I spent a few minute mulling over the kind of batteries I’ll need and the power requirements of the Arduino System.
First the battery used for solar powered system will most like be a lead-acid deep-cycle 12v rechargeable. These batteries are quite robust and can handle full discharges (unlike a nickle cadmium or a lithium polymer battery). An ordinary car battery would not take full discharges well.
Having said that, how much the system will discharge depends largely on the efficiency of the solar panel. I was looking at a 12v 5W panel that retails for fairly cheap.
As far as energy required is considered, I need a power system that will provide enough power for the Arduino, possibly an XBee Wifi shield, a relay, and to keep a solenoid charged for 5-10 minutes per day.
I have yet to gather the numbers for these things, and have yet to determine whether I can make the Arduino go into and automatically come out of sleep mode in order to conserve energy.
Also, stepping the 12V DC supply down to 4-5v could be a problem, if using a linear voltage regulator then a lot of the energy from the battery could be wasted as heat. Therefore it might be a good idea to consider a Switching Regulator, which is largely more efficient, but increases the cost of the system by about £12.
Calculating Power Consumption
Maths is not my strong suit. I find it difficult to remember formulas, and when I do remember them, my maths go all screwy and somehow many to get the wrong result anyway. So, as an interesting side project, how about creating an app for helpful formulas? A power calculator. I’m sure such a thing already exists, but I think I might build one anyway. Because then it’ll be mine and I’ll know how to do it for other things.
Ok, so its quickly becoming that this system will largely be theoretical. Mostly because the cost of a hose timer turns out to be not more than £5 for a rudimentary electronic timer. There is no way I can compete with that in terms of price for performance. However, it is still an interesting exercise allowing me to spend time learning about various possibilities.