Don't say trickle charge - the technology has changed! I've used a Battery MINDer Plus for years due to its ability to remove sulfate from batteries. I see they've come out with a newer model (1510) that has lights to show you where the battery is in the charging cycle, similar to the CTEK. I recently got a great deal on the CTEK Multi US 4.3 (same one re-branded & sold by BMW for more $$) & like it, too.
http://www.batteryminders.com/products/
https://smartercharger.com/battery-chargers/
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2015 BMW 228i in EstorilBlau II Metallic / Terra Dakota Leather - 6-speed manual, M Sport Package, Cold Weather Package, Lighting Package, Premium Package, Harman Kardon premium sound, Anti-theft alarm system, Blue Metallic Matt Accent Moldings, Aluminum Hexagon Interior Strips, Anthracite Headlining