whereis the battery in my car?

What is your question?

i Recently bought this car used. SInce then, the key remote has quit working and no longer locks the doors. I believe its an after market key because theres no Mercedes Symbol on it. Also, some times when I try to accelerate from a complete stop, the car barely moves. Finally, I cant find the battery. If you could help me out with these problems I would greatly appreciate it. Thank you