E46 diamond shaped keys have rechargeable batteries. They seem to last right around 8-9 years. The battery in my current E46 died at 6 years and the one in my older car died around 8 year mark.
Anyhow, the key will only change when in position 2. I tried experimenting to see if it'll change in pos 1, but no luck.I have big doubts it'll charge in off pos, but I havent tried.
If the battery doesnt hold charge its done, even if you take a 6 hour drive and charge the fvck out of it, it'll drain if you leave your car undriven for a weekend. Ask me how I know, lol.
I simply bought a Panasonic VL-2020 Lithium battery off eBay for both my cars, cut the key open, resolder and superglue back on. Worked perfectly.
Or you can always spend $200 at the dealer.