Why Is Your Old Phone Battery Swollen? | iFixit News (2024)

Tell me if this sounds familiar:

You just got a shiny new phone! Your old phone still works, so you chuck it in a drawer as “backup” and promptly forget about it. A year later, you’re cleaning out that drawer and your hand bumps into something… misshapen. To your horror, you pull out your old phone, its battery bulging so much that it’s pushing the screen up from the edges. What caused this abominable transformation, and why…WHY did it happen??

Believe it or not, it’s pretty common to find swollen batteries in forgotten electronics. In this article, I’ll explain what causes batteries to swell, how swelling affects battery performance, and what you can do to prevent your stored electronics from suffering this terrible fate.

A pouch of volatile stuff

Most modern gadgets use lithium-ion polymer batteries (li-po for short). A li-po battery consists of a dense roll of thin metals and plastics coated with complex chemicals. This roll goes into an aluminum pouch along with a measured squirt of gel-like electrolyte. The pouch is then vacuum packed and heat-sealed to be airtight, creating a finished battery.

The gel electrolyte within the battery serves a crucial role: it’s the ocean for the lithium ions to swim in. No ion flow = no energy flow = no battery. Theoretically, the battery pouch is airtight, so the amount of gel electrolyte remains stable.

Alas, reality doesn’t follow theory. The gel electrolyte sealed within the battery can break down and “decompose” into gas. The gas (which consists mostly of carbon monoxide/dioxide) is trapped within the airtight pouch, which swells up as the gas builds pressure. While it looks scary, the pouch acts as a safety mechanism to contain the volatile gas mixture, preventing it from venting into the environment.

The electrolyte decomposition is mostly irreversible—there’s no way to turn the gas back to gel. But it doesn’t mean the battery’s unusable. Whenever you charge and discharge the battery, a tiny bit of electrolyte turns into gas. The battery will still work just fine. However, if a significant amount of electrolyte decomposes into gas, the “ocean” in which the ions swim through dries up, and the battery fails catastrophically.

These conditions will accelerate electrolyte decomposition and battery swelling:

• The battery gets really hot (>90 °C)
• The battery gets overcharged (bad/out of spec chargers)
• The battery is physically damaged, shorting out internal layers
• The battery gets deeply discharged for a long period of time—the reason stored electronics often have swollen batteries

Your phone lies to you (in a good way)

What does it mean to “deeply discharge” a li-po battery? Most modern electronics have a Battery Management System (BMS). This circuitry sits between the battery and the device, protecting the battery from harmful situations (including being deeply discharged).

The BMS cuts the battery off after it’s drained to a certain threshold. The battery meter on the device may show “0%”, but the BMS leaves 10-15% energy in the battery to prevent irreversible damage and swelling. So, don’t worry about draining your phone to “0%” as you watch the latest iFixit teardown on YouTube! The battery is not deeply discharged. As long as you charge your device shortly after, you’re not harming the battery in any significant way.

Too drained to keep going

What does harm a li-po battery is if it dips below “0%” for a prolonged period. A li-po battery will self-discharge, and given enough time, deeply discharge itself. As the battery rests in a deep discharge state, the electrolyte breaks down, turns into gas, and the battery poofs up. This is why it’s common to find swollen batteries in disused electronics.It’s also why some phones won’t recharge if they’re dead for a while—the phone detects the deep-discharge state and “locks out” the battery for safety.

How long does it take for a deeply-discharged battery to poof up? Depending on the battery capacity, build quality, and specific chemistry, this can take weeks or years. Even if there isn’t swelling, it doesn’t mean the battery’s undamaged—it’s possible that the pouch is no longer airtight and the gas has vented out.

Extra nerdy reading: A deep-dive between battery layers

The specific reason a li-po battery swells when it’s deeply discharged revolves around the solid electrolyte interphase (SEI). The SEI is a film that’s formed where the gel electrolyte contacts the anode’s graphite layer. When the battery manufacturer charges the battery for the first time, some gel electrolyte in the battery turns into the SEI. This newly formed SEI acts as a “skin”, protecting the remaining gel electrolyte from decomposing into gas, while shielding the graphite layer from being damaged by the solvents in the electrolyte.

When a li-po battery gets deeply discharged, it’s the SEI that breaks down and decomposes into gas—that’s where the swelling comes from. Without the SEI, the delicate electrochemical balance that makes the battery work is lost, and bad things happen within the battery: The solvents within the electrolyte can now contact and embed into the graphite layer. The copper foil behind the graphite dissolves and dissipates through the graphite into the electrolyte, contaminating it. The contaminated electrolyte will grow metallic “dendrites” that pierce through the porous insulator, resulting in localized electrical shorts.

How to prevent your battery from swelling

If your battery is already swollen, follow these instructions.

To prevent a li-po battery from turning into a “spicy pillow” in your stored electronics, you have to prevent the battery from being deeply discharged. Yes, that means you’ll need to periodically check and recharge your old devices. I know—that sounds like a terrible hassle, so here are some tips to ease the burden.

If your device has a charge limiter:

Some devices (such as Dell laptops) allow you to set the battery charge level. In that case, great news! Set the device to charge to ~60% and just leave it plugged in during storage. This will keep the battery at an optimal storage condition.

If your device has a removable battery:

1. Charge the battery to ~80% and take it out of the device.
2. Tape over the battery contacts and store the battery in a cool, dry location where the battery won’t get physically damaged (very important!)
3. Set a calendar reminder to check it in six months.
4. After the elapsed time, install the battery, power on the device, and check how much battery is left. Use this information to approximate how long it would take for an 80% charged battery to drain to 20%.
5. Recharge the battery to ~80%, set a calendar reminder according to your calculations, and you’re set.

If your device has a non-removable battery:

1. Charge the battery to ~80%.
2. If possible, turn off all features that parasitically drain the battery (such as “device findable after power off”). Many laptops and tablets have a transport, repair, or service mode—set it to that mode if possible.
3. Turn off the device and store it in a cool, dry place.
4. Set a calendar reminder to check it. If the device has a small battery (such as wireless headphones), check the device after two months. For phone and laptop sized devices, check it after four months.
5. After the elapsed time, turn on the device and check the remaining battery charge. Use this information to approximate how long it would take for an 80% battery to drain to 20%.
6. Recharge the battery to ~80%, set a calendar reminder according to your calculations, and you’re set.