If a lithium‑polymer (LiPo) battery starts to swell, it’s telling you something important—stop and reassess. This guide helps you spot five clear warning signs, understand what each one means for safety and performance, and take the right next steps for RC packs and consumer devices in 2025.
How this is organized: Each sign includes (1) what to look for, (2) what it means, (3) what to do—plus quick do/don’t bullets, and (4) prevention tips. We keep the tone calm and practical, and link to authoritative guidance where it matters.
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Warning Sign #1: Visible bulging or device deformation
Risk: High (Severe if swelling is progressing or appears during charge)
What to look for
- RC packs: A pouch looks pillow‑like, won’t lie flat, or straps suddenly feel tight.
- Phones/tablets: Screen lift, case seam gaps, or a back cover that won’t sit flush.
What it means
- Swelling is gas buildup from internal reactions; the cell’s structure is compromised. iFixit notes that a swollen phone battery can push the display up and advises powering down and not pressing the screen back into place, then seeking professional service, as outlined in their 2025 guidance in the article What to do with a swollen iPhone battery (iFixit, updated).
- UL also lists “bulging… hissing, leaking, rising temperature, and smoking” as danger signs in workplace/everyday contexts; see UL Solutions’ lithium‑ion safety checklist.
What to do
- Do: Power down, remove from service, and place the device/pack on a non‑flammable surface in a fire‑resistant container (LiPo bag, vented metal box, or ceramic pot). Keep away from combustibles. For phones/tablets, schedule professional removal; avoid DIY if you’re not trained.
- Don’t: Press a lifted screen back down, puncture the pouch, bend it, or keep charging “to test it.” NFPA’s consumer tip sheet reinforces stopping use of damaged or overheating batteries; see the NFPA lithium‑ion safety tips for consumers (2024–2025).
Prevention tips
- Avoid heat stress and deep discharge. Store at about 3.7–3.85 V per cell in a cool, dry place and use balance charging. Spektrum/Horizon materials emphasize conservative cutoffs and balance charging; see the Spektrum Lithium Battery Safety PDF (manual).
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Warning Sign #2: Rising internal resistance and noticeable voltage sag under load
Risk: Moderate → High (High if combined with any physical swelling)
What to look for
- Shorter run time, early low‑voltage cutoffs in RC models, sluggish throttle, or your charger reports higher IR than usual. A pack that used to cruise now triggers LVC in half the time.
What it means
- Higher internal resistance (IR) indicates cell aging or damage. That translates to more heat and greater chance of gas‑forming side reactions under stress. RC educator Oscar Liang details using IR and sag to decide when to retire a pack in When to retire a LiPo battery, and his LiPo battery guide explains how sag reflects health.
- Manufacturer guidance warns against deep discharge that accelerates degradation. Spektrum’s safety sheet states “never discharge Li‑Po cells to below 3V under load,” supporting conservative LVC settings; see the Spektrum Lithium Battery Safety PDF.
What to do
- Do: Retire the pack from high‑current uses (e.g., drones, RC cars) and monitor temperatures closely. If IR rise comes with any swelling or heat, isolate and prepare for proper disposal.
- Don’t: Keep pushing high C‑rates “to squeeze value out of it.” Avoid using the pack where sudden sag could cause crashes or brownouts.
Prevention tips
- Right‑size your pack for current draw, keep charge rates modest (about 1C unless the manufacturer specifies higher), and always balance charge. Maintain conservative LVC (often >3.3 V/cell under load in demanding RC use) to reduce stress.
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Warning Sign #3: Hotter‑than‑usual battery during charge or use
Risk: Moderate (escalates quickly if heat pairs with bulging)
What to look for
- The pack/device feels hotter than normal to the touch, stays warm long after use, or your charger logs elevated temperatures.
What it means
- Heat suggests higher IR or side reactions. Charging or operating outside a moderate temperature window shortens life and increases hazard potential. See Battery University’s overview of temperature effects in Charging at high and low temperatures (Battery University).
What to do
- Do: Stop charging/using immediately. Move the battery to a non‑flammable surface to cool, then inspect for swelling or damage. Resume only after evaluation and only if temperatures return to normal.
- Don’t: Charge unattended, charge inside a model, or charge on combustible surfaces. Horizon Hobby’s guidance stresses LiPo‑compatible balance chargers and safe charging setups; see How to charge RC batteries (Horizon Hobby).
Prevention tips
- Charge at roughly 1C unless the datasheet allows more, ensure airflow during use, avoid hot cars/sun exposure, and let packs cool to ambient before recharging.
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Warning Sign #4: Tight fit, pressure on components, or parts misaligned
Risk: High (hidden damage risk to screens/PCBs)
What to look for
- Straps that suddenly need extra force, foam that’s compressed, buttons that stick, trackpads that “click” unevenly, or a camera module that’s misaligned. In phones, even a slight lift of the display is a red flag.
What it means
- Mechanical pressure often precedes obvious bulging and can break delicate parts. iFixit cautions against compressing a swollen device and recommends powering down and professional removal; see What to do with a swollen iPhone battery (iFixit).
- RC and drone makers advise avoiding stress and inspecting batteries before use. See DJI’s lithium battery safety notes and Horizon’s E‑flite LiPo warnings on not crushing or puncturing cells.
What to do
- Do: Remove the battery if safe to do so without bending or squeezing it. Store in a fire‑resistant container until you can replace or dispose of it properly.
- Don’t: Force the battery bay closed, stack gear on top, or “strap it tighter.”
Prevention tips
- Build in clearance in DIY projects, use proper trays/foam that don’t compress cells, and inspect mounting hardware that can gradually tighten.
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Warning Sign #5: Chemical/plastic odor, audible hissing, or visible pouch damage
Risk: Severe (immediate hazard)
What to look for
- A sweet/solvent‑like smell, any hissing or venting sounds, tears/creases in the pouch, or visible residue.
What it means
- These are signs of compromised containment with flammable gases present. UL explicitly lists “hissing, leaking, rising temperature, and smoking” among danger indicators; see UL Solutions’ lithium‑ion safety guidance.
- U.S. authorities consistently advise immediate stop‑use and isolation for hazardous lithium products. The Consumer Product Safety Commission’s recall language routinely directs consumers to stop using affected products right away; see a 2025 example in the CPSC recall notice language.
What to do
- Do: Evacuate people from the immediate area if active venting is suspected, increase ventilation, and avoid inhaling fumes. Place the battery on a non‑flammable surface away from combustibles and contact your local hazardous waste/recycling program for next steps.
- Don’t: Puncture, crush, or “deflate” a swollen LiPo. Don’t put it in household trash or curbside recycling.
Prevention tips
- Protect packs from impact, use proper cases, and stop using packs after crashes until a careful inspection. Replace any pack with torn laminate or exposed foil.
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Safe handling, storage, and disposal (2025 quick guidance)
- Charging: Use a LiPo‑compatible balance charger, on a non‑flammable surface, never unattended. Keep charge rates around 1C unless the manufacturer allows otherwise. See Horizon’s RC charging basics and general temperature limits from Battery University’s overview.
- Discharge/use: Configure conservative Low Voltage Cutoff and avoid deep discharge below ~3.0 V per cell under load; many hobbyists choose >3.3 V/cell for margin. See Spektrum’s lithium safety manual note on minimum voltage under load.
- Storage: Store around 3.7–3.85 V/cell in a cool, dry place; don’t leave packs fully charged for long periods. Spektrum Smart batteries document auto‑storage features around ~3.9 V/cell; see an example product page like the Spektrum Smart G2 auto‑storage behavior.
- Disposal: In the U.S., do not place Li‑ion batteries in household trash or curbside recycling. The EPA (updated 2024) recommends taping terminals or bagging batteries individually and taking them to HHW programs or certified recyclers; see EPA’s frequent questions on lithium‑ion batteries. Damaged/defective/recalled (DDR) batteries often require special handling—programs like Call2Recycle’s DDR guidance and kits explain how they’re managed. Always call ahead.
- Emergency: If ignition occurs, prioritize evacuation and isolate nearby combustibles. Use appropriate extinguishers for surrounding materials and follow local fire authority guidance. NFPA provides public safety tips; see the NFPA lithium‑ion safety tip sheet.
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Quick, printable checklist
Use this one‑screen checklist in the field or shop. If any item is “Yes,” stop using the battery and follow the actions above.
- Looks puffy, won’t lie flat, or device shows screen lift/case gaps — Risk: High
- IR is rising or voltage sags early under load — Risk: Moderate → High
- Runs hotter than usual during charge/use — Risk: Moderate
- Feels tight in its bay or presses on components — Risk: High
- Smells like solvent, hisses, leaks, or pouch looks damaged — Risk: Severe
Immediate steps for any “Yes”
- Power down and isolate on a non‑flammable surface
- Do not puncture, compress, or charge again
- Place in a fire‑resistant container away from combustibles
- Arrange replacement and contact appropriate recycling/HHW facility
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FAQs
Can a puffy LiPo be “fixed”?
- No. Swelling comes from internal gas generation and structural damage; you cannot safely reverse it. iFixit and safety authorities warn against puncturing or trying to “deflate” cells; see iFixit’s swollen battery guidance.
Is it ever safe to use a slightly swollen battery?
- Treat any physical swelling as a stop‑use condition. At minimum, retire it from high‑current demands and prepare for replacement. UL’s danger‑sign list (bulging, hissing, leaking) underscores that swelling is a hazard; see UL’s safety checklist.
What storage voltage should I use?
- For most LiPo packs, about 3.7–3.85 V per cell is a good storage target. Many RC systems and “smart” packs automate this near ~3.9 V. Always verify your pack’s datasheet. See Spektrum Smart auto‑storage behavior.
How do I dispose of a swollen LiPo in the U.S.?
- Don’t throw it in the trash or curbside recycling. Tape terminals or place the battery in an individual plastic bag, and take it to a household hazardous waste site or a certified recycler. The EPA’s 2024 update explains this in Frequent questions on lithium‑ion batteries. Damaged/defective/recalled batteries may need special handling; consult programs like Call2Recycle’s DDR information.
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If you remember only one thing: Swelling is a stop‑sign. Power down, isolate, and replace—then adjust your charging, storage, and current‑draw habits so it doesn’t happen again.
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