The E53 does not use battery power in the key to send signals to the EWS (engine immobilser) system. It uses a radio frequency field around the key slot on the steering column to power a radio frequency ID chip (RFID) inside the key assembly, that then sends a unique ID signal to the EWS system. If the ID received by the EWS system is pre-programmed to be accepted, the EWS will allow the engine to start.

Again, the battery in the key plays no part in this process. The battery in the key is purely for the remote lock/unlock features, which are entirely seperate from the engine imobiliser system.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)