One Major Way Smartphone Batteries Can Catch Fire—and How to Prevent It
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One Major Way Smartphone Batteries Can Catch Fire—and How to Prevent It

Lithium-ion polymer batteries, also known as lithium-polymer, or li-po for short, are awesome little pouches of energy that power our beloved smartphones, laptops, and tablets. Any portable gadget that requires lots of continuous power probably has a li-po battery as its heart. (Devices that require large bursts of power such as power tools and e-bikes tend to use hard-shell lithium-ion batteries, which behave slightly differently from li-po batteries.) Li-po batteries are one of the most energy-dense electrical storage options available, and under normal conditions, they’re perfectly safe to handle. 

Two Li-Po batteries walk into a bar. A fight breaks out and they both get stabbed. One gets spicy, while the other stays icy.

Yet stories of exploding batteries, rare as they may be, have captured the public’s imagination for reasons that are quite understandable. But what really happens if you’re fixing your phone, your tool slips, and you accidentally poke the battery? Images of scary explosions and chemical burns flash before your eyes—if you hurt the li-po in any way, it feels like a fiery response will be an inevitable outcome. 

The iFixit team is here to mythbust the li-po’s explosive nature, and, in the process, show you how to prevent a wounded li-po from going postal. To do so, we’re going to dissect and stab a few batteries—for science

The Anatomy of a Li-po Battery (a.k.a. “Spicy Pillow”)

A li-po battery is like a tightly packed fruit roll-up. Multiple layers of ultra-thin metals, plastic, and chemical slurries are stacked together, rolled tightly into a flat rectangle and stuffed into a foil pouch. The pouch is filled with a polymer-based electrolyte (hence the lithium polymer name) and sealed, making a single complete battery cell. Multiple cells are connected together in parallel or in series to make larger batteries. For a look at how these batteries are made, check out this in-depth video of an iPhone battery factory in action. 

There are three major layers in a li-po battery: the cathode (+), insulating separator, and the anode (-).

From left to right: the cathode (+), insulating separator, and anode (-)

The cathode (+) is a thin sheet of coated aluminum. The coating imparts different charge, discharge, capacity, and cycle characteristics and boasts fun names like Lithium Manganese Oxide (LMO) and Lithium Cobalt Oxide (LCO). The common denominator with all the coatings is lithium, a low-density metal that is a key factor in making the battery work.

The insulating separator is a flexible, semi-permeable sheet that physically separates the cathode (+) and anode (-) layers, while allowing lithium ions to flow through. Without it, the cathode and anode would touch each other and cause a short-circuit. 

The anode (-) is a thin sheet of copper coated with a carbon material—normally graphite of some sort. 

All of these layers are soaked in a gel-like electrolyte, which gives the lithium ions a medium to flow in. No ion flow = no energy. The electrolyte consists of a mixture of lithium, solvents, and additives—the amount of electrolyte strongly affects how much energy the li-po battery can store. The exact composition is different with every manufacturer and is a closely guarded trade secret for each. 

The actual electrolyte is hard to show, so we opted for a stand-in electrolyte: toothpaste!

The spicy ingredient isn’t the lithium 

When a li-po battery catches on fire, it’s not the battery’s lithium content touching air/moisture that ignites the battery. Rechargeable li-ion batteries have very trace amounts of metallic lithium—not enough to supply the “oomph” necessary for ignition (unlike the non-rechargeable primary lithium batteries, which have quite a bit of metallic lithium and can ignite from moisture contact.) Rather, it’s the solvents in the electrolyte that are prone to fiery bouts. 

Normally, the electrolyte is safely insulated from things that can set it off, but sometimes things (usually pointy) can change that. Let’s see what happens when something punctures a li-po battery. 

Play-by-play of a thermal runaway

When you puncture a li-po battery, you break through the insulating layers and cause a localized short-circuit. All of the battery’s stored electrical energy wants to course through this short, and the resulting current flow superheats the spot—this is how a car’s cigarette lighter works. This hotspot vaporizes the surrounding electrolyte, which turns into CO2 gas mixed with very volatile solvents. It also heats this gas, and if the mixture reaches the flash point, the gas ignites! The heat from the fire vaporizes more electrolyte, creating a chain reaction known as thermal runaway. Boom! Battery fire.


When we punctured a fully charged iPhone 12 Pro Max battery, for instance, it immediately swelled up and glowed like a hot metallic croissant. Smoke and gasses spewed out the sides, and the vapors quickly caught on fire: 

Once the thermal runaway begins, it’s very difficult to stop it. Like white-hot coals, you have to cool the reaction to a point where the electrolyte is no longer self-generating fuel and heat. The common approach to lithium-ion battery fires is to douse it with large amounts of water or wait for the battery to burn out, as seen in this Tesla Emergency Response Guide

25% or (Com)bust

Since it’s so difficult to put out a li-po battery fire, it’s imperative to prevent it from happening in the first place. You can drastically reduce the chance of a thermal event by draining the battery to 25% or less. Without the stored potential energy, the battery has a difficult time generating the heat required to ignite the electrolyte, even when there’s a short-circuit. Puncturing a less than 25% charged battery might generate sparks and smoke, and the battery could get really hot, but it’s unlikely to catch on fire and enter thermal runaway mode.

To see for ourselves, we stabbed a 25% charged iPhone 12 Pro Max battery multiple times. The battery got hot (~120°C) and shot out some smoke and sparks, but that was about it.

This is why you’ll see a battery discharge note in our repair guides. We want to make the repair as safe as possible, and discharging the battery makes a big difference. Don’t skip this important step! 

For your safety, discharge the battery below 25% before disassembling your phone. This reduces the risk of a dangerous thermal event if the battery is accidentally damaged during the repair. If your battery is swollen, take appropriate precautions.

iFixit repair guide warning

Note that discharging the battery for safety works only with smaller li-po batteries—like those found in phones and laptops (laptops have bigger batteries, but they’re also composed of smaller, individual li-po cells.) Tablets such as the 12.9” iPad Pro contain two large li-po cells. There may be enough stored energy to ignite the battery even if it’s discharged to below 25%. If you’re working around larger li-po cells, discharge the device to 0%. Don’t worry—you won’t damage your battery if it’s drained for a short time.

Also note that for most modern devices, li-ion batteries aren’t truly drained when your device shows 0%. When the battery’s low, the BMS (Battery Management System) board cuts it off when it drops below a voltage threshold—normally around 3.3 V. This leaves about 10-15% energy left in the battery, which prevents it from being irreversibly damaged. Just don’t leave it at 0% for weeks on end, or the battery can self-discharge below this safety threshold and become irreversibly damaged and swollen. 

Help! I punctured a battery!

If you puncture a li-po battery during a repair, don’t panic! A punctured battery rarely explodes, but it can vent super hot fiery gas and spit out shards of hot shavings, which can cause severe burns. Inhaling the smoke and solvent is unhealthy, but rarely fatal.

If the battery doesn’t immediately react (because you’ve cleverly drained the battery and it was a minor jab), monitor the battery for five minutes. If it doesn’t warm up after five minutes, you can continue working on your device, but don’t reuse the battery. Even if the jab didn’t breach the outer pouch, the battery may have suffered internal structural damage and is a potential safety hazard.

If the battery warms up, set the device on a fire-resistant surface and give it a day for the battery to fully discharge. Once it’s discharged, the battery will be inert and cool to the touch. Remove and dispose of the battery properly. Don’t reuse the battery.

If the battery begins to thermally runaway, treat it as a Class B fire (the vaporized solvent is primarily fueling the fire). Use fire-resistant tools like a metal spatula to push the device onto a fire-resistant surface like a metal baking sheet. If possible, move the device outside to a safe location where the battery can burn itself out.

If you can’t move the device, you can smother the fire with sand, cool the reaction with a foam, ABC, or CO2 fire extinguisher, douse it with a large amount of water, or cover it with a fireproof container. Even if you extinguish the fire, the battery will continue to smolder like a hot lump of coal. Monitor it until it fully cools down. Once it’s inert, dispose of the remnants properly

Toasty Bonus: Free Battery Fire Wallpapers

As a reward for making it to the end of the post, here’s a wallpaper of a scorched iPhone 12 Pro Max. To get these as your background or lock screen: Navigate to this page on your phone. Tap a wallpaper to view it at full resolution, then save it to your photos. Open the Settings app, select “Wallpapers” and then “Choose a New Wallpaper.” 

iPhone 12 Pro Max Melted Internals Wallpaper

iPhone 12 Pro Max Melted Internals Wallpaper (Dark Mode)

Hopefully these images will be the closest you’ll ever get to a battery fire! Practice safe battery handling procedures and remember: Only you can prevent battery fires.