Two things to notice here: (1) According to this page, it’s not enough to drain and then charge—you have to start by charging it fully. And, (2) “full discharge” is ambiguous. The figure seems to indicate that the full-discharge flag will be set at 10%. But thinking about that makes no sense. The whole problem that’s trying to be solved is that the % reading is inaccurate. How does one know when they’ve drained the battery “below 10%” if their battery reading is inaccurate? They don’t! For example, we’ve installed numerous batteries that eventually gave a “low battery” warning and then continued to work full steam for hours on an indicated battery charge of 1%. In short, “calibrating” a battery by draining it “below 10%” is futile. It’s like giving someone a car with a broken fuel gauge and telling them to drive until the tank is ¼ full.
What may actually be going on here is, the graph above is meant to show the actual chemical state of the battery and not the % indicated in iOS or MacOS, which can be quite different. User-facing software may indicate a battery charge of near zero when the actual chemical state of the battery is closer to 10% charge. This is done deliberately to prevent the battery from ever discharging below a safe level where battery damage may occur and the system may not be able to reboot. In short, the system always shuts itself down with a little bit of charge left in the battery as a safety measure, but it doesn’t show that reserve amount to the user.
> The low battery warning is purely implemented in the device software as a means to prevent possible data loss whilst using it and is completely independent of the battery management system.
> Even if you let your device run until it shuts down automatically due to lack of battery charge the battery management system will still keep the battery charge at a high enough level to prevent damage to the battery pack.
> The battery gauge that you see displayed on the screen is basically the amount of USEABLE charge the battery has and NOT the absolute total charge of the battery. This is why you can change the battery low warning to any percentage you choose – it’s not there to protect the battery (that’s done automatically by the battery management system) it’s there to give you enough time to save your work or connect the charger.
> Therefore, if you intend to calibrate your device battery you need to let it run down past the warnings until it shuts down automatically BEFORE recharging, otherwise you may not discharge the battery sufficiently to register the battery management systems discharged flag, thus rendering your attempt to calibrate the battery incomplete.
Remember there are two different (but connected) systems at play, the battery management system, which monitors and controls the health of the battery and the software user interface (and associated power control software), which reads data from the former to display an indication of battery charge status and level and respond to various flags (like shut down when the discharge flag is set).