Do you have to completely discharge your smartphone’s battery to preserve it?

The batteries of our smartphones are used almost daily. Are there good charging practices to try to preserve them a bit? Completely discharging them does not in any case have this effect.

Despite the rapid development on the smartphone side, the problem of the still too short battery life persists. Until recently, Xiaomi offered a solution: charge a phone in just 8 minutes. One can wonder if this is a good idea, but a priori, a quick charge done in good intelligence is not a priori harmful for your battery.

Today, all smartphones are equipped with lithium-ion batteries. The longevity of such components, discharged and recharged almost daily, can possibly be preserved by some good charging practices. A question often comes up: is it recommended to let your smartphone fully discharge? Such a practice is useless if you want to protect your battery a little. To understand it, you have to know how this equipment works.

A battery is the transformation of electrical energy into chemical energy, summarizes Mathieu Morcrette, research engineer in solid chemistry at the CNRS and director of the Reactivity and Solids Chemistry Laboratory at the University of Picardie Jules Verne. These chemical transformations are controlled via the external circuit, that is to say via the electrons which pass from one electrode to another. “

Ions, electrically charged particles, result from this transfer of electrons. The ions circulate inside the battery, at the level of the electrolyte (which separates the two electrodes). ” A battery is the control of a spontaneous chemical reaction, when it is discharging. This chemical reaction is reversible: it goes in the opposite direction when we inject electrons », Continues the engineer interviewed by Numerama. In the case of smartphones, the ion that moves from the positive electrode to the negative electrode during charging (and back again during discharging) is lithium ion.

An open smartphone with the battery visible. // Source: Pixabay (cropped photo)

What are the elements that can degrade the system? ” They are side reactions to reversible chemical reactions. These reactions are activated by temperature. Any significant heating of the phone will degrade the cells », Explains Mathieu Morcrette. In other words, anything that can cause the temperature of the battery to rise risks degrading it more quickly. This is why, as much as possible, you should try to keep your battery at the lowest possible temperature.

The ideal state of charge? Between 20 and 80%

The ideal would be, for this, to try to always keep a state of charge between 20 and 80%. In other words, you should neither fully charge your smartphone, nor let it discharge to the end.

It is understood that the phenomenon can take place during charging. ” When charging a battery, the voltage of pole + increases and that of pole – decreases, so the difference increases », Describes the engineer. This voltage difference can degrade the electrolyte. ” The higher the state of charge, the more the battery will degrade He continues. Blame it on the famous parasitic reactions. A 100% charged battery is more likely to be degraded, compared to a battery that displays a 30% charge, for example. ” It is therefore better to avoid having your phone charged to 100%. “, Confirms the specialist, while noting that this use is obviously not the most practical on a daily basis.

There is no point in fully discharging your battery

The same phenomenon exists when you let your smartphone discharge to the end. ” When we go to the end of the discharge, or at the end of the charge, there is an increase in the resistance of the cell. However, if we increase this resistance, the phone will heat up more significantly. », Summarizes Mathieu Morcrette. It is for this same reason that, when you charge your smartphone, you may have noticed that it only becomes hot at the very end of the charge.

To avoid such heat dissipation phenomena, the advice would be to disconnect the charger when the battery is 80% charged, and charge it when there is only 20% charge left. In fact, this is not always the most practical: for charging, it requires regularly checking the phone’s display or having an idea of ​​the charging time not to be exceeded. For the discharge, it seems a little easier to anticipate (but it is also necessary to have a charger at hand when needed).

This is precisely what has been happening on iPhones since a recent update: the system will intelligently manage the charge to block it at 80% at night and resume it so that it corresponds to your usual wake-up time. You will find yourself with 100% when you wake up, but the last% will not have been loaded right away, in order to preserve the component.

At any rate, ” rThere is no point in fully discharging the battery the first time you use it. », Affirms Mathieu Morcrette. Not more than during the following uses.

Article originally published on April 26, 2020 and updated on July 23, 2021

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