solar module battery charging project

 Hello all,

I am going to a camping music festival this summer where there will be
no electricity, and I just had the idea of getting a small solar
module to power my battery charger for my digital camera.

So here are some specs:
Camera: Minolta DiIMAGE G500
Battery:  Lithium Ion NP-500, 3.7V, 820 mAh
Charger: BC-500, Input - AC from wall, Output - 4.2V, 0.6A (2.52 W)
http://www.kingbatt.com/product/id/wckn001_3.html  <- there is a
picture of a replacement charger (different specs though - 12V
output), as well as the battery at the top right.

I have a few questions about this - I am an electrical engineer and am
currently taking a solar electricity class, but I do not know much
about battery charging so (1) how feasible is this project? (2) would
the sun intensity variability (and hence solar module output power)
greatly affect the battery charging performance?  (3) Any good idea of
where I could find a solar module that matches the charger specs? If
no modules match the specs, would it be better to get one that was
rated above or below the V or I charger output values?

My initial idea was to buy an extra charger and battery from the site
above so I could hack it and wire the solar module directly to where
the output of the voltage regulator would be.  I don't expect to do
this with one battery/charger because I really would not like to break
it and be S.O.L.!  Also, I'm assuming any solar module wouldn't be too
expensive, probably about $30 tops, right (about $10/W if I'm not
mistaken)?

If anyone has any input, knowledge or general suggestions, I would
greatly appreciate them.  Thank you!!

Ray

  I also noticed that a cigarette lighter adapter comes with the
replacement charger.  Maybe it would be better to find a solar cell to
meet those input specs (12 V)?

 There are small 12V PV panels at Harbor Freight, Northern Tool, Pep
Bots, etc..

 On Fri, 11 Apr 2008 15:46:19 -0700 (PDT), "Ray D."


I agree with Roland's suggestion - but just to explain a little
further. Get the 12v "cigarette lighter" charger and also buy a
lighter socket (I guess they are available in electronics shops as
they are here in NZ). Then you need a "buffer" battery as suggested by
Roland. This could be a small 12v "gel cell", or you might use 10
cheap NiMH or even Nicad cells (we have a store here that sometimes
has 600mA nicads for as little as NZ$1 = US$0.70 each). Then look for
a small 12v (nominal) solar panel of maybe up to 5 watts - or possibly
about 2 - 3 watts would do. Its doubtful whether you would need much
of a "regulator", especially if you had a gelcell of 3 to 6 amphr
capacity and were charging your camera battery at the same time.

Another alternative is to dispense with the battery and just connect
the 12v charger directly to a 12v, 2 - 3 watt solar panel. Provided
the camera battery is plugged in, the panel will just supply whatever
current it can to the battery, and the regulation in the 12v charger
will limit current (if needed) for the battery.
You may not have full charging current, but its probably better to
keep it lower, than to have so much that the voltage into the 12v
charger could rise too high.
Just my tuppence worth.
Eric Sears

 Thanks for the input guys.  I was searching around more last night,
and although it might be a little more complicated, I was actually
thinking about building the charging circuit using a standalone
lithium ion battery charger chip from Linear Technology -
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1037,C1078,C1088,P8990,D6397

I could program the charge current to whatever the solar panel can
handle by simply using a resistor, and the Vcc input range is very
wide, from 0-10V (4-8 optimal).  Also, you can terminate the charge
current once the charging cycle has completed to prevent over-
charging.  I found a good match for a solar cell with Voc = 8V and Isc
= 310 mA.  Considering I would only charge this during peak sunlight
hours (with battery = 820 mAh), I think it would charge nicely to keep
my battery going throughout the couple of weeks I'll be away.