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

[they call me frenchy]

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
$1USD 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

 

[Landline]

<cough splutter> $US1.00
would not even get amorphous junk from Indonesia for that price for that
size

challenging, the solar panels will be 25-50% covered due to another
part of the design

interesting concept solar panels that charge while shaded

 

[H. Dziardziel]

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).

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
given.

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?

..

 

[they call me frenchy]

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.
************************************

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.

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?

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
given.

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.

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?

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

thx lots!,
frenchy

 

[they call me frenchy]

<cough splutter> $US1.00
would not even get amorphous junk from Indonesia for that price for that
size

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.

interesting concept solar panels that charge while shaded

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!
respectfully,
frenchy

 

[H. Dziardziel]

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.

For a reference and initial engineering estimate those are fine
with the understanding one should use actual specs from the solar
cell manufacturer.and note that such specs are optimal maximum
output in full direct sun. Actual cell output is always much less
and the voltage-current ouput curve will usually not be a good
match for the application for the simplest cell configuration
based on their maximum.outputs.

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.

Not quite, the panel needs to supply a certain voltage and current
for the application. So, _if_ the panel can output 3V, a 465mW
the panel _might_ do so at 155mA. Check the specs and again keep
in mind specs are for rarely acheived ideal sun and load
conditions..

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?

NiCad and NiMh cells, when charged at the standard C/10 rate,
require about 40% to 60% overcharge due to electrochemical
charging inefficiency, The higher the charge rate the lower this
ineffciency. 50% for these small capacity cells (1000mAh or
2500mAh) at C/10, is 100mA or 250mA is ckose enough.

So, the 1000mAh cells need 1500mA for a full charge. Take a look
at any AA or AAA cell, this standard rate is usually printed on
consumer packaged cells.

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.

Solar cell output fall drastically under such conditions so that
will need some spec and area sun research. Just reducing cell mAh
may not be enough since the solar panel voltage may be too low
under those conditions to charge any capacity NiCd NiMh unless the
panel has reserve capacity. Eg, a maximum 6V output through a
regulator-charger circuit to match the nominal two series cell 3V
charging voltage.. This would allow for low sun conditions.

Good luck

 

[they call me frenchy]

On Tue, 23 Aug 2005 11:10:12 +0900, H. Dziardziel

NiCad and NiMh cells, when charged at the standard C/10 rate,
require about 40% to 60% overcharge due to electrochemical
charging inefficiency, The higher the charge rate the lower this
ineffciency. 50% for these small capacity cells (1000mAh or
2500mAh) at C/10, is 100mA or 250mA is ckose enough.

I did not know this. Obvously I need to put myself through a crash
course in battery chemistry and charge/discharge ethics. Can you
recommend a resource that you particularly like? Like an application
PDF from Sanyo or Panasonic or similar?

Solar cell output fall drastically under such conditions so that
will need some spec and area sun research. Just reducing cell mAh
may not be enough since the solar panel voltage may be too low
under those conditions to charge any capacity NiCd NiMh unless the
panel has reserve capacity. Eg, a maximum 6V output through a
regulator-charger circuit to match the nominal two series cell 3V
charging voltage.. This would allow for low sun conditions.

Good luck

Excellent point about the solar panel voltage falling too low to
charge under non-sunny conditions. Likewise, I need to read
application notes/manuals on solar battery charging circuits. The
more regulation I include, the higher the hi-volume cost. As usual,
this will be an engineering balancing act.

Thanks again for your input and it would be great to get a few
recommendations for reference texts from you, because you obviously
know what you are talking about!

respectfully,
Steve French aka frenchy

 

[Paul]

A google search of "NiMH charging" brought me:
http://www.powerstream.com/NiMH.htm
among other hits. Amazing what you can find when you look.

 

[H. Dziardziel]

Look for the various application and charging manuals:
http://www.panasonic.com/industrial/battery/oem/
http://www.sanyo.com/batteries/lit.cfm
http://www.duracell.com/oem/rechargeable/Nickel/nickel_metal_tech.asp
http://data.energizer.com/SearchResult.aspx
http://power.ti.com

Excellent overall sources:
http://www.batteryuniversity.com
http://www.powerstream.com