Apologies for my ambiguous phrasing, but if your key fob looks like the one shown in this thread, the whole assembly is sealed and the entire fob must be replaced--not just a battery.

A knowledgeable mechanic, skilled in disassembling and re-assembling electronic gizmos, might be able to replace the battery on the fob without messing up anything else. However, the cost would probably approach the dealer replacement price--about US$200--and there is some risk that a repaired fob might work only for a short time.

 

I popped a new battery into my 2004 R53 Fob Key(it wasn't soldered on)
and followed the reprogramming instructions posted in the forum, but the remote still doesn't work.

The Mini Dealership wants $180 for a new key and $180 to reprogram it.
GEEZ .... Any solutions out there that has worked for anyone else?

Thanks

 

I am also in the same situation
Put in a new battery and both fobs stopped working. I have tried to reprogram them but no luck.
I doubt that the receiver has gone and its a 2005 model