stop charging laptop battery when full

[Jon Radel]

On 1/20/17 3:08 PM, Matthias Apitz wrote:
> El día Friday, January 20, 2017 a las 01:53:50PM -0500, Sergei Akhmatdinov escribió:
> 
>> On Fri, 20 Jan 2017 18:11:25 +0100
>> Matthias Apitz <guru at unixarea.de> wrote:
>>> I can ask the current capacity (and other values) of the battery with:
>>> [snip]
>>> While charging, is there a way to stop the charging when
>>> hw.acpi.battery.life reached 100 to protect the (LI-ion) battery?
>>
>> Such functionality should be provided by your battery's internal circuitry.
>> Think about it, it wouldn't be safe for any consumer to buy a laptop which
>> doesn't switch to DC power when the battery is full.
> 
> I'm not sure about this. Let's assume the power provided by the charger
> it withdrawn "somehow" from the battery and the laptop runs only on the power
> provided by the charger. The battery will now start to drain itself (due to
> its internal resistor value). When it will start to charge again, and
> how?

The somehow would be the rather sophisticated charging circuit found in
any recent laptop that was properly designed.  Yes, the battery will
slowly drain.  It will start to charge again when it hits whatever
threshold the charging circuit considers appropriate, which hopefully is
enough below the threshold used to stop charging to provide enough
hysteresis to avoid thrashing too much.  All of those values chosen by
engineers beaten about the head by marketing types who want to claim
maximal battery life, and management types who want to make sure the
battery lasts through at least the warranty period.

True, vaguely related story:  I was nearly driven crazy one night by my
iPhone plugged into an external battery pack right beside my bed.  The
"I've been plugged into power" noise is one of the few that is not
suppressed by the Do Not Disturb feature.  So I went to sleep while it
was happily charging.  When the phone was fully charged, it stopped
charging and the current draw dropped below the battery pack's threshold
for "I'm not really being used and supplying a trickle will drain my
battery, so I'll mostly shut off output."  At that point the phone,
convinced it was no longer plugged into power, ran on its battery until
it dropped below the appropriate threshold and started charging again,
at which point the external pack turned on output again and the phone
bleedled in my ear.  That kept going all night, never waking me up quite
enough to deal with it...I pieced it together the next morning.

> 
> And why all the hints say: Once fully charged, disconnect the laptop
> from charger?

I sincerely doubt that all the hints say this, merely a great number of
them.  Reasons for this I can think of include:

1)  Hints from people who don't understand how sophisticated charging
circuits have become, and

2)  Hints from people who don't realize that Ni-Cad and Li-Ion batteries
are very, very different, and

3)  Hints from people who understand what the charging tradeoffs are and
wish to do something different that what the engineers of the charging
circuit ended up doing, and

4)  Hints from people who are generalizing the need to fully discharge
periodically on some devices if you wish to re-calibrate the battery
monitoring.

In many cases I suspect it's hard to tell where the advice is coming
from.  Good luck sorting them out.

> 
> Will it lead to some endless charge/discharge cycle?

More or less.  But why the concern?

> 
> I was asking for some method to disconnect the laptop completely from
> the charger, either internally or, for example, by some external device,
> receiving the signal via Wifi and cutting the charger from the outlet.

I suspect that that would be a fun project for a Raspberry Pi hooked up
to a relay.  Actually, if you're going to go to that extent, you might
want to first have a good go at analyzing the current/voltage across
your charging cable, which would go far in determining what is really
happening with your laptop's charging--much more likely to be accurate
than speculation by random people like me on this list.  :-)

BTW, the more often, and the deeper you discharge your Li-Ion battery,
the sooner it will wear out. So disconnecting the charger every time you
hit full charge and then discharging the battery most or all the way
will shorten the battery's life.  See
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
for some actual numbers, followed by much argumentation in the comments
as to what it all really means.

Now, what would be really cool would be to build your little Raspberry
Pi & relay box to disconnect the charger at a lower than maximum voltage
so as to maximize battery lifetime, rather than maximizing the runtime
of a "full" charge.  See above referenced article for more.

-- 
--Jon Radel
jon at radel.com

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