Why Phone and Laptop Batteries Swell — and What Dendrites Are

Why a battery swells: it’s gas

A lithium-ion battery isn’t perfectly stable. As it’s charged, used and simply gets older, the electrolyte slowly reacts at the electrodes — the same side chemistry that makes every battery lose capacity (we explain it in our guide to battery wear). Some of these reactions give off gas. HP puts it simply: as lithium-ion polymer batteries age, “a small amount of gas is generated within the battery cell”, and in most cases the amount is minimal over the life of the battery.² Microsoft says that advanced expansion caused by excessive ageing is “most often caused by the formation of non-flammable carbon dioxide (CO2) gas”; Battery University lists mainly carbon dioxide and carbon monoxide.^4,10

Phone and laptop batteries are flat “pouch” cells: the electrodes are sealed in a soft foil bag instead of a rigid metal can. That saves space, but the gas has nowhere to go — so the bag inflates. Some of this is expected: Battery University estimates that small pouch packs can grow by 8–10 percent over 500 cycles. Manufacturers plan for it — pouch cells are made with a temporary “gas bag” that is cut off after the first charge, and Microsoft designs Surface devices with an enclosure meant to contain normal expansion.^4,9,10 Trouble starts when the swelling goes beyond that.

What speeds swelling up

• Age and cycles. Dell lists age and the number of charge cycles among the reasons batteries swell. Every battery produces some gas over its life; an old one has simply had more time and more cycles to do it.^2,5
• Heat. HP: long-term exposure to elevated temperatures “increases stress on Lithium-Ion Polymer Batteries and accelerates the chemical aging process”. Microsoft: devices charged or operated at high temperatures age faster. Dell names exposure to high heat as a cause of swelling.^2,4,5
• A battery that’s always full. HP singles out notebooks left plugged in, which keeps the battery at 90–100% for long periods and “can accelerate battery degradation and chemical aging”. Its infosheet on swelling names “long-term exposure to a high state-of-charge and elevated temperatures” as factors that can accelerate swelling — which is what HP’s charge-limiting Battery Health Manager is meant to counter. Microsoft notes that batteries kept at a high charge state lose capacity faster.^2,3,4
• Charging a cold battery, or pushing in too much current. When lithium can’t enter the graphite fast enough, Battery University explains, the unabsorbed energy “turns into gas buildup, heat and lithium plating”.¹¹
• Manufacturing quality. Battery University notes that some swelling can occur even with proper use and that “most is due to faulty manufacturing” — one reason the quality of a replacement battery matters.⁹
• Physical damage. Dell’s handling rules start with: don’t crush, drop or puncture a battery, and don’t bend it.⁵

What a swollen battery breaks

The gas isn’t what ruins the device — the pressure is. In a phone the battery sits right behind the screen; in a laptop it usually sits under the touchpad and keyboard. As it inflates, it pushes on whatever is in the way. Battery University: the pressure “can crack a battery cover open and in some cases break the display or electronic circuit”. HP: the deformation can be significant enough to affect the touchpad, keyboard and panel.^1,9

Typical signs: the screen lifts at one edge or a gap opens between the screen and the frame; the back glass cracks or lifts; the phone rocks when laid flat; a laptop’s base bulges or no longer sits flat; the touchpad stops clicking properly or the keyboard bulges. On a MacBook, a swollen battery is a module replacement — see MacBook repair.

At our workshop a swollen battery often arrives together with a broken screen, because by the time the gap was noticed, the display had already been pushed out or cracked. Caught early, it’s one part to replace instead of two: battery replacement starts from €50, a screen from €40.

Is a swollen battery dangerous?

The honest answer has two parts. HP states that “a swollen battery does not present a safety issue”, and Microsoft that expansion caused by excessive ageing “does not present a safety concern” — the gas is mostly carbon dioxide.^1,4 But the same companies tell you to stop using the device, and Microsoft adds that you should handle it with caution “to prevent putting pressure on or risk puncturing the battery cell”. Dell warns that “puncturing, bending, or crushing a battery can be dangerous”.^4,5

The real danger is a damaged cell. If something breaches the separator between the electrodes, an internal short circuit can heat the cell very quickly: Battery University describes temperatures reaching 500 °C, at which point the cell catches fire or explodes — “venting with flame”. Such failures are rare — Battery University puts the failure rate of a quality cell at better than 1 in 10 million — but a battery that’s already bulging, pressed on by a cracked screen or pried at with a screwdriver is exactly the kind not to take chances with.¹⁴

“Crystals” inside a battery: dendrites and copper bridges

You may have heard that a battery “crystallises”. The idea comes from older nickel-cadmium batteries, whose “memory” really was caused by crystals growing on the cadmium plate.¹⁷ Lithium-ion batteries don’t have that memory effect¹⁸ — but two kinds of metal growth can form inside them, and they matter far more than any crystal.

Lithium plating and dendrites

When a battery charges, lithium ions normally slip in between the layers of the graphite electrode. If they arrive faster than the graphite can take them — because the battery is cold or the charging current is too high — lithium deposits on the surface as metal instead. This is called lithium plating, and it can grow into needle-like dendrites.^11,20 Battery University warns that consumer lithium-ion batteries must not be charged below 0 °C: a sub-freezing charge plates metallic lithium on the anode, which “leads to a permanent degradation in performance and safety”; charging too fast at low temperatures can lead to dendrite growth, higher self-discharge and compromised safety.^12,13 Researchers at Sandia National Laboratories note that plating reduces capacity and “can pose a safety risk in the form of potential dendrite formation”.²⁰

This isn’t a new problem. The first rechargeable lithium batteries used pure lithium metal, and as they were charged, “thin whiskers of lithium grew from the lithium electrode. When they reached the other electrode, the battery short-circuited”. Getting rid of pure lithium metal — the step that made the lithium-ion battery workable in practice — is part of what won the 2019 Nobel Prize in Chemistry.¹⁹

Copper bridges after a deep discharge

The second kind forms when a battery is left completely flat. A device switches itself off before the cell is truly empty — the protection circuit cuts off at around 2.2–2.9 V per cell — but if it then sits in a drawer for months, the cell keeps slowly discharging.^15,16 Below a certain point the copper foil of the negative electrode can start to dissolve, and when the battery is charged again, the copper deposits and can form conductive bridges between the electrodes. Sandia researchers write that the copper collector “can dissolve during overdischarge and form microshorts on recharge”; a study at Tsinghua University traced internal short circuits after deep over-discharge to copper deposited on the electrodes.^20,21 Battery University advises against reviving lithium batteries that have sat below 1.5 V per cell for a week or longer, because “copper shunts may have formed inside the cells that can lead to a partial or total electrical short” — and when recharged, such a cell “might become unstable, causing excessive heat”.¹⁶

An early warning sign of both problems is a battery that drains unusually fast even when the device is switched off: Battery University notes that copper dendrites cause elevated self-discharge, “which could compromise safety”.¹⁵

What to do if your battery is swelling

1. Stop using it and unplug the charger. HP and Dell both say so, and Microsoft recommends you stop using a device whose battery has visibly expanded.^1,4,5
2. Don’t press, bend or puncture it — and don’t try to pry the battery out. Dell: if a battery gets stuck because of swelling, don’t try to free it, “as puncturing, bending, or crushing a battery can be dangerous”.⁵
3. Keep it away from heat, ideally on a hard, non-flammable surface.⁵
4. Have the battery replaced. A swollen battery isn’t something to fix — the cure is a new battery.¹ We remove and replace swollen batteries safely at our Limassol workshop. If you’re elsewhere in Cyprus, call us before sending the device: a swollen battery shouldn’t simply be dropped into a courier parcel.
5. Don’t put the old battery in household waste. Dell says swollen batteries should be returned or disposed of at an approved recycling centre.⁵

How to lower the risk

• Keep phones and laptops out of the heat — above all, out of parked cars and direct sun.⁶
• Don’t keep a laptop at 100% all the time: turn on its charge limit (where to find it).²
• Don’t store devices completely flat — leave them at about 50% if they’ll sit unused.⁷
• Don’t charge a battery that’s below freezing; let it warm up first.¹²
• Choose a quality replacement battery.⁹
• Replace a worn-out battery before it starts to bulge, and watch for the signs above.

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