A rechargeable battery can only be recharged a certain amount of times. Once its at the end of its life it will no longer hold a charge. You have really 3 options.

1) Buy new key from bmw dealer
2) Replace key in old remote (Requires cutting open the old key and special ordering the recharageable battery online only from an electronics website)
3) Find a blank key on ebay (Will be hard to find someone to cut it and program it, not really worth it, just buy one from the dealer works out to be the same price)