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Fully charging 2 AA batteries using solar panels that are in a 5" by 3.25" configuration...

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Posted by they call me frenchy on August 15, 2005, 6:24 am
Hello there,

I am new to this group and I plan on doing a thorough "google" search
about using solar cells to charge AA batteries.  To get me going
quickly in the right direction, can any of you comment on where I can
find a source for approx 5" by 3.25" solar panel assemblies that would
fully charge 2 AA batteries (NiMH or NiCD, approx 2300 to 2500 mAH) .
Ideally I would like to fully charge them from empty (or close to
empty without damaging them) on the most gloomy winter day that a
typical northern state (or Northern Europe) would experience in the
middle of winter (worst case scenario light supply for the design).

I look forward to learning more about solar as I work on this project.
Ideally the solution would be supplied from a sourcing company that
could give good price breaks for very high quantities (approx under
$USD for 2.5M to 10M annually).  Would I need to go overseas for such
a quantity, or is it possible to stay domestic?)

I really hope that this is possible within the 5" by 3.25" space
restriction that I am dealing with.  To make matters even more
challenging, the solar panels will be 25-50% covered due to another
part of the design.  Are there any good websites or resources that
discuss this subject matter?  I have electrical background, but I am
new to solar.

thank you,
Steve French aka frenchy

Posted by Landline on August 15, 2005, 8:08 am
<cough splutter> $US1.00
would not even get amorphous junk from Indonesia for that price for that

interesting concept solar panels that charge while shaded

Posted by they call me frenchy on August 22, 2005, 3:33 am

OK, I have reduced the size requirements down to 76.2mm [3"] x 50.8mm
[2"])  I still hope to get close to this price figure in very high
quantities, but the design electronics and functionality take
precedence.  So I appreciate your comments.

Yes, indeed design restrictions are a bummer.  For this reason, we
decided to go with a smaller solar module size that is completely
unobstructed and to reduce the AA capacities to 1000mAH (there are two
batteries).  My challenge now is gloomy days.

Thanks for your response.  Believe it or not, it helped me!

Posted by H. Dziardziel on August 16, 2005, 2:43 pm
 On Mon, 15 Aug 2005 02:24:52 -0400, they call me frenchy

There is no time frame given for the recharging so let's start
with the standard charge for 2500mAh cells that have been fully
discharged ( <1V per cell).  That is 250mA (1/10C) for 14 to 16
hours.  To do that the charger needs to output at least 1.6V per
cell to allow for cell I^2R losses and electrochemical voltage
rise during charging.  

An electronic charge controller for one or two NiMh cells greatly
complicates requirements due to the charger electronics needs.
Can any panels of the size you give provide this voltage and
current in full ideal sun conditions?   Goodluck finding solar
panels for that (3V+ at 250mA) with the size and cost you have

However, the scenerio described suggests that 15 odd hours at
250mA won't even be cumulatively available for perhaps even
several days in a row so a much hgher mA rate is needed, say 1/2C
or 1300mA for one hour.  Well, need we say more?


Posted by they call me frenchy on August 22, 2005, 3:25 am
 H. Dziardziel,
Thank you very much for your responses!!   I give you some new
information here enclosed in **********  and my responses to your
comments are embedded into the thread below.

New info:
Since my original post, I have modified my design based on your input
and a host of other reasons, we have changed the Solar module size to
76.2mm [3"] x 50.8mm [2"].  We have also reduced the capacity of the 2
AA batteries to 1000mAH.

So, since a typical solar module has an efficiency of approx 12%, this
means that it can provide 120W/sq. m in direct sunlight.  Based on my
area above, this is 464.5mW in direct sunlight.  Since I will be
charging 2 AA batteries, the charging circuit will need >=3v.  Using
3v, the solar module would be able to output 464.5mW/3v = 154.8mA.
Since I would like the battery capacity to be 1000mAH, this solar
module will be able to charge this battery capacity in 1000mAH/154.8mA
= 6.46Hrs of direct sunlight.  Right so far?

But since I would like to be able to charge the batteries in winter
and gloomy conditions, I would probably serve myself well to reduce
the battery capacity to 500mAH.  How do I calculate for % loss of
solar power during gloomy conditions?  Certainly it is NEVER reduced
to 0% is it?  I should design my worst case scenarios around Seattle,
etc.  but exclude Alaska from my calculations.

On Tue, 16 Aug 2005 23:43:26 +0900, H. Dziardziel

I dont follow you when you say 250mA for 14 to 16 hours.  That would
be 3500mAH to 4000mAH capacities, but we were talking about 2500mAH
capacities at this point.  I plan to have an undervoltage detector
circuit that turns the load off when the batteries reach a certain
discharge level, so that I do not damage the batteries.

Regarding your 1.6v per cell requirement.  Hmmm... that is
interesting.  I had one supplier of solar modules come back to me with
a 5" by 3.25" solar module with ratings of 3V 400mA.  You are saying
this would not be good?  That the voltage is too low?

Lets forget the cost for now.  I am planning to slowly trickle charge
the batteries over the course of one day of sunlight (even winter
days).  I was planning to have a photo-detector circuit to turn power
to my load on and off when the sun is up.... and then an over & under
voltage protection circuit to keep the batteries protected from
over/under charge.  These electronic circuits could be implemented
with extremely low power consumption cant they (assuming that the
solar module voltage output is nominal and requires no regulation).  I
was planning to check out the ultra-low power offerings from Maxim
when I got to that point.

Indeed I will have to make a disclaimer that the product will not work
optimally in certain conditions such as continuous days of extended

thx lots!,

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