I don't think that BMW wants you replacing the battery,because on all models 2001 and up battery is unreplaceable. Owner's manual text says it is charged continuously. See attachment. Also read that if the key does not work, replace the key.
There is a possibility of the battery eventually discharging if the driver leaves the vehicle parked up for a long period without removing the key from the ignition lock completely.
I did see this bulletin, maybe it helps:
Vehicles affected: E36, E38, E39, E46, E52, E53, E83, and E85 with electronic immobiliser 3 (EWS 3)
If the key is not removed from the ignition lock completely (semi-removed status), there is a critical distance between the key and the ring antenna where the EWS will detect statuses permanently changing between "key inserted" and "key removed". This will result in a permanent closed circuit current of approximately 800 mA.
In the event of a complaint, explain to customer that he should either leave the key fully in the ignition lock, or remove the key from the ignition lock completely.
Parts replacement does not provide a remedy
The customer complains that the remote key does not lock/unlock the vehicle - the battery in the remote control is discharged
Vehicles affected – all models 2000MY-on E38, E39, E46, E52, E53, E60, E65, E66, E85.
1) Battery needs recharging
2) Key is defective
Analysis of remote control keys returned in warranty show that many are not defective. Only the battery needs to be recharged. Note that without use, the battery will only retain its charge for approximately 1 year. The Owners Manual advises that the key must be used at least once per year (driving for an extended period) to maintain its state of charge.
In case of complaint, check with the vehicle owner if the problem key is the key regularly used, or the spare key.
If the spare key is the problem, and a short drive puts sufficient charge in the battery to make it operational again, do not replace the key, but explain to the customer that the key must be used occasionally (driving for an extended period) to maintain its charge.
If the regular key is the problem, replace the key. http://tech.bentleypublishers.com/se...ual-3Sedan.pdf