Next time you are in a Auto zone or like automotive parts chain store ask them about the battery exchanged vs purchased and the longevity. Or even better an RV dealership or RV service shop. A stored car like your BMW as you described (sad to have a nice car like that and not drive the sheet out of it) is an slight exception somewhat to the rule regarding battery lifespans. It is also a 2006 lot less on board electronics that are voltage dependent even when ignition is off.

And as I have suggested on the thread maybe re-read a couple of my responses . LOL I know that you are a 12 volt system wizard DIY and all but..... LOL

 

Nope. Never claimed to be a 12v system wizard. Just sharing my experiences. The older Z4M's like mine have electrical issues too, but mine (so far) isn't succumbing to them. I know that when I charge it every month or two, it has drained down quite a bit each time, but comes back nicely.

 

By what you just said you have a damaged battery. if you did a load test and reported honestly you may learn that!
Most automotive Batteries have a recharge life which is almost never talked about on forums because it gets to technical.

 

Average lifespan of modern car batteries is about 3 years. Some die sooner, some last longer.