You need a smart charger that can float the battery once it reaches full charge. A trickle charger does not do this. Battery Tender brand are cheap and they're what BMW rebrands. My friend has a Schumacher INC 700a that has a wide variety of features, and he's very happy with it. Obviously that's in a different price range, but it exceeds BMW's specifications for an in house charger.
'99 528i ('98/12 build). 282,000km BY29428/Royalrot
Breaking My Wallet since 2009
Mods: Stoptech SS brake hose, 280piece toolkit resting on trunk floor, Beisan VANOS seals, increasing amounts of duct tape and glue
'99 540i (grandfather's)