Porsche Trickle Charger?

[9fourteen]

I had my 2006 boxster sitting in the garage since December. I bought the trickle charger from Porsche and had it plugged in the whole time. This morning I decided to get the car out to start driving it. It started right up. I drove it, had it parked for a couple hours, drove it home and parked it in the driveway. When I went to put it back in the garage it wouldn't start. The starter clicked as if the battery was dead. I pulled the battery and put it on the charger. When I hooked it up the charger showed it only had about 6 volts on it. I'm guessing that it must have had enough juice to start it a couple times before it died. I only drove it about 10 miles both directions this morning so it probably wasn't enough to charge it back up. Does that sound normal?

Is that the proper use of the trickle charger? Did I do something wrong? For long durations, is that trickle charger not sufficient?

I think next year I'll pull the battery before I store it for the winter and just charge it up when I'm ready to get it out.