly puffed pack — the safer method is the resistive discharge via light bulb, outdoors, with the pack in a fireproof bag or metal container.
Once fully discharged, submerge the pack in a bucket of salt water — two tablespoons of table salt per gallon of water. Leave for 24–48 hours. This depletes any remaining electrochemical potential and neutralizes the electrolyte. The bucket should be placed outdoors or in a well-ventilated garage, not indoors.
After the salt water soak, the pack is safe for standard battery recycling. Best Buy, Home Depot, Staples, and most auto parts stores accept LiPo packs via their battery recycling programs. Call ahead to confirm — acceptance policies vary by location. Never place a LiPo in household trash or a general recycling bin; the risk of compaction-caused fire in a collection truck is real.
Store during the discharge process in a fireproof LiPo bag — especially for Grade 3 or 4 packs. A metal paint can with the lid set loosely on top works well outdoors. If you cannot discharge safely the same day, store the pack outdoors in a fireproof container, away from structures, until you can.
For the complete charging workflow, see how a balancing LiPo charger works. For a battery checker to track cell health between flights, see RC battery balance checkers.
10-page PDF — equipment checklist, LiPo safety protocol, pre-flight routine, and the 5 most common beginner mistakes.