Mine showed green too. Yet I was seeing the battery symbol in the dash and cranking semed a bit slow by cold temp. There was no point waiting seeing it fail.
That's what I did yesterday. Walmart's EverMaxx 94R is good fit and has a 3-yrs replacement warranty. 108$. Dealers want 350$+ (installed).
I'd just pop in a new $110 WalMart battery myself to save hassle
2007 E90 AW 323i Step | Lowered 1.25"/1" | BMW Performance Exhaust | PBX | Debadged | Scangauge II | Style 162 18" & 161 17" wheels & rear 15mm spacers