OLPC News 2008-01-26

[Benjamin M. Schwartz]

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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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