Operation without battery (GTA02)

Andy Green andy at openmoko.com
Wed Jul 23 09:16:30 CEST 2008


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Somebody in the thread at some point said:
| Andy Green wrote:
|
|> Jeffrey, if you are interested about what the battery experiences, there
|> are a bunch of goodies from the Coulomb Counter in the battery
|> accessible down /sys/class/power_supply/bat, just cat them.  These tell
|> you the battery's view of what is going on directly.
|>
|> # cat /sys/class/power_supply/bat/current_now
|>
|> is particularly interesting, this is the flow of current out of (+ve) or
|> into (-ve) the battery in uA.  These values come fresh from the Coulomb
|> Counter each time, but that device itself updates its registers only
|> every few seconds.
|
| Is there a Wiki page or docs that explain what each register is and
the units it refers to?
| especially in the .../power_supply/bat/... area which seems to have
very pertinent information.

I think I will start one.  There's some real gold down the /sys mine if
you are interested in meddling.

| Do the values appearing in the /sys/classes/... files differ anyway
from what the hardware provides
| or is it tweaked by the kernel?

Some of the coulomb counter data is actually directly provided in very
funny units relative to a sense resistor outside the chip (20mR in our
case), the driver does the recommended computation and normalizes them
to uV and uA as required by power_supply APIs.  And temp in 0.25 Kelvin
steps --> Celcius in 0.1 steps.  But this is basically scaling up
operation and you see otherwise unmassaged data.

The datasheet is open

http://www.ti.com/lit/gpn/bq27000

as is the driver of course

http://git.openmoko.org/?p=kernel.git;a=blob;f=drivers/power/bq27000_battery.c;h=7020608e6e901f3e9b46fea911dfd41ed31f7877;hb=stable

and you can see there are even more registers in there that don't map on
to power_supply stuff.

- -Andy
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