It really depends upon the type of alarm you have. If it is factory it will be no big deal. At the most you will have to hit the clicker once and you will be good to go but I doubt that will even be necessary as I change batteries in Volvos all the time and have no problems with factory stuff. It is the aftermarket stuff that arms itself when it see and interuption in the power supply. When you reconnect the battery most systems will go off and run the horn and lights, etc. So if you have an aftermarket alarm just make sure that you have the alarm remote handy when you reconnect the battery.

I have to agree with the solar charger idea, I don't know if it will be parked in the sun or close enough that you could run the panel out to a sunny spot but that would be a good option too. If you were to run a trickle charger I would think you would only run 1-2 amps absolute maximum charge. I have seen those solar panel style battery chargers in a few places like Harbor Freight and they get less expensive all of the time.

Mark