This blog post explains the scientific reason why smartphone battery life gets shorter despite larger battery capacities.
If you look at recently made smartphones, you’ll notice their batteries are significantly larger than before. Despite thinner designs and larger screens, battery capacity continues to increase. So why? Why does the battery life seem to wear down the more we use our phones? Why does the battery level drop from 100% to 80% after just 10 minutes of watching a video with Wi-Fi on? Furthermore, why does the battery level slowly decrease even when the phone is not in use? These phenomena are among the frequent frustrations we encounter in our smartphone usage experience. Honestly, I’ve been using my smartphone a lot lately, often swapping out the battery multiple times, and I’m experiencing this shortening battery life firsthand. For all of you out there using phones, including myself, who get annoyed by this reality yet remain curious, I aim to explain the scientific and materials engineering reasons behind it in an easy-to-understand way.
First, to understand the various factors affecting battery life, we need to know “what’s inside a phone battery and what’s happening there.” As you know, a phone battery is a cell. There are two types of cells: primary cells and secondary cells. Primary cells are exactly what they sound like – the kind you find in toys, like ‘dry cell batteries,’ which can only be used once and cannot be recharged. The type of battery I’m referring to, like the ‘mobile phone battery,’ which you can use, charge, and use again, is a secondary battery. In other words, a mobile phone battery can be charged and discharged.
Now, let’s take a deeper look at the structure of the battery. If you look at your battery, you’ll see the word Li-ion. This means that inside the batteries we use, ions of a substance called lithium (Li) are moving around. Here, an ion refers to a state where an electron becomes unstable and seeks another partner, much like someone who was happily coupled and stable but became single and now feels unstable and busy looking for another partner, or conversely, someone who is anxious because they are dating too many people beyond their capacity. The lithium ions mentioned here are in a state where electrons have escaped, so they constantly strive to find another partner within the battery. So, how does the movement of these ions affect the battery’s operation?
Lithium ions play a central role in storing and releasing electrical energy within the battery. They travel between the battery’s positive (+) and negative (-) electrodes, storing energy during charging and releasing it during discharge. Inside the battery, there is an electrolyte solution that facilitates the movement of lithium ions, and a separator that regulates the flow of electrons as the ions move between the different electrodes. Without this separator, a direct short circuit would occur inside the battery, potentially causing it to explode. Thus, while the battery’s structure appears simple, it is actually designed with great complexity and precision.
So what exactly happens inside? As mentioned earlier, lithium ions move back and forth between the (+) and (-) electrodes in search of another ion. However, this movement isn’t random; it follows a specific pattern. When the battery is fully discharged and recharged, lithium ions leave the material forming the (+) electrode, accompanied by electrons. The lithium ions then travel from the (+) electrode towards the (-) electrode, settling into the gaps within the crystals forming the (-) electrode.
Conversely, when the battery is used (during discharge), lithium ions and electrons exit together, just as during charging. The lithium ions move from the negative electrode to the positive electrode, settling into the crystal gaps of the material forming the positive electrode. The electrons are transferred to the positive electrode via a different pathway.
That covers what the components of a cell phone battery are and what reactions occur inside them. Now, let’s discuss “why does the battery life shorten the more you use your phone?” Factors determining the lifespan of a lithium-ion secondary battery include the state of the electrodes and the state of the electrolyte. As lithium ions move between the two electrodes, those electrodes repeatedly expand and contract. The reason is this: when a lithium ion leaves one electrode, the space it occupied becomes vacant, causing the electrode to contract. Conversely, when that ion finds a new partner, the new partner electrode must make room to accept the new ion, causing it to expand. This repeated expansion and contraction fatigues the electrodes, leading to performance degradation over time. Consequently, the battery’s overall capacity gradually decreases.
Additionally, the electrolyte inside the battery undergoes chemical changes over time, hindering ion movement. During this process, impurities form within the electrolyte, which also causes the battery’s electrical performance to decline. Another factor shortening battery life is the influence of external temperature. Using or charging a battery at high temperatures accelerates the chemical reactions in the electrolyte and increases the risk of damaging internal components.
Thus, the fundamental cause of reduced battery life is that the more you charge and discharge the battery, the more fatigued it becomes, shortening its lifespan. It’s like an office worker who becomes fatigued from repetitive tasks, loses sleep, and eventually damages their body. Now that you understand why battery life shortens, please treat your battery as you would your own body. Furthermore, I hope you find ways to actually extend battery life—like how I go to the gym every morning to build a healthy body—so you can enjoy your battery for a longer time.
