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Who hasn't found themselves in this situation? Your battery is in the red and your phone displays a measly one percent of battery. Always in a nice moment: during a meeting, when you are at the supermarket and reading your shopping list or when you are lost in the car and you are guided using your favorite GPS application. You anticipate an imminent shutdown of your device, but, to your surprise, that last percent seems to last much longer than expected. What technical mystery lies behind this phenomenon ?
An imperfect battery management system
Omnipresent in our smartphones, tablets and laptops, lithium-ion batteries are today very reliable: they heat up little, hold the charge well and remain stable if used properly. On the other hand, it is their internal management system which is not free from imprecision: the Battery Management System (BMS)< /strong>.
« All lithium-ion batteries, whether they equip a telephone, a laptop or even an electric vehicle, use a battery management system or BMS. This is what attempts to measure the state of charge, because in reality, when you go from zero to 100%, it's not quite true  “, explains Kent Griffith, professor at the University of San Diego and drum specialist. Concretely, when it is displayed 100% on your screen, this does not necessarily correspond to an actual full charge.
Indeed, if the BMS displays 100%, it is above all to preserve the life of the battery. “100% is what is set to give you a certain amount of battery life. If you were to overcharge it or the manufacturer set a higher limit, this battery might not last as long. For example 500 charge cycles instead of 1,000 and it could be dead in a year, a year and a half ” Griffith says.
So the percentage displayed on your screen is not an exact measure of the remaining charge< /strong>. To estimate the state of the battery, the BMS is based on the voltage measured at its terminals as well as its state of charge (SOC, State of Charge). “[…] So basically, when your phone hits 1% and lasts a long time, in some sense it's just an error in the system battery management » indicates the specialist. A phenomenon which tends to get worse as the battery ages.
The charging rate indicated on the phone is therefore only&# 8217;a more or less precise indication, we must not trust it blindly. Although this is still quite close to reality, the BMS actually uses theoretical and empirical models to gauge the autonomy of your device. Its estimates may also fluctuate depending on the ambient temperature, the age of the battery or the charge or discharge current. Here's why that last percent sometimes seems endless.
- The display of the remaining battery of a phone is managed by the BMS.
- This system estimates the remaining autonomy based on the battery voltage and its SoC.
- This is why it is sometimes imprecise and the last remaining percent can seem endless.
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