I pulled my nice 996 out of the garage and shut it off to go grab some stuff inside. When I came back it was so dead the interior lights didn't even turn on. Forget about closing the windows or locking the doors!
I was really impressed it went from mostly OK to deader than dirt in seconds. I put it on the charger every weekend, no power out there so I can't use a maintainer.
I found an AGM battery on sale at NAPA and it's good as new.