Hi. For my first non-trivial arduino project I am trying to make a solar powered arduino duemilanove. For that I will use a 12V 7Ah gel cell battery.
Seems like overkill for running an Arduino, but maybe your overall project is larger and needs that much current capacity...
For properly charging the battery on standby with the solar panel, it needs a constant voltage supply of 13.7 to 13.9 volts at 20 °C with a temperature compensation of -20mV/°C. At least that is what the specification sheet says.
OK.
What are the possibilities for controlling the voltage from the solar panel to the battery depending on the temperature?
You could do it, but it is probably overkill for all but the most extreme cases.
Am I being exaggerated by trying to regulate the voltage depending on the ambient temperature?
Depends on your final application. In most general cases, yes.
What should I do when the solar panel supplies less than the needed voltage (night)? Should I "disconnect" it with a transistor/relay or can I still charge the battery with less than the specified voltage?
A simple rectifier diode acting as a "blocker" will work to keep the battery from discharging back into the panel. If you buy a panel that is specific for charging a 12 VDC battery, most all of them have this built in, but it is trivial to add it otherwise.
I charged a similar battery at Burning Man using a small panel I picked up at a yard sale; I had to add a blocker diode, but I put the panel in the sun during the day and ran a small flourescent work light at night for my tent - it worked fairly well.
If you absolutely need the most performance, you can buy small DC solar charge controllers for a battery that small for around 20-30 dollars (US) - you'll likely spend far more time and money trying to design and build your own (unless your goal is to learn how to do this, of course). The controller is designed to regulate the charge from the panel into the battery depending on light levels, load levels, etc and prevent back-discharge at night.
