BMW Questions? Ask a BMW Repair Expert for Answers.
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these keys are sealed and are not intended to have the battery replaced. You can cut the key open and solder in a new battery, but a) this requires a decent amount of skill and b) you risk damaging the key so that it no longer is able to start the car.
there is no possible aftermarket key you can but that will start your car, period end of story. if you want a new key, it has to come from a BMW dealer.
if you would like to send your key out for somebody to replace the battery and verify that the battery in fact is bad, I would check out this site: https://www.bmwgm5.com/Key_Repair.htm
if you would like info to attempt battery replacement yourself (not advised) check out this site: https://www.youtube.com/watch?v=e1o9vH3mgmg or http://www.bimmerforums.com/forum/showthread.php?1356266-DIY-E46-Key-Battery-Replacement or http://www.bimmerfest.com/forums/showthread.php?t=315311
.Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle.