I know there are other ways to do this, but I want to use an arduino!! The project: a battery charger to keep 4 vehicle batteries topped off during periods of inactivity (months). To make it harder, there is no power in my barn where the vehicles are stored and it would cost me about US$1800 to have a trench dug and power run. So, I was thinking about a solar cell or two hooked to an arduino that routes power to the batteries based on their state of charge. I have a good handle on the methods of charging the batteries and have done several complex projects with arduinos, but now I've got the additional problems of the arduinos not having power for long periods and having to wake up properly then do something that could be interrupted by clouds. All while running unattended in a barn that actually has rats from time to time.
I don't understand things like keeping the arduino off line until the solar cells provide enough power to run it. How will I know if there is enough power being supplied to actually float the batteries at 13.5V? I can measure the voltage easily enough but what happens as it drops late in the day? That's the kind of stuff I'm wondering about. I'm not in a hurry to start this project because I have a couple of projects going right now, however I'll be ready to start in a couple of months. I live in Arizona, USA so sun is not a concern.