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From: Theo <theom+news@chiark.greenend.org.uk>
Newsgroups: comp.mobile.android
Subject: Re: is my phone ON or is it OFF?
Date: 27 Apr 2024 10:16:52 +0100 (BST)
Organization: University of Cambridge, England
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Jörg Lorenz <hugybear@gmx.net> wrote:
> On 25.04.24 15:55, Frank Slootweg wrote:
> > Jeff Layman <Jeff@invalid.invalid> wrote:
> > [...]
> > 
> >> The charging circuitry is within the battery itself. It controls the 
> >> rate of charge and, in particular, monitors the temperature of the 
> >> cell(s). If it gets too high it will stop the charging. Note though, 
> >> that the connection from the phone's charger socket (lightning, USB, or 
> >> whatever) goes through some of the phone's circuitry, so the phone knows 
> >> the state of charge of its battery. That way its % charge can be 
> >> displayed on the phone's screen, and battery-usage apps can let you know 
> >> what's being used and how long the phone might last before recharging is 
> >> essential.
> > 
> >   The fact that the phone is charging and how full its battery is, is
> > also displayed/displayable when the phone is switched 'off'.
> > 
> >   Ergo, the phone is never really off. It's either awake or sleeping
> > during normal use or in cold standby when the user switched it 'off'.
> 
> Bullshit!
> The OS is not booted when the Android is turned off.

AIUI it is, kind of.  When you plug in the charger with the phone off, the
phone starts.  The bootloader then launches the Android kernel with the flag
androidboot.mode=charger

Instead of doing the full Android boot, that causes the kernel to launch a
charger UI application that shows your battery percentage on the screen or
an animation (which comes from files on your OS partition).  The charger app
doesn't allow you to otherwise interact with the phone and other services
like the radios aren't running, but the SoC is booted and running software. 
The SoC is also doing standard power management, ie the charging process
here likely looks very similar to charging when the phone is turned on
(because it is, in essence).

The full OS services are not running, but the Android kernel and the charger
app is.

Theo
(usual caveats: my understanding may be out of date, different vendors may
do their own thing, etc)