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So when I bought my 2000 328i last month the buttons on the key didn't work. I just assumed the battery was dead. The dealership only gave me 1 key, so I figured I would buy another key and keep the "dead" one as the spare. But I was reading through my owners manual, and realized that the battery in the key is charged when its in the ignition. So now I'm wondering if there is a bigger problem than just a dead battery! I'm also curious how this relates to the car alarm. It doesn't work. I assumed I just couldn't arm the alarm because the battery in the key was dead. But now I'm not so sure. Any advice or opinions are much appreciated. Thanks!
 

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Welcome to the forums. The keys have been discussed here before regularly. I assume there are two possible solutions to your dead key. Although I can't be sure about the alarm, I think it could be that it needs to be set by the key.


#1 - The battery in the key has 'gone bad'. They do honestly lose their ability to hold charge over the years and the ignition will not be able to put power into the battery to make it work. You can learn how to swap the battery
in this Youtube video complete with AWFUL music
Make sure you use the correct Panasonic VL2020 as the one in the video (and probably your key) is non-rechargable which will cause the issue. Don't forget you might need to code the remote functions after this!


#2 - The key itself is damaged. In which case you can either replace the key with an eBay special, program the remote functions to operate your car and move the transponder from your key into an eBay special. You may not want to move the transponder yourself as it requires incredible soldering skills and you only have one key, but it'll still be cheaper than the BMW $800 replacement.

Good luck
 

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And it would be wise to get another key. Even a cheaper non-remote key that will at least start your car and open the doors manually.
 

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Can someone please explain how the heck the ignition charges the key battery?! I know it does it, I just don't understand how the current paths to make it happen.
 

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This article goes into more depth about it but it seems to be induction from the car side with the coil in the key that is easy to break off the circuit board when you crack a key open. The coil in the key is also used to amplify signals from the transponder back to the ignition.
 

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Can someone please explain how the heck the ignition charges the key battery?! I know it does it, I just don't understand how the current paths to make it happen.
Its charged by transformer action. There are two coils or inductors, one in the key and one in the steering column. When a coil has current passing through it, it creates a magnetic field. When this magnetic field collapses it creates an electrical current. The column has a varying current going through its coil creating and collapsing its magnetic field. The keys coil is in the range of this magnetic field and it creates current flow in the keys coil. This current in the keys coil is used to charge the battery.
 

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I used the battery replacment instructions. Cut the key open, replaced the battery. Everything worked as stated, except the "lock" button. Not sure what went wrong there. I was just happy to have any function. Fast forward 3 weeks or so, and the remote functions die completely for a day or so, and then come back. This happens regularly now. Anyone else have this issue?
 
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