Why does the mobile or smartphone battery swell? We investigate!

The image of a smartphone with a lifted or swollen screen is often a symptom of a well-known issue: a swollen battery. This phenomenon is not only alarming to look at but also potentially dangerous due to the risk of damaging the device (or even causing a fire in extreme cases!). Although it is sometimes thought that a swollen battery is a manufacturing defect, in most cases, the swelling is due to wear and usage conditions. In this article, aimed at mobile repair and maintenance enthusiasts, we will explain the technical reasons why batteries swell, excluding the possibility of a manufacturing defect. We will examine in detail how charging habits, heat, impacts, and other factors can cause this problem, and we will discuss a special case: old phones with damaged charging circuits, which may inadvertently overcharge the battery. Let?s dive in!

The phenomenon of a swollen battery: internal degassing

First, let?s understand what happens inside a swollen battery. Lithium-ion batteries (the common type in our phones) generate energy through chemical reactions. When these reactions fail or become ?out of control,? they release gases inside the cell, which have nowhere to escape due to the battery?s hermetic seal. The result: the battery swells like a balloon due to the internal pressure of these gases. This failure in chemical reactions can be triggered by several reasons unrelated to manufacturing, mainly:

• Overcharging or overvoltage: charging the battery beyond its safe limits.
• Physical damage: impacts or punctures that alter the internal structure.
• Natural aging: over the years and charge cycles, internal components degrade.

In all these cases, a process called ?degassing? (the emission of internal gases) occurs, causing the cell to swell. Next, we will explore each cause in more detail.

Battery aging and chemical wear

Like all consumable components, batteries have a limited lifespan. With each charge and discharge cycle, the internal materials of the battery gradually degrade. Over time, the efficiency of the chemical reactions in the cell decreases, and internal residues form (e.g., insulating layers on the electrodes) that prevent the battery from functioning at 100% capacity. This natural chemical deterioration can lead to the accumulation of gases inside the cell, eventually causing a visible swelling. This is an expected process after numerous charge cycles and is usually seen in devices with several years of use. In fact, sometimes there seems to be no apparent reason for the bulging other than the passage of time and intensive use: simply, the battery has reached the end of its useful life and starts to fail.

It is worth noting that very intensive use of the device can accelerate this aging process. For example, if we subject the phone to constant charging and discharging (many hours of screen time, demanding games, etc.), the battery will experience more thermal and chemical stress, accumulating wear in less time. This is why swollen batteries are common in older or heavily-used smartphones. In summary, an old or ?worn-out? battery is more likely to swell simply due to the natural wear and tear of its internal components.

Overcharging and poor charging management

Overcharging is another major cause of battery swelling. It occurs when the battery continues to charge despite being already full, that is, when it is forced to accept more voltage or current than it should. If you habitually leave your phone plugged in long after it reaches 100% (e.g., charging it overnight every day) or use an inappropriate charger that doesn?t ?cut off? in time, you might be overcharging the battery without realizing it. When charging beyond the recommended limit, parasitic chemical reactions are triggered inside the cell. Instead of storing energy normally, the battery begins to produce undesirable byproducts (gases) due to these altered reactions. These gases accumulate, and the result, once again, is a swollen battery.

Why does overcharging happen? Fortunately, all phones have protection circuits to prevent it. A charge management chip (part of the BMS, Battery Management System) monitors the cell voltage and cuts off the current input when the battery is sufficiently charged (typically around 4.2V per cell). This way, under normal conditions, the battery should not overcharge because the device itself stops charging once it?s full. The problem arises when this system fails or is mishandled: if for some reason the control does not cut off the flow in time, the battery will keep receiving voltage even when ?full,? entering a state of continuous overcharge. Just a small excess of voltage can cause damage; in fact, once the ?safety value? (~4.2?4.3V per cell) is exceeded, the battery can suffer permanent damage and will start to swell quickly.

That?s why it?s so important to use appropriate chargers and avoid unnecessary charging. In the next highlighted section, we will explore a particular case: what happens in old phones when the charging circuit fails and how this can lead to a swollen battery in a short time.

? In-depth: Damaged charge management chip in old phones

Imagine a phone with several years of use that has suffered a drop or simply electronic wear: it is possible that its charge management chip (controller) no longer works properly. This chip (or charging circuit) is responsible for regulating how much current and voltage the battery receives during charging, ensuring that safe limits are not exceeded. If the chip is damaged, poorly calibrated, or its sensors (e.g., the battery thermometer) fail, the consequences can be severe for the battery:

• Constant overcharging: The most typical failure is that the circuit does not ?realize? the battery has reached 100% and continues to supply voltage. In a malfunctioning old device, it might happen that even with a full battery, the system does not cut off the charging energy. This means the battery is always slightly overcharged, which greatly accelerates harmful reactions inside it. The electrolyte (the internal liquid) degrades due to continuous excess voltage and starts to break down, releasing gases that swell the cell.
• Failure in current control: Another possible situation is that the damaged chip does not regulate the current (amperage) properly. It might supply higher currents than necessary all the time or fail to reduce the charging speed when the battery is almost full. This generates excessive heat and mechanical stress on the battery, also contributing to its swelling. A healthy circuit typically switches to ?trickle charge? mode when the battery is near 100%, feeding it only small impulses to keep it full. If that fine control fails, the battery receives too much energy in the final stage of charging, causing overcharge.
• Erroneous readings and unbalanced charging: An old device with faulty sensors might believe the battery is at a lower percentage than it actually is, and thus tries to keep charging to reach a fictitious 100%. Similarly, if the chip does not measure temperature correctly, it cannot stop charging when the battery is too hot. These incorrect readings lead to wrong decisions: essentially, the phone ?mistreats? the battery without realizing it, keeping it in unsafe conditions during charging.

In summary, a defective charging circuit can send more energy than necessary to the battery and fail to stop charging when it should. This results in chronic overcharging: the battery is exposed to voltages or currents above its tolerable range for prolonged periods, triggering rapid internal gas generation. Therefore, a very old or damaged phone is more likely to swell the battery?its protection systems may have failed. If you notice that your phone keeps ?charging? indefinitely or gets excessively hot while plugged in, it could indicate a problem with the charge management chip. In such cases, the battery is at risk and is likely to swell. It is advisable to check the circuit or, more practically, replace the battery (and the controller if possible) before a major incident occurs.

(Note: Fortunately, these catastrophic charging system failures are not very common, as manufacturers implement multiple levels of protection. Both the main charging chip and the battery?s own protection circuits would have to ?fry? for uncontrolled overcharging to occur, something rare but not impossible. Therefore, especially in old devices, don?t ignore signs of overheating or strange behavior while charging!)

Use of poor-quality chargers and cables

Not all chargers are equal. Using low-quality, non-original, or damaged chargers or cables can contribute to the battery swelling over time. These cheap accessories often do not properly regulate the charging current or voltage, causing the battery to receive fluctuations or spikes beyond the recommended levels. A counterfeit charger, for example, might not ?cut off? power even when the phone is already at 100%, or deliver a current with a lot of noise and instability. In practice, this equates to stressing the battery with overcharging or ?dirty? electrical supply, which leads to?you guessed it?abnormal chemical reactions and internal gases that swell the cell.

The solution is simple: always use high-quality chargers and cables, preferably those that are official or certified by the manufacturer. Original chargers are designed to provide a stable and adequate current flow, working together with the phone?s charging circuit to prevent overcharging. For example, many modern chargers stop supplying power or reduce it to a minimum once they detect that the battery is full. On the other hand, with a generic charger of dubious origin, that control can fail. A damaged cable can also cause problems (intermittent connections, overheating at the port, etc.), so it?s wise to replace frayed or loose cables. In summary, taking care of the quality of your charging accessories helps maintain the health of your battery.

Prolonged exposure to heat (and extreme temperatures)

Heat is a direct enemy of batteries. Exposing your phone to high temperatures for prolonged periods?such as leaving it on the car dashboard under the sun, near a heat source, or simply playing demanding games while charging in a hot environment?accelerates the internal degradation of the battery. High temperatures cause the internal materials to degrade faster, which promotes gas production during the cell?s normal operation. In other words, a hot battery is much more prone to swelling than one operating at a moderate temperature. This is why, when a phone suffers from chronic overheating, its battery often becomes swollen prematurely.

What about cold? Extremely low temperatures are also bad for batteries, although their effects are different. Intense cold (well below 0°C) can interfere with the internal chemical reactions, causing the battery to malfunction or not charge properly. While cold itself does not usually swell the battery immediately, it can cause structural or chemical damage that later leads to problems when the temperature rises again. In any case, keeping the device within an optimal temperature range is crucial for battery health. Avoid leaving your phone exposed to scorching sun for hours, don?t forget it in a hot car, and don?t subject it to freezing temperatures. High-end smartphones often include protections (e.g., reduce performance or stop charging if they detect excessive heat), but it?s better to prevent: excessive heat in the long run almost guarantees a swollen battery.

Internal physical damage from drops or impacts

A strong physical impact?such as dropping the phone on the floor, accidentally sitting on it, or a heavy blow?can damage the battery internally even if the phone appears intact externally. Lithium batteries are delicate components; inside them are very thin layers separated by membranes. An impact or deformation can compromise that structural integrity, bending the layers or breaking the separation between them. When this happens, the battery may start to fail internally and generate gas. Interestingly, the internal damage is often not immediately visible after the drop. Maybe the screen and casing look fine, but a few days or weeks later, you notice the phone has swelled: the battery swelling may be the only visible sign of an impact that caused internal havoc.

For this reason, it?s important to note that not only overcharging ages the battery, but also physical accidents. If your phone suffered a heavy impact and you later see the back cover lifted or the screen separating from the frame, there?s a high probability that the battery has swollen as a result of that impact. Even micro-perforations inside (caused, for example, by trying to open the phone with an inappropriate tool) can introduce air or moisture into the cell and ruin it, generating gas. Conclusion: treat your battery with the same physical care as you treat the screen; impacts not only break the glass, they can also ?break? the battery dangerously.

(Reminder: if you suspect your battery is swollen due to an impact, do not puncture or crush it in an attempt to fix the swelling. A deformed battery is highly unstable; the correct action is to replace it as soon as possible at a service center, avoiding using the device in the meantime.)

Prolonged lack of use and improper storage

Did you know that not using your phone for months can also harm the battery? Although it may seem contradictory, leaving a battery unused for a long time in inappropriate conditions can cause it to swell when you try to use it again. This relates to battery chemistry: ideally, they should be kept active within certain ranges. If you store a device for a prolonged period, you should pay attention to how and where you store it:

• Charge level during storage: It is not advisable to leave a battery stored completely full at 100% for months, nor completely depleted at 0%. In both extremes, the battery suffers stress. The optimal state is to store it with an intermediate charge (around 40-50%) if it won?t be used for a long time. Batteries stored at 0% can over-discharge (deep discharge), falling to voltages so low that they damage their chemistry. On the other hand, storing them at 100% generates continuous voltage stress, accelerating degradation. Maintaining a medium level helps the battery rest healthily.
• Environmental conditions: The location where you store the phone matters. A cool, dry place is ideal for storage. If you leave the phone in a place with high humidity or heat, the battery can degrade faster even without use. For example, do not store an old phone in a hot attic or a damp storage room; better in a closet at moderate room temperature. Extreme temperatures during storage can cause swelling even without using the device.
• Time without use: All batteries slowly discharge over time (self-discharge). If it will be unused for many months, it?s a good idea to check and recharge it a bit periodically. If you leave it forgotten for a whole year, you might find it swollen or faulty when you try to charge it again, due to having fully discharged in the meantime.

In summary, prolonged inactivity also degrades batteries, especially if storage conditions are not good. There have been cases of old phones stored in a drawer for years whose batteries swelled without even being used. Therefore, if you have a spare phone that you don?t use, store it with the precautions mentioned (partial charge, cool location) and check on it occasionally. That way, you?ll avoid unpleasant surprises when you want to turn it on again!

Inappropriate charge cycles (unhealthy charging habits)

Our daily charging habits can significantly impact battery health. There are certain ?old school? practices that are unnecessary and even harmful for modern lithium-ion batteries. For example, do you always wait until your phone discharges to 0% before recharging it to 100%? Many people think it?s better to deplete the battery before plugging it in, but this is not optimal for lithium batteries in every cycle. In fact, it?s not necessary to completely drain the battery before charging it; doing so repeatedly can wear it out more. Experts suggest keeping the charge between approximately 20% and 80% in daily use, rather than using 0% or 100% extremes in every cycle. This avoids chemical stress on the electrodes and extends the lifespan.

Let?s look at some inappropriate charging habits and why they?re worth correcting:

• Always charging to 100% and leaving the phone plugged in: If you charge your phone to the maximum daily and leave it plugged in for hours (e.g., overnight, every night), you are subjecting the battery to a state of sustained maximum voltage. As explained in the overcharging section, although the phone may cut off the main current at 100%, it often applies small intermittent charges to maintain 100% (?maintenance charging?). Staying at 100% constantly accelerates anode oxidation and contributes to swelling in the long run. It?s better to unplug the charger once charging is complete rather than leaving it indefinitely. (Many modern phones already include nighttime modes that delay reaching 100% until just before you wake up, mitigating this effect?we?ll discuss this shortly.)
• Always discharging to 0%: On the flip side, frequently depleting the battery until the phone shuts down isn?t a good idea either. Deep discharges increase the internal resistance of the battery and can cause chemical instability. Occasionally calibrating the battery indicator with a full discharge is fine, but doing it daily will shorten its lifespan. Ideally, don?t let it drop below ~10-20% regularly, and recharge earlier.
• Constant micro-charging (plugging and unplugging frequently): Another habit to avoid is ?nibbling? on the charge: plugging the phone into the charger for a few minutes, then unplugging it, then plugging it back in shortly after, repeatedly. These short and frequent charges can add unnecessary cycles and also generate heat spikes each time. It?s better to let the percentage drop moderately (e.g., to 30-40%) before plugging in, and then do a continuous charge up to, say, 80% or 90%. There?s no need to charge in 5% increments all the time; that anxiety to see 100% only stresses the battery more.
• Excessive use of fast charging: Fast charging technologies make life easier, but constantly using maximum power charging (e.g., always using 40W, 60W, or higher chargers if your phone supports it) generates more heat. While manufacturers calibrate the speed to keep it within safe limits, fast charging produces more heat than slow charging, and heat, as we know, is harmful. My advice: use fast charging when you really need it (in a hurry to leave, etc.), but if you?re not in a rush, charging at a slow pace (e.g., from a PC USB port or with a standard charger of lower wattage) keeps the battery cooler and happier in the long run.

In summary, ?inappropriate? charging cycles are those that keep the battery under constant stress, whether by being always full, always empty, or charging recklessly. Adopting healthy charging habits, such as partial charging, avoiding keeping it at 100% all the time, and not swapping chargers constantly, will make your battery last longer and less likely to swell.

Modern battery protection technologies (iPhone 15 Pro, Samsung Galaxy S24 Ultra, etc.)

After reading all this, you might wonder: ?If so many things affect the battery, don?t new phones have protections to avoid these problems?? The good news is that yes: modern smartphones, especially high-end ones, integrate multiple battery protection technologies to mitigate risks of overcharging, excessive heat, and accelerated aging. But (and it?s a big ?but?), no protection makes the battery indestructible?they are still chemically vulnerable, only with electronic aids. Let?s see some examples using recent phones:

• Apple and the "Optimized Charging" (iPhone 15 Pro and family): Modern iPhones include a feature called Optimized Battery Charging. What does it do? Basically, it prevents the iPhone from staying unnecessarily at 100% charge for a long time. Through learning algorithms, the system learns your schedules; for example, if you usually charge your phone at night, the iPhone will quickly charge up to ~80% and then slow down or stop charging, maintaining that level for most of the night. Only just before you usually wake up, it will complete the charge to 100%. With this, it reduces the time the battery stays ?fully charged at 100%?. In iOS, you can see notifications like "Battery will finish charging by 7:00 AM" ? a sign that optimized charging is working. This feature, present in models such as the iPhone 15 Pro, aims to extend battery lifespan and prevent swelling from prolonged charging.
• Samsung and the "Battery Protection" mode (Galaxy S24 Ultra and others): Recent Samsung Galaxy phones offer a similar option called Battery Protection. When activated, the system limits maximum charging to 85% capacity instead of 100%. This way, even if you leave the phone plugged in "too long," it never reaches the absolute 100%, avoiding the final stress of overcharging. Samsung recommends this practice for users who tend to charge for many hours; keeping the cap at 85%, the battery experiences less overheating and stress. On a device like the Galaxy S24 Ultra, activating Protect Battery means you'll daily use only the 15-85% range, which has been shown to reduce degradation. It's a trade-off between slightly less daily battery autonomy in exchange for several more months/years of healthy battery life.
• Thermal management and advanced sensors: Both Apple, Samsung, and other manufacturers (e.g., Huawei, Xiaomi) have incorporated multiple temperature and voltage sensors within the device and in the battery itself. These sensors allow that, if the battery gets too hot, the phone automatically slows or stops charging and alerts the user. Additionally, modern power management ICs can balance the charge between multiple cells (in devices with dual batteries, for example) and reduce the charging current when the battery is already above a certain percentage (in many cases, charging speed is reduced after 70-80% to protect the cell). In short, recent phones are much smarter when charging: they closely monitor parameters to avoid abusing the battery. In fact, it has been observed that new smartphones handle charging more conservatively than a decade ago, aiming to minimize heat and overcharging.

Despite all these improvements, no system is infallible. Protections delay but do not completely prevent chemical degradation. With enough time and usage (or if we ignore recommendations and expose the phone to extreme conditions), even an iPhone 15 Pro or a Galaxy S24 Ultra could end up with a swollen battery. The difference is that, thanks to protections, it usually happens only after many more cycles than a phone from 5-10 years ago would have endured under the same circumstances. In any case, modern technologies do help drastically reduce the incidence of swelling due to overcharging or heat, as long as we use them correctly (there's no point in the 85% mode if we disable it to constantly seek that extra 15%!).

Conclusions and Final Tips

As we?ve seen, a swollen battery is usually the result of a combination of technical factors related to its use and environment, rather than a manufacturing defect. The natural aging will inevitably affect any battery over the years, but we can delay its effects by avoiding situations that accelerate degradation: overcharging, excessive heat, impacts, poorly managed long periods of inactivity, and bad charging habits. In a well-designed phone, internal protections will prevent immediate damage, but our daily habits make the difference in the long run.

To keep your smartphone battery healthy and avoid premature swelling, remember these key recommendations:

• Use quality chargers and cables, and disconnect the charger once the phone is fully charged.
• Don?t let your phone overheat (remove thick cases when charging if you notice excessive heat, avoid very hot environments).
• Don?t always wait until 0% or keep it always at 100%; the optimal range is to operate in intermediate charge levels.
• Enable battery care features offered by your phone (charge limits, adaptive charging, etc.).
• Handle your device with physical care, avoiding drops and rough handling, especially during DIY repairs.

And finally, if you notice your battery is already swollen, stop using it! Turn off the device and take it to a trusted service center to replace the battery as soon as possible. A swollen battery is a latent risk: it?s better to spend on a replacement than to regret an explosion or greater damage to your phone (and your safety). At our spare parts store and technical service, we are always ready to advise you and safely replace batteries.