I now offer a BMW Key Fob repair and FZV Antenna Amplifier repair
A lot of people have been contacting me about their BMW Key Fobs not working and want to replace the battery. I have worked on many keys and have yet to find one that had a bad battery. Most of the time the problem isn't the battery at all. It's a bad switch, cracked solder joint, broken joint or the key just needs to be resync'd to the car. If you do attempt to do the battery replacement, be very careful opening the battery. You can very easily damage the electronics inside the fob and hurt yourself. Here is a guy that broke off a component E46Fanatics Forum post# 815604 If you don't feel comfortable doing the battery replacement yourself you can send your key in to me to have the work done. Contact me at repairs@bmwgm5.com BMW key repair service. Again I have yet to see a key that actually had a bad battery.
First you need to get the key open. Use a Exacto knife or box cutter knife. BE VERY CAREFUL. Cut along the seam all the way around. I've slipped and cut myself and it's not pretty. The part over by the where the key ring goes around is tricky. You need to pry the two halves apart just a little with a small screwdriver and then cut along the seam that you normally would not be able to get to.
Once the key fob is open the battery. You will want to replace the battery. I sell the same battery that BMW used, the Panasonic VL2020 Vanadium Pentoxide Lithium battery, but, the lead pattern is different. I've tried to get the correct one, but, it was a custom P/N for BMW which you cannot buy from Panasonic nor can you buy it from BMW. There are some sellers in China selling a CR2020 or DL2020 battery with the correct lead pattern, but, those batteries are primary batteries or in other words, non-rechargeable, one time use batteries that must not be charged. Since there is no easy way to disable the charging function in the key the charging current will damage the battery and shorten it's life. Other sellers are selling CR2032 batteries that they claim work in the BMW feb fobs, but, there are two problems. The 2032 is a thicker battery than the 2020. First two digits are the diameter and the last two digits are the thickness. A thicker battery won't fit well. The second problem is the battery chemistry. Most really aren't CR-type batteries. They are LIR-type Lithium ion batteries. What's the difference? Well VL-type are 3.0 Volt batteries which needs 3.4 ±0.15 Volts to charge. The LIR-type are 3.6 Volt batteries which needs 4.2 Volts to charge. The key will generate a high enough voltage to charge the LIR-type battery resulting in a battery that is never really charged. Stick with the genuine VL2020 battery and do a little modification. Again I sell the correct battery either direct or through eBay Auction # 290518228279.
The left picture is of the orignal battery on the board (E90 key fob). Note how close the battery is to the board. The next picture is the VL2020 battery I sell unmodified. Now note the distance between the battery and the board. The lead needs to be cut with sharp cutters or a dremel. The rightmost picture is the modified battery. Solder the pin and cut off the excess length.
There are a couple different ways to go about connecting the other battery terminal. I'll briefly show the most generic way. Bend out the battery lead and place a piece of tape under it to insulate it from board. Then run a wire from the lead to the hole in the board. DO NOT SOLDER TO THE BATTERY itself. I know that it's tempting to remove the battery lead from the old battery and solder it to the new battery directly to the battery case, but, the high case temperatures will destroy part of the battery internals and greatly reduce it's life.
Now put the key back in the case and temporarily hold it together with a rubber band and try syncing the key to the car to make sure it works. Since the power was disconnected it will need to be resync'd to the car . Resync procedure.
That's it. Now glue the case back together. I use superglue, but, I've heard to JB Weld works well. In any case it'll look like crap. There is no good way to glue the key back together.