[Its.an.education.project] Ivan's latest blog entry on OLPC

[Edward Cherlin]

On Sat, May 17, 2008 at 8:32 PM,  <forster at ozonline.com.au> wrote:
> Wad
>
>> To the person using car batteries for storage --- bad idea, due to
>> cost over lifetime
>
> That's me.

It is my understanding that car batteries are widely used regardless
of that issue, because they are readily available. It is not enough to
have a better solution. We have to be able to deliver the better
solution.

> Yes agreed, but the car battery looks better over a 2 year life which may be more realistic for a technology project, also it can be procured locally
>
>> You also underestimated the battery capacity required ... Do you want to
> run a single school server... with two days of reserve capacity ... one
>> day reserve

We need a variety of figures, or better still a spreadsheet that will
allow us to put in the performance measures we want and tell us how
many batteries we will need to do it.

I estimate that 1 KW would be enough to run a school server and 100
XOs in full operation (100% computation, backlight on, peak power 8.7
W). I take that to define the base unit that we need. So 8KWh should
cover a school day, and somewhat less should cover evening use.

> I was thinking no server and not much reserve. I understand that a panel feeding directly into the OLPC's DC-DC converter may not start up, that's why I suggested a battery at all.
>
>> The OLPC Multi-battery charger provides the electronics and mechanicals
>> necessary to connect up to 15 batteries up to a 12VDC (or 90-240VAC)
>> supply.

http://wiki.laptop.org/go/Multi_channel_battery_charger

Charge 15 XOs simultaneously in 2 hours at 150 W-DC. This comes to .3
KWh per XO, or 4.5 KWh total. So we would need 30 KWh to charge 100
XOs, which would need to be done at least once a day, depending on
usage. I am told that car batteries run 30 Wh/kg (108 kJ/kg), so we
would be talking about a few kg of deep cycle batteries per system to
provide two days' charging capacity at moderate usage of the XOs
(mostly as book readers) and proportionally more for higher-power
uses.

These specs look quite inefficient. How real are they?

> Can you provide costings for the charger and tech specs?

Somebody on Peripherals should be able to say something about the
costs. Rsmith and Rafael Ortiz wrote that section of the Peripherals
page, and Kim Quirk made some sort of change some time last year.

The Wiki page has draft specs.

> Thanks
> Tony
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-- 
Edward Cherlin
End Poverty at a Profit by teaching children business
http://www.EarthTreasury.org/
"The best way to predict the future is to invent it."--Alan Kay