Swollen LiPo Batteries: Warning Signs and What to Do

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A puffed LiPo is not a pack with a cosmetic flaw. The makers' manuals treat any ballooning or swelling as a reason to stop using the battery for good. This page collects what the manufacturer and hobby-organization documents actually say, so you can act quickly and calmly: what to do in the first hour, how to dispose of the pack, and the charging and storage habits that keep packs from puffing.

This is a researched guide built from Horizon Hobby's battery manuals, the AMA's Flight School LiPo series, and Call2Recycle's guidance on damaged batteries. It is not a lab test, and it is not a substitute for the manual and support line of the brand of pack you own. Where two sources differ, the page says so. If a pack is actively smoking, hissing or hot, do not read: get everyone away, and treat it as a fire.

The short version

1. Stop charging or discharging immediately and safely disconnect the pack.
2. Put it somewhere open, away from anything that burns, preferably in a LiPo bag, and watch it. Do not touch it until it has cooled.
3. Do not use it again. Do not charge it, and do not try to discharge it yourself.
4. Contact the pack's maker and a battery recycler about disposal. Never put it in household trash.

Quick triage

What to do in the first hour

Horizon's current battery manual gives the procedure in plain language: if at any time batteries become damaged, hot, or begin to balloon or swell, discontinue charging or discharging immediately, quickly and safely disconnect the charger, and place the batteries in a safe, open area away from flammable materials, in an approved LiPo bag, to observe. After one hour, remove the batteries from service. It says not to continue to handle, attempt to use or ship them, and warns that damaged or swollen batteries "can be unstable and very hot," so do not touch them until they have cooled. An older E-flite LiPo guide gives the same sequence with a minimum observation period of 15 minutes. If you are not sure which applies, the longer one is the safer one.

A LiPo bag is a precaution, not a guarantee. Sellers of fireproof bags generally say as much, and the bag listed below carries the usual disclaimers. Use one because it buys time and contains a vent, but never treat it as a reason to charge unattended.

HOOVO fireproof LiPo bag

If there is a fire, Horizon says a Class D dry chemical extinguisher must be used and the batteries then placed in an approved LiPo bag. AMA Flight School adds that household extinguishers are not rated for LiPo fires and suggests keeping sand nearby, with a smoke detector and a fire extinguisher in the charging area. If electrolyte touches your skin, wash with soap and water immediately; if it touches your eyes, flush with generous amounts of water for 15 minutes and seek immediate medical attention.

Crash-damaged packs

E-flite's guide says to disconnect and remove the battery from the model after a crash, put it in a safe open area away from flammable materials, and observe it for at least 15 minutes. It also warns: do not place a crash-damaged battery in a car or any location that is not fireproof, because a pack that does not react after 15 minutes "may still catch fire when handled, up to many hours after the damage occurred." The best place for it until disposal is a fireproof container. This matters at the flying field, where the temptation is to toss the pack in the trunk and deal with it at home. If you also need to repair the airframe, our foam repair guide covers that side of a crash.

What to avoid

• Do not fly it, even once more. AMA is blunt: "Never use a puffed pack."
• Do not charge it. Horizon says continuing to charge or discharge a battery that has begun to balloon or swell can result in a fire.
• Do not discharge it yourself. The older recommendation to run a swollen pack down through a light bulb or other load is not in the makers' instructions, which say to stop charging or discharging a ballooning pack and remove it from service. Ask the maker's support line how to handle it.
• Do not puncture, crush or bend it, and keep metal tools away from it. E-flite specifically calls out screwdrivers and hobby knives.
• Do not leave it in a hot car or in sunlight. E-flite warns that a battery stored in a hot car can be damaged or catch fire.
• Do not put it in the trash.

A note on salt water. Some club lore, and one AMA youth lesson on LiPo safety, describe soaking dead or ballooned packs in salt water for two weeks before disposal. That method is not in the Horizon manuals, and the recyclers' instructions are different. This guide does not recommend it. Follow the maker and the recycler, who are the people who will handle the pack afterward.

Disposal

Horizon's manual says to dispose of batteries in the manner required by your city, county, state or country and to contact its product support for more information. Call2Recycle's guidance for batteries that are "bulging, leaking or otherwise damaged, defective or under recall" is that damaged packs need special handling:

• Call2Recycle offers containers approved in the US and Canada for transporting damaged, defective or recalled household batteries, which you request through its customer service.
• While waiting, bag each battery separately and keep it in a coffee can with kitty litter in an inaccessible location. At minimum, bag it and keep it somewhere nobody will bump it.
• Tape terminals and bag batteries for drop-off, and contact customer service before dropping off a damaged battery.

AMA Flight School, for its part, says swollen packs can be recycled if they are bagged, their terminals are protected and the structure is intact, and that Call2Recycle accepts LiPo packs at drop-off sites across North America. The two statements differ in tone, so the careful route is to call before you go.

Why packs puff, and what the makers say to avoid

Makers do not publish a chemistry lecture on swelling, but they do list the conditions that damage a pack. Those are the ones to control:

• Overcharging. E-flite's guide says Li-Po cells should never be charged above 4.2 V per cell (8.4 V for 2S, 12.6 V for 3S, 16.8 V for 4S, 25.2 V for 6S), and never trickle charged.
• Over-discharging. E-flite says never discharge a cell below 3.0 V under load (9.0 V for 3S, 12.0 V for 4S, 18.0 V for 6S) and to set the ESC low-voltage cutoff at 3.0 V per cell. A pack that has been over-discharged should be removed from service. AMA's youth lesson uses a more conservative 3.2 V, which is an even safer floor.
• Heat. Horizon says never to let a pack exceed 160 degrees Fahrenheit during discharge, and to store packs between 40 and 120 degrees. AMA's series gives 140 degrees as the danger threshold: if a pack feels too hot to touch, it probably is.
• Physical damage. Dents, punctures and shorts, including a brief short, can cause permanent damage to the pack.
• Sitting at full charge. AMA says a pack stored at 4.2 V per cell at room temperature loses about 20 percent of its capacity in two or three years, versus about 10 years at storage voltage in a refrigerator.

If a pack sags and your motor cuts out mid-flight, that is a separate warning sign worth reading about in our motor cutting out guide, and an ESC set with the wrong cutoff can contribute, which our ESC guide covers.

Prevention: charge, store, inspect

Charge safely. Per E-flite and Horizon: use a charger designed for LiPo packs only, set to the pack's cell count; charge at no more than 1C (so a 3200 mAh pack at no more than 3.2 A); check the pack's voltage before every charge and do not charge one below roughly 3.1 V per cell (9.3 V for a 3S), because it has been over-discharged or has failed; do not "top off" a pack at or near full charge; let a pack cool to ambient temperature after discharge; always use the balance lead; charge each pack individually. E-flite also says to charge in an open area, not on flammable materials, never inside the model or a full-size vehicle, and never unattended. AMA agrees: don't leave a pack charging unattended.

Store at half charge. Horizon says to store packs at approximately half their rated capacity; E-flite lists the half-charge voltages: 11.4 V for a 3S, 15.2 V for a 4S and 22.8 V for a 6S, which is 3.8 V per cell. AMA's series gives a range of 3.8 to 3.9 V per cell for storage longer than a week. Keep packs in a non-conductive, fireproof container, never loose together where terminals could touch, and verify voltage on any pack that has been out of service for more than six months. A charger with a storage mode takes the guesswork out; the ISDT 608AC, for example, is listed with charge, discharge and storage modes for 1S to 6S packs.

ISDT 608AC balance charger

Inspect and monitor. Before the first charge of a new pack, E-flite says to check for loose leads, broken shrink tubing and other irregularities, and to check that each cell reads between 3.5 and 3.9 V (a new 3S should read 10.5 to 11.7 V). A cell voltage checker with an alarm helps you spot a problem after a flight; the CAMWAY tester listed below shows cell and total voltages and has a default alarm of 3.3 V that can be changed. It is sold as a five-pack, which is handy if you run several packs.

CAMWAY LiPo voltage tester with alarm

Retire on schedule. AMA's series says to retire packs that have lost 20 percent or more of their capacity or have swelled. For sizing, C-ratings and battery choices in general, our LiPo battery guide is the place to start.

Frequently asked questions