Keeping the auxilliary battery charged when the car isn't driven?

I had not driven this '02 in a few weeks and when I tried to start it the battery was dead. After charging the battery it tested out fine with a commercial battery tester even though it's 6 years and 150,000 miles old. I anticipate there will be times ahead when it will sit for weeks or more than a month at a time so short of disconnecting the battery, I was going to connect a VW solar maintainer I have directly to the battery. There is no electricity where I store the car so I can't use a plug in device. Would there be any problems using this? I have one of these on a Cadillac and it starts right up after sitting 3 or 4 months.

 

This is the type I'm using: eBay Motors: VW Solar Panel 12V Battery Charger OBD II boat rv car (item 330286504185 end time Nov-20-08 18:42:04 PST)

I bought a half dozen a while ago for the cars I don't drive too often. They plug into the OBDII connector on VW's and suction cups mount them on a window.

 

I would connect directly to the battery with alligator clips and run a wire from the trunk, outside, and in through the back door and put the charger on the front dash or window. I went through this with another car by plugging into the OBDII connector and it turned out that it kept some computer awake continuously and actually drained the battery. VW must have some program in the car's computer that keeps it asleep while this is connected.