Did I miss any indication that you've tried to reinitialize the key after letting it recharge overnight in the ignition?

BTW, the key does charge in the ignition in position 1-3. And if one chooses to do this...I highly recommend that you hook up a battery charger to the jump start locations in the engine bay..because if you allow the car's battery to die with the key in the ignition...there are multiple threads on multiple BMW forums where doing this has killed the key permanently...for some reason if the car battery dies with the diamond shaped key in the ignition...99% of the time the key will never start the car again for some reason. :dunno:

Here's more info about the diamond shaped keys:

I did it using Tom's DIY, for my E39 key.

If you do it properly with a soldering iron it takes no longer than 10 mins.

First remove the old solder connections with some solder wick. The battery should fall out now.

You can find the correct rechargeable battery on ebay (with correct welded legs)

Then just solder the new battery connections in place.

WOW!
Thanks guys for the overwhelming response.
I dont think it needs reprogramming, as I hit the unlock button on it yesterday and it worked. But it didnt work again.
I think Toms DIY is the answer. Thanks again.
Ben.

Ok, so I cut my key apart, (and my thumb), got the guts out of it and went out to the car and hey presto, the damn thing works everytime! put it back in the casing and no go, well worked once or twice. Take it out of the casing and it works faultlessly. Go figure?
I guess Ill change the battery as what else could I do.
Thanks again for all the help everyone.
Ben.

Well you've already cut the key open, but save yourself some trouble and send it to this guy for a proper repair. I tried to repair mine and like yours it worked for a few weeks then died, the guy in the link repaired my key for 40 or 50 bucks, well worth it. I then went ahead and bought a new key from my local dealer for $160, now I have two fully functional keys!:D I see your in the land down under, hopefully the shipping won't make it cost prohibitive.

BMW General Module 5 (GMV, ZKE V)

Did this last night as one of my key fobs wasn't working.
A $7 battery off ebay, $2 tube of epoxy and 15 mins work.

I reprogrammed it and it works 100%

What a fantastic DIY. Thanks.