My phone is almost five years old and the battery maintains its useful life at 85%. This is how I got it

My smartphone, a iPhone 13 Pro Maxis almost five years old and its battery retains 85% of its original capacity. It is not coincidence or good luck. It is the result of understanding what happens to a battery of lithium ions every time we charge it and consciously apply a handful of habits that slow down its degradation.

Most users assume that a mobile phone’s battery is condemned to degrade without remedy in two or three years. And to a certain extent it is true: every lithium-ion battery loses capacity over time. But the speed at which it does so depends largely on how we treat hernot just its internal chemistry.

In this context, we are interested in understanding two specific physical phenomena, voltage stress and thermal degradation, because they are the ones that wear out battery cells the most. And we also need to know the guidelines I have followed for almost five years to minimize them. They are not magic tricks. They are direct consequences of the electrochemistry that occurs inside the battery.

What happens to a battery every time we charge it

A lithium-ion battery works by moving ions between the cathode and anode through an electrolyte. When we charge the phone up to 100% we force the cathode to reach its maximum voltage potential. This high voltage state oxidizes the electrolyte in contact with the cathode and favors the dissolution of transition metals from its crystalline structure. This phenomenon is known as voltage stress, and it is cumulative: the longer the battery spends in that state of maximum voltage, the faster it degrades.

A quick note before moving forward: that layer that forms on the cathode is called CEI (cathode solid electrolyte interface or cathode electrolyte interphase), and is the first cousin of the SEI (solid electrolyte interface or solid electrolyte interface) that grows on the graphite anode. At sustained high voltages, the IEC thickens uncontrollably, irreversibly consumes active lithium and reduces the usable capacity of the cell with each cycle.

For this reason, it is worth using the 80% loading automation offered by both iOS and Android. In fact, I don’t charge my phone at 100% unless I know I’m going to need full battery life that day. Keeping the battery in an intermediate voltage range (between 20% and 80%) dramatically reduces the time the cells spend at the extremes of voltage where molecular wear is most aggressive.

The second big enemy is heat, and this is where thermal degradation comes into play. Parasitic chemical reactions do not depend on voltage alone; They also accelerate with temperature, and do so in a non-linear manner. As a guideline, an increase of about 10 degrees Celsius can reach double the speed of certain degradation reactions in the electrolyte, a behavior consistent with the Arrhenius equation that governs the kinetics of any chemical reaction.

This is why I avoid ultra-fast chargers in summer, especially if my phone is exposed to the sun or inside a thick case. Charging at 45, 65 or more watts generates considerable internal heat, and if that heat is not dissipated well (and in environments at 35 or 40 degrees Celsius it is not dissipated well), the cell temperature can approach 40-45 degrees Celsius, a range beyond which electrolyte degradation and cathode dissolution accelerate noticeably. I definitely prefer slower charges at 15 or 20 watts, even if they take longer: the battery appreciates it in the long run.

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