Hey I Finnaly got plates on my e53

Only the car was in a parking lot for a while. And wasnt turned on for a while.
Now the key is weak. It randomly started the alarm. And now when I press the key it makes a little beep noise and try's to open the doors. Only it feels like its not enough power. Would this be the key fob or battery in car. I dont have secoud key to try it out with. Please help. Thanks