How to recharge the BMW Key Battery
BMW keys are charged while in the ignition. From position 1 (KL R) and 2 (Run) position the key's battery will be charged using inductive charging. If you have a low battery and would like the key charged up without driving the car simply connect an automotive battery charger your car's battery pos/neg jump start terminals in the engine bay, then insert the key into the ignition and turn it to position 1 and leave it overnight. The charger will prevent your car battery from dying while the key gets charged.