Thought I'd make a single thread for tracking.
My E63 complained of low battery in the fob. This is a newer style where the back simply clips on. It is released by pushing the key into a catch and releasing the back cover. This fob (no IR transmitter) has one battery. It actually had a 2025 which was kind of loose fitting. I believe it wants a 2032 so I ordered some. I found an old new 2032 to test fit and it needs to be jammed in and then pried out. But a single 2032 sounds right and the general Internet seems to agree.
My S550 uses the older tray style and I believe uses two 2025s.