My wife's BMW battery would die once a month. I trickle charged overnight 2 amps. Brought to AutoZone for a test. They said it was fine. Car was charging fine. This went on for a year. I finally broke down and bought a Battery. Never had an issue since. My opinion was a bad cell or low cell (sealed battery). Good luck and keep us posted.
That's kinda what's going on here. I keep it on a trickle charger yet when I'm out driving the car, after several stops the battery gets weak enough that it won't start the car. It's maddening.