I’ve always used only one of the two keys my car came with, and in doing so the unused key’s battery died. Apparently they’re rechargeable, and get inductively charged when in the ignition slot in the car. Even with a dead internal battery, you can still drive the car with them. Neat.
- Buy a new keyfob shell from Amazon: https://www.amazon.com/dp/B084WSN9C9
- Buy a new battery. Avoiding counterfeit batteries from Amazon, I bought a Panasonic battery from Digikey, an electronics distributor. This way I could be confident I was actually getting a battery from Panasonic. Link: https://www.digikey.com/en/products/detail/panasonic—bsg/VL-2020%2FHFN/119936
- Carefully run a sharp blade around the perimeter of the old keyfob shell, within the line separating the upper and lower half. Example here: https://youtu.be/Vsld1DtGW_M
- Remove the PCB from the keyfob shell.
- Desolder the battery terminals and remove the dead battery.
- The tabs on this battery were at 180º instead of the required 90º, so I had to improvise:
- Bend the negative terminal outward, parallel to the battery bottom surface. Then bend the thin part of the terminal upward and snip it so it does not go past the top surface of the battery.
- Attach a thin lead wire to this terminal, long enough to reach the negative terminal on the PCB. Doesn’t have to be the exact length, there’s plenty of room for the wire to snake around a bit.
- Scrunch the positive terminal so the thin part starts flush with the bottom surface of the battery. The wider part of the terminal will want to go past the PCB edge – this will make it impossible to close the keyfob shell. So bend any excess terminal above the thin portion upward so it clears the PCB edge, but does not go past the top surface of the battery.
- Place some durable, non-conductive tape or other material on the PCB, where the battery will lie. The original battery terminals were sized so the battery sat just above the PCB surface and didn’t short out anything. We haven’t that luxury, so we need to insulate it ourselves. I used 3 layers of thin Kapton tape. A piece of Post-It note would probably work, too.
- Solder the positive terminal of the battery to the PCB at the lower-left corner.
- Solder the negative lead to the negative terminal on the PCB at the mid-left side. Keep the wire away from adjacent components.
- Place a small bead(s) of hot glue on the negative lead, and at the battery side such that it fixes it to the PCB.
- Carve out the curved ridge in the bottom of the new keyfob shell so it doesn’t pinch the negative lead.
- Place the PCB switch-side down into the top half of the keyfob shell.
- Smash the top and bottom halves of the keyfob shell together.
- You are now a genius, and you saved ~$250.