I have a charger I bought at Walmart for my 230SL that I drive a few times a year. It is the "tender" type of charger. It charges at a low rate until it reaches a certain voltage and then turns itself off. When the voltage goes below a certain value it turns on again. This is the recommended way of maintaining a charge on a car battery that is not used much.

Because it turns itself on and off automatically, you can leave it plugged in all the time. I installed a plug to the outside so I can plug it in without opening the hood.

It is a Vector Smart model VEC080 and costs about $25. Works great!