I have the same problem with my 430SC. The battery drains if the car is left standing for more than two weeks or so. I do have an electronic charger that trickles without overcharging but have now invested in a Battery Brain http://www.batterybrain.co.uk/
This is able to detect when the battery is getting low and automatically cuts the connection to the car. It saves enough juice to start the engine again protecting you from being stranded and your battery from being damaged. To reset you simply flick a switch and the power is back on.
A brilliant and simple to install solution
