BMW master keys have built-in batteries that can unlock the car remotely. The battery recharges when the key is inserted in the ignition. Changing the battery for a BMW key is a simple matter. Unfortunately, BMW doesn’t give instructions in its manuals for this simple procedure. However, you still can do it yourself.
Purchase the correct battery for your car model. Because of the variety of BMW keys and car models, the table is not recreated here. The E39 takes a CR2016, for example. The 325CI takes a VL2020. A dealership should be able to tell you what battery size you need.
Alternatively, open the circular cover over the battery to read the size number off the back of the battery. Have the battery ready for the switch before you open the key, since you have to accomplish this task within 60 seconds to avoid having to reprogram the key.
Unscrew the two screws at one end of the fob, if your BMW is a 2001 model or older. Then, pull open the fob and replace the battery. Accomplish this in less than one minute to avoid losing the encryption.
Pry open the circular cover over the battery. Undo the two screws with a Phillips screwdriver. Swap out the battery and then replace the screws. Accomplish this step in 60 seconds on working keys to avoid losing remote-entry programming.
Use a pocketknife on other keys. Stand the key with the metal part pointed up and drive a pocketknife down into the seam between two plastic parts. A penknife won’t give you enough leverage to do this. Make the separation in the seam carefully. Pull out the battery. Put in the new battery. Seal the seam closed with glue for plastic. Perform this step in 60 seconds to avoid having to reprogram a working remote.
Your battery may not need changing. Go for a long drive with the key--the battery recharges while in the ignition slot. It may work again after extended recharging.