Android and Linux

[kentborg-KwkGvOEf1og at public.gmane.org (Kent Borg)]

About the Android battery...

I think I have learned a tidbit about the care and feeding of lithium
ion batteries by watching how the Nexus One behaves.

First a few observations:

- First, the green "I'm charged!"-light comes on at 90% not 100%

- Second, the top 10% or so of my battery maybe seems to get used up
quickly.

- Third, on further observation, the phone doesn't always charge when in
the top 10% of capacity, sometimes it waits until it drops to 89% before
it starts charging.

So it seems Google thinks the top 10% is its margin for managing things
and therefore we should consider only 90% of the capacity to be for our
use and think of the top 10% as bonus but not guaranteed. I am guessing
Google is doing this to prolong total battery life, not to maximize each
charge.

I used to think lithium ion life was a function of three parameters:

- The number of "degree days", that is: A, how old the battery is (days,
weeks, months since manufacture) and B, how warm it is kept when idle.

- C, how many watt-hours have been put through the battery.

(OK, throw in abuse cost of trying to use or charge the battery when
cold, and whether it is really frozen while in storage--which is bad--or
merely kept at about 0C cold while in storage--which is good.)

Now I think there is another parameter that is important: Something
about the cost of frequently topping it up to 100%.

- I don't know if packing in the last few watt-hours is somehow expensive,

- or whether reversing the chemistry from discharge to charge and back
is expensive and so should be avoided when only a few percent will be
put in*,

- or whether putting the last couple percent in the battery is somehow
cheaper when it is part of putting in the last 10% rather than putting
only the couple percent.


* So putting in 2% in the top case might be no more damaging than
putting in 2% in the middle case--but in the top case, the benefit can
only be 2%, while in the middle it might be a longer charge.


Anyone know?


Lithium ion battery information is kind of secret: because treating them
wrong can be physically dangerous, the raw cells are very difficult for
individuals to acquire, and so the knowledge of how to care for them is
not widely spread. Heck, because cells and charging electroncs are so
tightly bound there might be a bunch of trade secrets in that marriage.

Also, I have heard that part of how Toyota manages the Prius battery is
to only operate in the middle of the capacity, that top and bottom
10-20% is not to be used. Yes, this is NiMH chemistry, but there might
be some similarities and general purpose complexities in the extremes.


-kb

P.S. My Panasonic subnotebook seems to have a really big "100%", I can
sleep for sometime and not have it dip to 99%. Possibly it is fudging
the reports, that the top several percent are maybe all reported out as
"100%" and they are doing tricks similar to those of the Nexus One, but
they keeping them better hidden.