BMW 3 Series key fob battery

Key with remote in the head

1. Here the key blade and remote are one piece. You open the plastic head, not the blade.
2. Many key heads split along the side seam. Start at the small notch and pry a little at a time.
3. Replace the cell with the same face showing, and make sure the rubber keypad sits flat before closing.
4. Press the head together until it clicks and test the buttons. The transponder chip that lets the car start is separate from the battery, so starting should not change.

The owner's manual says this remote has a rechargeable cell. It charges while the key is in the ignition or the car is driven. There is no coin cell to buy; if it no longer holds a charge, a dealer replaces the remote.

Smart key (push-button start)

1. Most push-button fobs hide a metal backup key. Press or slide its catch and draw the key out first.
2. The gap left by the metal key usually has a pry point. A slow twist with a thin flat tool (tape on the tip saves the plastic) opens the two halves.
3. Pop the old battery out with a toothpick or plastic pick, not a metal blade, and seat the new one with the + side matching the old one.
4. Snap the halves back together, slide the metal key home, and press each button, then try the start button from the driver's seat.

A CR2032 is 20 mm across and 3.2 mm thick. CR2032, CR2025 and CR2016 share the same width but not the same thickness, so a swap between them can leave a loose contact or a case that will not close.

A CR2450 is 24.5 mm across and 5.0 mm thick, much fatter than a CR2032. A thinner cell will rattle and lose contact, so buy the exact size.