[Power] Good day for a solar panel test

[Nicholas]

Dear Power OLPC list.

Tested the panel in mid Winter sun with high cloud. 7w & 7.2w using
30ohm and 20ohm resistors respectively to calculate the power output.
Looks good. Will do more tests with different power resistor values
when a clear day presents itself.

On 7/12/12, Richard Smith <richard at laptop.org> wrote:
>> First thing to note is the power cable is short but it is not far from an
>> ideal length IMO.
>
> Yep.  That's going to be fixed in the production units.  Proposed
> length is 1.5 meters.  Just like the previous GP panel.  What do you
> consider the ideal length?

I use mainly external battery chargers (charging NiMH or Li-ion cells
by solar in some sort of external charging device) so a short cable is
fine for me. Perhaps 200-300mm longer than the original length is good
for that purpose. However, charging the laptop in the sun is not the
intended purpose here, nor is an external charger.. So, that leaves a
longer cable. It was mentioned to me privately that 5m is a useful
length for a solar panel cable for simultaneous charging/using if the
v-drop problem could be sorted out. There are a couple of volts
headroom with an XO compared with other 12v devices (XO charges at 12v
other devices, 12.8 -> 14v?) so perhaps a long long cable could be
useful. I personally don't know

>
>> Too long is a pain I've found in the past.
>
> How so?  It needs to be long enough that the panel can go in the sun
> and the user or XO in the shade.  The samples that are getting used in
> Madagascar highlighted this because they were concerned about having
> the XO in the direct Africa sun.  It gets very hot and charging makes
> it even hotter.  The laptop is only rated for 50 degC operating temp.

Coiling, uncoiling.. Can be a bit tiresome, got so many cables around
here it seems. It's always great when you find a cable just the right
length..

As far as the longevity of this panel, the heat generated from the
black nylon backing is what I worry about. Nylon for outdoor use is
black to offset UV degradation of the fabric? Hard to do any
meaningful tests for this--it's cold here. I have a 5w 6v panel
developing bubbles in the encapsulate at the level of the silicon
cells. The panel is black (it originally came with a fabric surround
which made the problem even worse and was gotten rid of within a day)
and it's only about a year or two old and it heats up something wicked
in the sun (even without the fabric surround). This particular panel
is originally rated for 800mA, puts out 600mA now (I think Isc). Could
be using a cheaper type of encapsulate. It could possibly be PET from
memory.

I also have a Voltaic Generator laptop bag, this also heats up
somewhat in the sun, even with the bag opened to let air in when
charging. The panel is silver with blue cells but the bag is black
nylon. I would not charge a laptop in this bag in the sun for an
extended period of time even with the bag open--limits the bags
usefulness.

Last thing. Kind of stating the obvious but a 15 watt panel *is*
really useful, I can see this panel being co-opted for all sorts of
uses, household lighting, cellphone charging.. Perhaps not the best
alternative but one alternative to a really long cable is a 1.5m cable
pre-spliced with generic connectors and extra connectors for making up
extra cables including a spare couple for making up an extra long
cable if needed?

Regards, Nicholas