OLPC News 2008-01-26

[linaccess at yokoy.de]

nice idea.
the heater could be an add on, just for the extrem cold areas.
yokoy

On Sat, 26 Jan 2008 15:22:57 -0500
"Benjamin M. Schwartz" <bmschwar at fas.harvard.edu> wrote:

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> Walter Bender wrote:
> > 5. Batteries: Carla Gomez Monroy reports from Mongolia that the
> > batteries are not lasting as long as expected. The extreme cold was
> > the first suspect. Richard had Carla collect data via olpc-logbat and
> > ran some tests of his own in the freezer (which isn't as cold as
> > Ulaanbaatar). These data, along with a closer examination of the
> > GoldPeak data sheet, make it pretty obvious that batteries don't work
> > so well in the –20 to –40C range. The extreme cold makes the output
> > voltage drop considerably. The result is that at around the 50%
> > capacity mark the voltage is so low that the low-voltage cutoff kicks
> > in and shuts the laptop off. Richard does worry about when the
> > children take their XO laptops outside while suspended; the power
> > dissipation (and thus self-heating) is at its lowest. It may shut off.
> > The question to work out with our battery vendors is that is it OK to
> > de-rate the low-voltage shutoff when it's so cold. Will this do any
> > damage to the battery?
> 
> - From the uninformed crazy ideas department:
> There's a temperature sensor in the battery compartment.  If the EC sees the
> battery temp hit -15C, and at the same time the voltage is, say, 10 mV above the
> low-voltage cutoff, it could _resume_ from suspend, in order to increase load on
> the battery, and therefore increase self-heating.  This would raise the battery
> voltage and extend battery life.
> 
> Mathematically, if effective battery capacity is reduced by cold, and
> temperature is increased by drawing current from the battery, then there may be
> a domain in which drawing current increases the battery's effective capacity.
> This approach would work even better if there were a heating element, or
> something that could be used as such, near the battery.
> 
> - --Ben Schwartz
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