There are many threads regarding battery tenders/maintainers and a lot of different opinions.
I have always used a Delran Battery Tender Plus 1.25-amp 12-volt output charger on my other vehicles.
After reading the BMW manual for my M550, it states to only use chargers that have a constant charging voltage of 14.8 V.
To be safe, I ended up buying this BMW charger, which I believe is made by CTEK:
https://www.bmwpartsdirect.com/oem-p...er-61432408594