Why do newer battery conservation methods for laptops and cell phones bring the charge up to 80% for a period of time and then fully charge the device?

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Why do newer battery conservation methods for laptops and cell phones bring the charge up to 80% for a period of time and then fully charge the device?

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This has to do with how a battery is constructed. A battery consists of two metal foils with an electrolyte separating them. If you charge it too fast it can heat up damaging the electrolyte and short circuiting the battery. Also if you apply too high voltage you can get sparks between the foils through the electrolyte which also damage it. So battery chargers have to limit both the current and the voltage. As the battery is in its main charging state it is limited on current. However as the battery gains charge its voltage increases so the charger have to apply higher and higher voltage to get to the current limit. And at around 80% charge it will have reached the maximum voltage that the battery can charge at. So it can not deliver the full current as this would require too much voltage. The charging current will therefore slowly go down and the battery charges slower and slower until fully charged.