Mine unfortunately is not replaceable, being an E53. What I do to avoid them failing on me is swap keys about every 3 months to avoid one sitting for and extended time and ruining the charging cell.

Not sure if the E70 is the same, but you may try taking the weaker key for an extended drive or leave it in the ignition in position 1 with a battery charger on the car. (I’ve read that you can also set them on a wireless rapid charger for your phone and leave them overnight- never done it myself or seen it work, but rumor has... dunno)

Either way, good luck [emoji106]