DIY Guide: E46 Key Fob Battery Replacement

Is your BMW E46 key fob becoming less responsive? Do you find yourself pressing the buttons multiple times, or needing to be right next to your car for it to work? It’s a common issue for E46 owners, and often the culprit is a dead or dying key fob battery. While some might believe the urban legend that the key fob battery recharges while driving, this is simply not true for the E46. There’s no inductive charging or direct electrical connection to recharge the battery. The good news is that replacing the battery in your e46 key fob is a straightforward DIY task that can save you a trip to the dealer and unnecessary expense.

This guide will walk you through each step of the E46 Key Fob Battery Replacement process. We’ll cover everything from safely opening your key fob to resynchronizing it with your BMW. Let’s get your key fob working like new again!

Tools You’ll Need

Before you begin, gather these tools to make the e46 key fob battery replacement process smoother:

Vice-Grip: To securely hold the key while opening the fob.
Paint Scraper: A thin, sturdy scraper to start separating the glued fob halves.
Sharp Knife: For carefully slicing along the seam of the fob to break the glue seal.
Small Tipped Soldering Iron: Essential for desoldering and soldering the battery leads.
Silver Solder: For creating strong and reliable solder joints.
Small Chisel or Pocketknife: To help pry off the old battery leads.
Wide Jaw Pliers (Channel-Locks): To hold the circuit board steady during soldering.
Rubber Band: To gently apply pressure to the pliers, freeing up your hands.
New Battery: Panasonic VL2020 battery with or without leads (more on this below).
Plastic Glue: To re-seal the key fob securely.
Clamps or Rubber Bands: To hold the fob halves together while the glue dries.

Step-by-Step E46 Key Fob Battery Replacement

Opening the Key Fob

The BMW E46 key fob is notoriously glued shut, requiring a bit of careful effort to open.

Secure the Key: Hold the metal key part firmly with the vice-grip. This gives you a stable base to work with.
Start with the Scraper: Insert the paint scraper into the seam near the key ring end of the fob.
Slice and Pry: Carefully use the sharp knife to slice along the seam of the plastic fob casing. Work your way around the perimeter, gently prying with the knife or scraper as you go. Take your time and avoid applying excessive force to prevent damaging the plastic. The goal is to crack the glued seal without breaking the fob housing.

Alt text: Close-up showing the process of opening a glued BMW E46 key fob using a paint scraper and knife to separate the casing halves.

Identifying the Battery

Once you’ve opened the fob, you’ll see the internal components.

Locate the Battery: The battery is located on one side of the printed circuit board (PCB). In the original key fob, it’s typically a Panasonic VL2020 rechargeable battery.
Note the Leads: The original battery often has custom leads tack-welded to the positive (+) and negative (-) terminals. These leads are soldered to the PCB.

You have options for replacement batteries:

Battery with Leads: You can purchase a Panasonic VL2020 with pre-attached leads. This simplifies the soldering process. However, as noted in the original article, shipping costs can sometimes be high due to restrictions on lithium batteries.
Plain Battery (without Leads): A standard Panasonic VL2020 battery without leads is also suitable. You’ll reuse the existing leads from the old battery, which some find easier for soldering in this application.

Desoldering the Old Battery

Now, you need to remove the old battery from the PCB.

Secure the PCB: Grip the PCB firmly with the wide jaw pliers. Use a rubber band wrapped around the handles of the pliers to maintain gentle pressure and free up your hands. Important: Do not overtighten the pliers, as you could crack the circuit board.
Melt the Solder: Apply the tip of your hot soldering iron to the back of each battery lead where it’s soldered to the PCB.
Remove the Leads: Wait for the solder to melt completely, then gently pull the lead free from the PCB. Repeat for both leads.
Detach Old Leads from Battery (if reusing): If you are using a plain replacement battery, use the chisel or pocketknife to carefully pry the custom leads off the old battery. Be gentle to avoid damaging the leads if you intend to reuse them.

Alt text: Detailed view of the internal circuit board of an E46 key fob, highlighting the battery well, positive and negative solder points, and the white insulating pad.

Soldering the New Battery

With the old battery removed, it’s time to install the new one.

Prepare the Leads: If you are reusing the old leads, ensure they are clean and properly shaped to connect to the new battery and PCB. If using a battery with pre-attached leads, make sure they are correctly positioned.
Solder the Negative Lead: Solder the negative lead to the bottom (non-writing side) of the new battery. Use a small amount of solder to create a secure connection. Avoid using excessive solder, as space inside the fob is limited.
Solder Negative Lead to PCB: Re-solder the negative lead back to its original position on the PCB. Note that the negative lead through-pin on the PCB is typically smaller than the positive one – double-check the orientation.
Position the Battery: Once the negative lead is connected, the battery should sit snugly in the center of the white insulating pad on the PCB.
Solder the Positive Lead: Solder the positive lead. You can solder to the battery end first or the PCB end, whichever is more convenient. Ensure a good solder joint at both ends.

Closing the Key Fob

With the new battery soldered in, it’s time to reassemble the key fob.

Clean the Fob Edges: Clean any frayed edges of the plastic fob housing to ensure a clean mating surface for gluing.
Test Fit (Without PCB): Before applying glue, test fit the two halves of the fob together without the PCB inside to make sure they align correctly.
Reinstall PCB: Place the PCB back into one half of the fob.
Apply Glue: Apply a thin, even bead of plastic glue to the perimeter of one fob half.
Clamp the Fob: Carefully align and press the two halves of the fob together. Use clamps or rubber bands to hold the fob halves firmly together while the glue dries completely, following the glue manufacturer’s recommended drying time.

Re-syncing Your E46 Key Fob

After replacing the battery, you’ll need to re-synchronize your key fob with your BMW E46. Follow these steps for each key fob you have:

Get in the Car: Enter your BMW E46 and close all doors. Ensure the doors are unlocked.
Insert and Turn Key: Insert the key into the ignition and turn it to position 2 (ignition ON, all dashboard lights illuminated, but engine not started).
Remove Key: Remove the key from the ignition.
Press and Hold Unlock: Immediately press and hold the unlock button on the key fob.
Press Lock Button Three Times: While holding the unlock button, press the lock button three times in quick succession.
Release All Buttons: Release both the unlock and lock buttons.
Listen for Lock Response: The car’s central locking system (CLS) should respond by locking and then unlocking the doors (or just the driver’s door in some cases). This indicates successful synchronization.
Repeat for Other Keys: Repeat steps 4-7 for any other E46 key fobs you need to synchronize.

Important Note: Any key fobs that are not re-synchronized during this procedure will not function correctly until they are included in a re-synchronization process. You’ll need to repeat the entire sequence for all keys if you add a key later.

Conclusion

Replacing the battery in your E46 key fob is a rewarding DIY project that can restore your key fob’s functionality and save you money compared to dealership service. By following these steps carefully, you can successfully complete the e46 key fob battery replacement and get back to enjoying the convenience of your remote key. Remember to take your time, be patient, and double-check each step to ensure a successful outcome.