Even new car batteries like to be used more than every 2 weeks for short
periods.  Also, you may be in a cold climate...which wouldn't help the
situation.

Best solution I found was to install one of the very small "microprocessor"
controlled "trickle chargers" that sense when the battery voltage drops a
certain amount and automatically turn on and bring it back to full charge.
These are about the size of a notebook PC power converter (small, squarish)
and the last one I used I simply wire tied to the side of the battery,
inserted the leads into the battery poles, and it had its own "AC plug".
When I parked the beast, I simply opened the front hood and plugged in an
extension cord.  Worked perfectly for over 3 years.

Porsche also sells one that fits in the cigarette lighter plug...and I
understand it works, but it's (of course) Porsche priced.

Regards from Texas!