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DIY: replacing key fob battery

617K views 477 replies 246 participants last post by  Shonky 
#1 ·
Ok so here is my first attempt at a detailed "DIY" thread, also if you mess up your own key trying to do this don't yell at me!:argue:

So I have the 2 key fobs that came with my car when I bought it used. One I use everyday and the second one has had a dead battery since I got the car. When I called the dealership they wanted like $260.00 for a new one. I did a search around here and found you could get deals and what not and possibly get it down to around $90-120 bucks.. still way too much in my book since I already had a working key.

After seeing a thread on here where another member had taken apart a key fob from a car he had already sold I decided what the hell the key was already dead anyways...

Here is the key before I tried anything..



I then proceeded to take a matt knife and xacto knife and basically carefully cut the key open along the seam where the top and bottom joined up.



Next I took a small chisel and broke the spot welds on the battery that connected it to the rest of the circuit board.



Once I had the battery out I looked at it and took the model number "a panasonic VL2020 3v" I found the exact same battery on mouser.com for $4.73. Once I got the battery is was a simple matter of soldering the new one in and making sure everything fit back together.




Before I glued it up I rubber banded the thing together and went out to my car to see if I could program it.

Put the working key in the ignition. Turn to position 2 for like 1 second then back and remove the key.

Press and hold the "unlock" button then press the "lock" button 3 times, you should hear all the locks cycle through.

Now take the non working key and press and hold the "unlock" button then press the "lock" button 3 times.

Then turn the ignition on and off again to end the programming....

I took the car for a drive to charge up the battery. Once I was certain it was all working I glued up the key again with some super glue....

Here is a pic of the key being glued...



While definitely not a hard or complicated "DIY" project I did save myself about $200 and had fun while doing it...
 
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#363 ·
MINIzguy: Not a great idea. The circuitry is still there to charge the battery and so your non-rechargeable battery will be getting charged every time you drive. At best it will probably leak eventually.

mraghvani: If the keys are truly blank, you can program the key using a special device (an EWS programmer can read/write keys and the EWS). However to program a key, you need to pull the EWS out of the car to get the codes to program into the key. EWS programmer is about $150-200 and is not a BMW tool.
 
#364 ·
Hey Shonky,

The second key, when bought off eBay might have been programmed before as I was also given a 6 digit code (I have no idea what its for). I then removed the circuit board and placed it into a blank, which was then cut and programmed with the 2 button method. All works, just not starting the engine :hmm:

Is the EWS stuff pretty easy to go thru ?

Do you know of any links on a 'How to' for blank keys ?

To go thru the EWS bits, is the BMW Scanner 1.4.0 go enough to use ?

Thanks in advance
mraghvani
 
#365 ·
To recode or read the EWS (to program keys) you have to remove it from the vehicle. You need a special EWS tool to read/program keys and to read/program the EWS.

6 digit code? I don't know what that would be for. The keys (as in the numbers/codes inside the key and EWS) are much longer than that. The circuit board is mostly the remote control section. The actual RF ID bit is usually separate in a corner near where the metal part of the key comes out of the plastic so it's close to the antenna inside the ignition switch. It's a self contained part and looks like a small integrated circuit chip with a corner cut off but with no pins. That's what the car talks to. Like this

 
#366 ·
Also, reprogramming the EWS was never intended to be possible by BMW. Hackers have just figured out how to do it.

BMW's plan was 10 pre-programmed keys in the EWS. If you needed a new key, they had a database of the pre-programmed codes and could just manufacture a new key and send it to you based on the VIN without touching the car.
 
#367 ·
Thanks for the info and appreciate the answers.

So basically I have no choice but to go to BMW and get a 2nd key ordered which will start the car. Ah well as least I'll have a spare which unlocks / locks the car but doesnt start it :(

Damn and I even bought the OBD cable and software, thinking I could give it a try.
 
#369 ·
#373 ·
Helpful hints!

If you have some of theses problems, ive made a list of ideas and hits to make this DIY easier!
*EVERYONE SHOULD CHECK TO MAKE SURE THE SOLERING POINTS DIDNT BREAK ON THE BACK OF THE MICROCHIP BOARD. MINE WAS, AND I COULD HAVE AVOIDED CHANGIN A PERFECTLY GOOD BATTERY!!! All I needed was a small re-solder of the Negative tab where it connects to the board.


1."Fob is hard to open or I keep cutting my fingers"
-Try to use an actual Carpenters Knife with a NEW razor, and only let the blade out ONE click. This makes the blade easier to handle and theres less risk of going so deep you cut the microchips/ so ive seen. And take your time..

2."Im having trouble with getting the old battery out"
-Use the razor flat against the battery and slice the spot welds. Take your time.

3."Key still doesnt work, tried reprogramming and it didnt work."
-Make sure the battery is making good contact and is not reversed. My battery is positive side up/..

4."How sould I re-seal the key"
-CrazyGlue/ or Gorrila Glue. Run a small bead around the bottom part of the FOB, then lay the top part with the buttons down to the glue. Use a zip tie around the butt of the key if yours isnt broken. Then clamp the front, Not on the buttons. Use this because othe things will ooze inside the key as they dont dry fast.

Futhermore.. I did not remove the OLD tabs, Nor did I solder ANY tabs to the new battery. I used a generic 2025 and just slid it between the old tabs. The pressure of the outer case keeps it tight enough to ensure good contact.

EASY is, as EASY does :thumbup:
 
#374 ·
Using a normal battery instead of a rechargeable is possibly not all that wise. An electronics engineer would know that.

The circuitry is definitely there to charge the battery and eventhough you have put a non-rechargeable battery in, it will still charge it which is probably the reason it is still operating. A normal battery will take an amount of recharging to a point where it will probably leak.

Also The 165mAh rating comparison is invalid. This is based on one charge. You are comparing the non-rechargeable running from 100% to 0% once (how it should be operated). The rechargeable battery can do this many hundreds of times.

All that said, my old E36 remote batteries where never replaced in the 6 years I had it using one key only. The E36 definitely did not charge the remote battery in the key. Not really sure why they changed to rechargeable actually.
 
#375 · (Edited)
eh.. not too worried about the battery melting like a junior mint. all i know is ive made it around the $280 stealership whammy. i think the battery will work just fine. The charge will fade away over time and need to be changed a lot sooner, im guessing, but im happy if i get a month outta it. remember to be poor you sacrifice proper operation for function... yes i know the rechargable is $2 more.. every penny counts !!
 
#376 ·
Genuine batteries seem to last in the 5-10 year range. Replacing the battery with the right one should give you that much again. Even if your non rechargeable is lasting a year before finally dying, you're a long way behind after saving your initial $2. If you get a month making you happy, you'll be a long long way behind.

Also it won't "melt". Depending on the battery chemistry it may leak something corrosive potentially damaging the electronics.

Your money, your car and your key though, so good luck.
 
#378 ·
I love this thread

I had a spare key that the previous owner said was broken. I was going to cut it open and replace the battery, but wanted to try synchronizing it first. I followed the steps outlined, but still wasn't getting a response.

I then tried the synchronization process using the working key first, and viola. I now have two working keys.

The best part being that the previously non-working key doesn't have the annoying child lock programmed into it.

Thanks :)
 
#385 ·
Re: horizontal or vertical mount

They are selling the horizontal (flat) mount on eBay. I bought horizontal mount last time, it wasn't great. The horizontal mount tabs are 180 degrees apart, the vertical tabs are 0 degrees apart (with some spacing), and the key is expecting a battery with terminals 90 degrees apart, so neither is "right". After trying a horizontal mount, I think the vertical mount will be easier to modify to work. Mouser shows a horizontal mount battery for both types (it's wrong). Digikey has the right pic for vertical mount:
http://www.digikey.com/product-detail/en/VL-2020/VCN/P082-ND/17263

These tabs might be bent to fit the circuit board's 90 degree apart placement. I'll be buying these from now on. Don't worry about using a "cheater" wire between one tab and the circuit board if you have to. If I remember I'll report back on how the vertical tab mount worked.
 
#387 ·
#386 ·
Wish I'd found this thread last week - both my keys have been working/not working intermittently and so I have been locking manually and eventually the central locking failed. I took to our BMW specialist who seemed confused and have now had the car for 6 days. They fixed the central locking and thought that would fix the keys (even though I told them it wouldnt as keys werent working before) They then replaced the "diversity unit" but still wasnt working so have now programmed a donor key and decided that both keys were broken. Seems weird that both keys could break and now thinking if I'd reprogrammed the keys before I took it into garage it would be working now and saved me £600!!!!! I think Ive been ripped off and am so fed up. Any advice please or is it too late now Ive let them spend the money. I just want my car back now and they say hopefully before the weekend!
Thanks
Anna
 
#388 ·
My remote suddenly stopped working yesterday after about 2 years of keyless bliss. I asked google and it found this thread. So I read the first page of this thread, then sliced open the key (clean job, no damage to key or fingers, yay!)

I started to order a new battery but I was horrified that shipping was 2x the price of the battery so I read some more before hitting 'buy'. I found a post that mentioned good battery but bad contacts, so I checked .. the battery has 3.1v normally and low 2's when a button is depressed. So the battery seems good.

The solder joints look good visually and the positive feels stuck on pretty well. I haven't pulled it out to check the negative.

Does anyone have suggestions on how to further diagnose between (1) battery contacts, (2) a bad fob circuit board, (3) something in the car (FZV amp?), or (4) something else? I'm afraid that I don't have a whole lot of time or equipment to be tinkering too much, but on the other hand, I'd hate to just start replacing things at random.

Thanks in advance for any advice. This thread has been a huge help already! :)
 
#389 ·
It really shouldn't drop in voltage that much when transmitting. Have you driven around with it for a reasonable time to charge the battery? The battery should have enough charge to work initially

Also have you re-synced the key to the car since replacing the battery?

If you soldered it in, there really can't be bad battery contacts. The spot welds to the battery are either connected (or not)
 
#390 ·
tdm, Shonky is right, a 1/3 drop in voltage is unacceptable. If the voltage is less than ~2.7 under load, a signal is probably not even being produced. My O-Reilly's has a "key transmitter tester" tool that lights up when it detects a keyfob signal. You could see if a local car shop has that tool, but even so, the frequency might not be correct with so low of a voltage. If your key IS sending a signal but your car isn't unlocking, that's more proof that the battery is too weak.

I think you really need to replace the battery. One of the keys I've opened actually had a broken pin from the battery mount so it was obviously an open circuit, but another had a weak battery like yours, and replacing the battery fixed it. I agree that the shipping is high (you sound cheap like me!). I bought a few batteries on the last order, and have fixed two keys from it, so it helps spread the cost.

Another tip. I have learned the hard way to sand smooth the rough edges left by cutting open the key before gluing them back together. Two of the keys I fixed broke apart when their respective keychains were dropped on cement. Sanding the edges smooth and flat using a sandpaper block resulted in (so far) break-free repairs.

Finally, if your SO uses one of the keys you fixed, explain to (her) how to put the circuitboard back in the shell and tape/rubberband it in to get the car to start so she's not stranded if she drops it and it breaks when she's out.
 
#391 ·
My two key fobs stopped working today. Jumped on Ebay and ordered two Panasonic batteries for $14.90 which includes free shipping.

So I could just program one key fob at a time and the other one doesn't have to be working correct? Hopefully I could be lucky like one member here where he just replaced the battery and didn't have to do any further programming.
 
#393 ·
replacing battery in key fob

great diy, great thread. I opened up the key fob, no problem. I detached the old battery by using a jacknife blade to gently pry under and break the welds on positive side. I then lifted ( bent) that positive moon shaped connector arm vertically and out of the way. I then lifted the old battery up to expose the welds on the negative side and I used the knife to break the welds on neg side. The battery came away then. I then straightened out ( re-bent) flattened the pos. and neg. connector arms that I had twisted during removal of old battery. Both pos. and neg. conn. arms are still soldered attached to the circuit board. Kind of like a sandwich. So, when I receive the new battery, why can't I just slide it in between the pos. and neg. arms ( top and bottom), maybe use epoxy to hold the new battery in place and be done? Did I miss anything?
 
#394 ·
as noted prev., I have the old battery out. When I opened up the fob, I noticed a whitish film or corrosion on the circuit board in the area of the squiggly (antenna) line and going up the board a bit. I tested the old battery & got 3v. I placed the old battery back in the battery clamshell holder and tested the actual battery contacts on the circuit board, 3v. Good. So I have battery contact. I then put the tester probes on each side of the little push button switch that opens the doors (there are 3 switches). I had my wife use a toothpick and push down that switch, nothing, no reading just 000. I then did the same on the center switch, got 3 v. I didn't test the last switch. I wonder if I have a bad "door opener" switch due to past corrosion? I will still install the new battery when it's delivered but corrosion may have damaged the circuit board. I'll keep you posted.
 
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