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[Discuss] [ OT ] iPad battery

Jerry Feldman wrote:
> My question is does it hurt to keep the iPad plugged in even
> when it is at full charge. AFAIK, most computers and smartphones have
> circuitry to prevent battery damage and over charging.

Not having first hand knowledge of the iPad charging circuitry, I can
only confirm that your assumptions match my expectations for a modern,
battery powered device. If this were not the case, you'd have heard
about lots of people complaining about ruined batteries resulting from
them forgetting to unplug their iPads.

> Basically in general the worst thing you can do with a Lithium Ion
> battery is to run it all the way down, and most systems [will] shut down
> at about 20%

Roughly correct.

Additionally, you maximize the life of a Lithium Ion battery by
minimizing the number of charge-discharge cycles it experiences, which
is finite.

And adding some confusion, it is recommended that Lithium Ion batteries
are stored long-term in a partially discharged state.

It's a bit of a compromise, but the easiest way to maximize the life of
your Lithium Ion batteries is to keep them perpetually fully charged.

> One caveat is that the charger is always drawing power, so the electric
> meter is running even when the charger is not plugged in to the device,
> which is more of a green issue than anything else.

Chargers are getting better in this respect. Even an old-school linear
power supply will draw less power once the battery has charged and the
charge circuitry turns off charging, but they still use a fair bit
(relatively speaking) when sitting idle. Modern switching power supplies
improve upon that, and they make it easier/cheaper for the device being
powered to tell the supply to shut itself down, reducing idle draw to
near zero. I'm not sure if any main stream consumer devices make use of
this latter capability yet.

> My mother was told not to keep it plugged in all the time.

Either obsolete advice or for the green reasons you cited.

Many phones have on-screen messages to not just detach the charger, but
unplug it from the wall precisely for the power savings. Ironically it
would have had far greater impact 10 years ago when most "wall warts"
were inefficient linear supplies.


Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
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