EricP3: Do you drive your car frequently? Is it still on the original battery? If yes to both, and you've not changed your driving habits lately, then I'd say you're due for a battery. Across several vehicles and a couple decades, I've typically seen batteries die at around the four year mark. I heed the warning signs if any show up, as it's no fun getting stranded when the battery finally goes toes-up.
A trickle charger is good when a vehicle typically sits for multiple days between being driven; i.e. if your BMW was a weekend fun car that you took out on Saturdays, but otherwise it sat in your garage. Also they're good when putting a vehicle up for a while, such as storing a motorcycle for the winter.
Is it worth eking out a few more weeks on this battery, waiting until it fully fails before replacing it? The most recent battery I replaced died with no warning; I ran a couple errands at lunch one day, pulled into a coffee place on the way back to work and when I came back out my car (non-BMW) was 100% completely dead electrically. Thankfully I was in the same plaza as a repair shop and sorted it out easily, it could have been a lot worse. Imagine if it were Saturday evening with your date in the car? (or wife & kids if appropriate).
