How to replace the key fob battery of an E89 (no comfort access)

[Varixto]

My car came with two key fobs. The one I have always been using and a spare one, which looked brand new (it had the BMW stamp at the back). With it, once I gained access to the car with the good keyfob, I could start the car but could not lock/unlock it.
Owners of E89's with comfort access can simply replace the battery like in most other cars. However, I do not have said feature. My keyfob recharges itself simply by using it (it gets recharged by the fob slot in the car). The problem is that if you do not use your spare key for a long period of time, it will lead to the battery completely losing its ability to recharge. This is the problem I had.

If your car is like mine, the keyfob is sort of sealed and you need to pretty much destroy it, get the motherboard out to desolder the old battery and solder the new one! What a faff! (Thank you BMW)

I tried the electric toothbrush charger trick to try to recharge the battery but had no luck so I decided to investigate about this issue. Eventually I came across a post in another forum where a member had replaced the battery (luckily he had posted some pics.) and it did not look to tricky so I thought that I would give it a go.

The first thing I did was to buy some stuff to solder/desolder as I did not have anything at home and I had never soldered anything before. Then, I bought the various other bits and pieces (I had some of the stuff like the dremmel and the small vice).

COMPONENT/TOOL LIST

- Desolder sucker / pump (around £6 @ Amazon)
- Desoldering wire (this is optional, as I think the pump is better for this job)
- Soldering iron 40W (less than £15 @ Screwfix)
- Lead-free soldering wire (around £4 @ Halfords, I think)
- New rechargeable battery (Panasonic VL2020 - less than £7.50 @ Amazon)
- 11mm BMW logo stickers (£3.50 for 2 @ eBay)
- Small vice (to hold the fob/motherboard in place).
- Small dremmel (quicker) / stanley knife

1) NECESSARY + OPTIONAL PARTS, MATERIALS AND TOOLS...

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2) OPEN THE KEYFOB I used the dremmel to open the key fob and reveal the motherboard with the soldered battery. The arrows show the points you need to desolder in order to be able to remove the old battery.

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The front of the motherboard looks like this. I have highlighted where the battery is soldered to the motherboard (on the other side).

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3) DESOLDER THE OLD BATTERY
For this, I used the bottom of the key fob as a plate in order to be able to use the vice to hold the motherboard in place and desolder the battery. The desoldering is quite a fiddly process... You need to place the tip of the iron to melt the tin and then quickly use the pump to suck it out. You will probably have to do it two or three times per tab to be able to remove all the tin and remove the battery.

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Once you desolder both points / tabs, you can remove the old battery. You can see in the picture the two holes left after desoldering. These are the holes for the battery tabs.

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NOTE: When you buy the battery, make sure that the tabs are at a 90 degree angle, like in this pic.

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4) SOLDER THE NEW BATTERY Place the new battery in place. Put in on top of the old base of the keyfob, which you will use as a plate so that you can use the vice without damaging the motherboard. Solder both tabs. This is a bit fiddly, as the tabs are really tiny. I used my right hand for the iron and my left hand for the wire. I tried to make sure that the battery tab was also hot and then melted the wire while still touching the tab ever so slightly.

NOTE: This is the final product... Note the partially melted bit of that chip, it was like that when I opened the keyfob so it was like that from factory! However, it works fine.)
I am sure that if a professional has a look at the soldering, he/she will say that it's rubbish but... it did the trick!

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6) ASSEMBLE THE KEYFOB + PLACE LOGO STICKER FOR MIDDLE BUTTON The last part was, obviously to assemble the keyfob and place the logo sticker for the middle button. I chose a different logo because from now on I am going to try to use each remote equally so that the battery doesn't die again. The different sticker will help me recognise which is which (and I also think it looks quite nice!).

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Considering that when I went to my local BMW dealer, they told me that they would just make a new keyfob at around £245, I am happy with the result.

I hope that you have found this useful. Any questions, just ask!

[ARCS]

Sent my key off to these guys on ebay. 2 day turnaround for £17. Hasn't missed a beat.

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[Varixto]

Sent my key off to these guys on ebay. 2 day turnaround for £17. Hasn't missed a beat.
Screenshot_20250705_225532_eBay.jpg

Thanks for the reply. I had seen that on eBay and for a while I was considering it as an option. However, I enjoyed learning how to do it by myself. Now that I know how it's done, if I need to do it again I'll be able to do it much quicker and for much less, since I have all the tools and components needed.
In addition, I now have a soldering iron and all the gear, should I need to do any other job. My post might be more useful for someone who has all the tools.