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[Reply #15 on:]

Say you consume 60mA, leaves you with 200mA left to charge your batteries. If you use 2500mAh batteries it would under perfect conditions theoretically take 12,5 hours to charge your batteries.

You have to calculate as followed: If we say (for simple calculations) that you consume 65mAh than that would be 25% of your solar panel capacity.
So for every hour that you use your Arduino 3 additional hours of usage are being stored in your battery (under perfect conditions). Say you have a day of 5 hours effective light, than you have a charge of 15 additional hours in your battery (4 hours short).

Personally I would double or triple your charging current (more panels in parallel).

A note on the choice of your panel. That one is rated at 8V which is in a way unnecessary high. This will be dissipated in heat by your regulator.
Another option is the use of Solar garden lights and make your own array. They are quit cheap on Amazon and each lamp has its own build in rechargeable battery that you can use http://www.amazon.co.uk/GARDEN-SOLAR-POWERED-STAINLESS-LIGHTS/dp/B0046Z6THI/ref=sr_1_26?s=outdoors&ie=UTF8&qid=1331491858&sr=1-26 (you would need a bunch of them)

[Reply #16 on:]

Thanks for your help with this. It does make a lot of sense what you calculated...
I did choose the 8V panel because of the voltage drop over the LM317 which, if I remember correctly, is about 1.7V. So I would need slightly more than 6.7V from the panel in order to get a stable 5V output. 8V was the closest higher voltage I could find.

I'm now beginning to think that solar powered will not be feasible for this project. The size of the panels I would need is just too big. This was supposed to be a small outdoor temperature and humidity sensor. I can't have the solar panels cover half the garden just for that one sensor..

So I  think I have to power this from mains or find a way to drastically reduce the current this sensor draws. But I guess even a 3.3V 8MHz Arduino Mini will not reduce the current enough. Now I do wonder how these commercial wireless weather station sensors can work for many months on 2 AAA batteries....

[Reply #17 on:]

Point taken :-)

If they are wireless they might use a different kind of transmitter. Maybe a lower transmitting power (shorter range) or different transmitter (IR) or higher frequency? Or a combination of solar panel (as both light sensor and power supply) where the battery needs an additional charge once every month?

The maximum capacity of AAA (NiMH) lays around 1100mAh (some no name asian brands claim higher).
Asuming the batteries are placed in series (as the device might not operate on 1.2V) this would mean 1100mAh devided by 720 hours (30 days x 24 hours) = 1.5mAh which seems to me like a very low power consumption.

In my opinion it is not be possible to have a wireless sensor comunicating with the base unit for a few months on 2AAA batteries without some kind of additional power supply.

[Reply #18 on:]

http://www.ebay.co.uk/itm/180651377637?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
it should work? Even if the panel only delivers half that max current, it should still be enough to keep the batteries charged on an average UK day?

I've a roof full  of polycrystalline panels rated at 2500 watts peak, at the moment in brightish overcast conditions, they are producing 345 watts.  In dull overcast conditions they go down to double figures or even nothing at all in thick black cloud and drizzle (and absolutely nothing when its dark).  They only get close to their rated capacity in full on sunshine.  Small panels will have a similar performance (granted I have an inverter to make it AC and grid tying it, but they don't consume that much). so that 260 mA would probably be running about 40 mA today. (Barely enough to run a bare arduino let alone charging anything).

[Reply #19 on:]

Wow, that's extreme. Thats a factor of 7,24. I would have expected more. Well in that case you would need a panel rated at 500mA just to get around 65mA under te same weather conditions and around 4A output to charge your battery on the side. At 8V that would be 32W. In that case you might as well spend some extra money and buy a controlled charger so you get the max out of your panel.

[Reply #20 on:]

Light intensity is logarithmic, it takes only a small perceived change for it to drop right off. 

Having just checked up, my panels are mono-crystalline, it makes little odds, it just means they are slightly smaller for the same power output.  They behave just the same. 10 of these :

http://www.swithenbanks.co.uk/shop.php?action=full&id=12312. 

[Reply #21 on:]

How much draw does an Arduino take in a power-saving mode that lets time be tracked? If it's only got to read a thermometer once an hour then power usage can stretch a good long ways.

Again, with solar panels you can reflect or magnify more light onto the panel if there is light to reflect or focus. Those crystalline cells can take direct sunlight in space, they should take a few x sunlight even at the equator. Even a face-tanner reflector should get some increase on a solar panel. 

[Reply #22 on:]

battery -> http://www.pololu.com/catalog/product/2226

If I have a solar panel array delivering 9 volts and 260mA nominal, and my project only pulls 70-90mA, would the remaining 170 or so mA get wasted, or charge the battery?

The cells are rated 9V @ 130mA nominal, I'm expecting closer to 8.5@85 out of them, thus why I'm paralleling them.

FWIW, the regulators have a 15V max input.

How would I set this up? moduli fotovoltaici I know I'll need a diode between the solar panels so one doesn't backfeed the other, and another diode before the array so the battery doesn't backfeed the panels.

I've seen solar battery/load tenders for 3V3 Li-ions, but nothing for 9V NiMH's.

edit: found this -> http://ghurd.info/dc.html

I've emailed the designer to ask how to modify for 9V.

This is great option for small usage

[Reply #23 on:]

Not sure what you are trying to do, but if all you are building is a low power temperature sensor which will transmit
data a short distance, these things draw very little power, but you cant use an arduino to do it as its total overkill
for whats needed and the power consumption is too high.
Ive just built a clone of an Oregon THGR810 battery powered temp sensor , and it draws only around 200 microamps
on average , peaking to around 10 ma when transmitting, which means around 1 - 2 years on a pair of AA batteries.
Look at using a small micro power microcontroller which can go into sleep mode when not transmitting.

[Reply #24 on:]

I think Undrad might have a different project than the original poster of this topic.
9V NiMH charger circuits are easy to find on google for example:
http://www.circuit-finder.com/categories/power-supply/battery-charger/713/nimh-and-nicd-battery-charger-circuit
In your exampel the 170mA would charge the battery.