power consumption after shutdown
Richard A. Smith
richard at laptop.org
Sat Feb 14 14:24:50 EST 2009
david at lang.hm wrote:
>> The embedded controller was. Something needs to be watching for a
>> power button press in order to know when to turn on.
> by default the wireless card remains alive to participate in a
> potential mesh network, disabling wireless should give you a lot more
In power off the the WLAN is not powered up. Only the EC remains powered.
> from the XO. Self-discharge rate has a dependency on storage
> temperature as well.
The self discharge of LiFePO4 is quite low. The numbers thrown about in
the marketing docs are between 3-5%/month. Based on the values in the
datasheet I calculate ours to be 3.8%/month at 25 degC. However, its
highly temperature dependent. The datasheet suggests that at temps >=
45 degC its 33%/month and at 60 degC 100%/month.
This has be somewhat experimentally verified. There were a few boxes of
laptops stored in a temp controlled environment for at least a year and
when I examined those batteries they still had plenty of life but,
batteries sent to Jordan that were stored in a hot warehouse arrived
> I did this just now, on a unit running Q2E27, and another running
> Q2E30, the current is 24mA at 12.9V powered from a large sealed lead
> acid battery with nothing else attached. Two days of this would be
> 1.15 amp hours (Ah). Three days would be 1.73 Ah.
While running on eternal power the EC does not attempt to enter a low
power mode. This is because its monitoring the battery. This is
sub-optimal while on things like solar power so at some point this may
roll up on the "Fixme:" list but theres a lot of conditions that have to
be dealt with. For now Ext power == EC on full.
When powered off and on battery power only the EC attempts to to go into
the STOP state where power consumption is much less. Not as low as it
should be (see below) but much less.
> days later I insert the AC adapter into the (still closed) XO, after
> the 'power' light comes on, it goes yellow. [I've seen this both
> when I was running 'joyride' on the XO, and when I was running
> 'staging'.] I take this to mean that *something* was draining some
> power for the two days the XO was sitting in its "shut down" state.
As mentioned above the EC attempts to go into STOP mode when the power
is off and not plugged up to external power. The data sheet indicates
that STOP mode is supposed to be in the 10s of uA range. Measurements
taken (this morning against my EC dev tree) via instrumented XO indicate
that its 4mA. Studying the datasheet that suggests that rather then
going into STOP mode the EC is only going into IDEL mode. In IDEL the
instruction fetch is turned off but the clocks and timers still run.
4mA is listed as the nominal draw in that mode.
Looking at the EC code the go-into-STOP-mode rather than the
go-into-IDEL-mode bit is getting set. So the code is _trying_ to go
into STOP mode but its power draw suggests otherwise. I'll have to
study it deeper.
In anycase @ a 4mA draw the EC will see the net ACR diff at about
3.1%/24hours so across 2 days you would have probably discharged enough
for the EC to enable the charger to top up the battery.
Richard Smith <richard at laptop.org>
One Laptop Per Child
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