[Power] Haiti power questions

[Richard A. Smith]

>>> Full charge for XO1 measured at 156 minutes at 7.75 amps DC, but the
>>> voltage was a bit low
>>
>> sorry I meant 0.775A
>
> That must be continuous current draw then. I though you gave me a number
> for total current used during that period. Sorry, I should have read
> your linked description of the experiment.
>
>>>
>>> So that gives us, @12V, 93Wh per XO delivered over a period of 2.6h, or
>>> 35.77Wh per XO per hour, to achieve a full charge on the XO.
>>
>> 9.3 Wh per hr or 24.2Wh per charge?
>
> That sounds right if you drew .775A continuously for 2.6h.

Its difficult to express the XO-1 charging time as a single entity.  It 
changes from battery to battery, depends on what the user is doing with 
the XO at the time, and can vary based on ambient temperature.  I 
usually say at least 2 hours and up to 3 if the XO is on.
2 hours will get you most of the way.  The switch to constant voltage 
charging mode happens around that time.

XO-1 powered off will be the fastest charge and if the XO is on with the 
camera running then the slower.  If you also have a bunch of high power 
USB devices plugged up at the same time then even slower.

Tony: there's something not quit right with the end of your graph.  It 
should look closer to the way this chart ends.

http://dev.laptop.org/~rsmith/XO1-chg.png

Rather than the steep cutoff you show. Perhaps you don't have the time 
resolution to display it.

The above graph data is from my power logging scripts that read the 
battery for multiple runs of 12 different batteries.  The XO's were 
sitting idle.

The average of the above runs is 22Wh but that's at the battery. 
Assuming 90% for the XO DC/DC converter (its 89%-91%) and you get 
24.4Wh.   Pretty darn close to your number.  The roll off at the end 
does not contain a large amount of energy wrt to the entire charge and 
you appear to have a average battery rather than one on the high or low 
side.

My 26Wh estimate was throwing in a couple of extra watts for whatever 
error sources the measurements may not really account for.

<from other mail>
> You are only expecting 3 hours of use per day?  That changes things a bit
> too.  I was considering 6 hours when doing my mental calculations as to
> array and battery sizes.

No. I was indicating that usually its charging while its on in and that 
3 hours would pretty much cover that condition.  I don't know the 
workflow for the area in question and can't tell you how many hours of 
use they will have.

The number is a reasonable estimate if all 50 XO's came in at once and 
plugged up to charge with children using them in etoys.  Since we were 
talking about the peak delivery requirements of such a system its a 
estimate of the load that would generate that peak requirement.

-- 
Richard A. Smith  <richard at laptop.org>
One Laptop per Child