Solar power & batteries : diode vs driver

Hello,

I'm quite new in electronics. But I want to build an autonomous device logging on a sd card the temperature outside. My idea is to reproduce a standalone arduino, with chip pre-programmed on my arduino uno board, and to power it with 2 3.7V batteries coupled to some solar cells delivering less than 4-5 V. My question is about how to couple batteries, solar panel and arduino to both power arduino and allow battery charging. (Arduino is not important to me, it could be anything needing power).

I dismantled a garden light and see that solar panel was basically connected to batteries with a simple diode. But I also viewed that dedicated drivers exist to put between batteries and solar panel.

My understanding is that driver is probably a better solution for battery longevity, but diode solution is working well too. Am I right ? Do you have a deeper understanding of this, or a previous experience to share ?

Thank you !

The diode solution isn't great, it'll heavily overcharge on bright days (if its got enough voltage for the dull days). Few battery chemistries tolerate that well, completely no-go for lithium which require over-charge and over-discharge protection.

Thank you for those details.

My batteries are 18650 Li-Ion, so I guess I should look for a correct driver. I didn't find anything on Sparkfun and found only charger for 3 or 4 batteries at the same time on Ebay.

Any idea where I could find a driver for 2 batteries 18650 ?

Thank you for your help.

You can use the Adafruit charger to charge one 3.7 V Li-ion battery properly from even a garden-lamp type solar cell module (those batteries can explode and burn if not properly charged). http://www.adafruit.com/products/390

Then, use something like a Pololu boost converter to produce 5 V for the microprocessor circuitry. http://www.pololu.com/catalog/product/798

Seeedstudio has fantastic bargains on small solar modules.

Thanks,

but as far as I know, concerning the boost converter, arduino board may be unstable with input voltage less than 7V (http://arduino.cc/en/Main/arduinoBoardUno).

I have seen some devices on ebay from China combining both charging regulator and booster, for 2 18650 batteries. I will try these out.

Loic_B:
but as far as I know, concerning the boost converter, arduino board may be unstable with input voltage less than 7V (http://arduino.cc/en/Main/arduinoBoardUno).

It is a bad idea to use the barrel jack power input on an Arduino in a solar powered system. The built in linear regulator wastes the input voltage in excess of 5V as heat, which is extremely inefficient. You can power an Arduino by applying +5V to the pin normally used for regulated +5V output. Or if it has a USB port, use that.

If you are determined to waste energy, Pololu and others sells boost regulators that provide any voltage between 4 and 25 V. Of course, the boost regulators are not perfect (in the range of 85-95% in efficiency at modest power levels), but this isn’t as inefficient as dropping, say 8 V to 5 V, by heating the environment.